A glossary of the words we use about trees

Our articles use a fair number of words without stopping to explain them: abscission layer, included bark, mast, tylosis. Here they are, each in a sentence or three, with the article that treats it.

Roots, trunk and wood

Adventitious root

A root that forms where roots do not usually form: on a submerged stem, on a trunk from a dormant bud, or at the stump of an old root that died back. Flood-tolerant trees grow them in quantity when the water comes up and the original fine roots die, and the capacity to make them tracks flood tolerance closely. See How floodplain trees survive high water.

Aerenchyma

Thin-walled tissue full of gas spaces, built in roots and shoots, that lets oxygen diffuse down from the shoot to a root sitting in water. Tolerant species pair it with a barrier to radial oxygen loss, which keeps that oxygen from leaking out through the sides of the root before it reaches the growing tip. See How floodplain trees survive high water.

Branch collar

The slight swelling at the base of a branch, where trunk wood and branch wood overlap a little more each year. Inside it is trunk tissue with a protection zone that resists decay; outside it is branch. The cut goes just outside the collar and the branch bark ridge above it, never through either. See Pruning at the branch collar, and when to make the cut.

Burl

A hard, rounded, knotty swelling on a trunk or a big limb, wood the cambium laid down faster, for longer each season and in no set direction. Infection by bacteria, viruses or fungi is the usual suspect, but what started any one burl is seldom known. It rarely harms the tree. Cutting it off leaves a wide, shallow wound with no protection zone at its base to hold back decay. See Burls: what makes the big knobs on a trunk, and do they harm the tree?.

Chambered pith

Pith broken by thin cross walls into a row of hollow chambers, which shows when a twig is sliced lengthwise with a knife. Black walnut has it. Where the walls cross a solid core instead, as in black gum, the pith is called diaphragmed. See Winter identification: telling native trees apart by bark, buds and twigs.

Chlorenchyma

Plant tissue whose cells carry chloroplasts. In a stem it forms a narrow band, usually a tenth to three tenths of a millimeter thick, immediately under the outer bark. Nothing about it shows from the surface: scrape a few cells of cork off a gray trunk and the green is there. See Photosynthesis without leaves.

Corticular photosynthesis

Photosynthesis in the green tissue under a stem’s outer bark. Because the corky outer layer resists gas diffusion, the chloroplasts mostly work on carbon dioxide the stem’s own living cells have already respired. The result is less carbon leaving the trunk rather than more coming in. Researchers call that stem recycling photosynthesis, and reserve stem net photosynthesis for green stems that take carbon dioxide from the air through stomata. See Photosynthesis without leaves.

Girdling root

A root that circles the stem instead of running away from it and thickens until it strangles the trunk. A buried root flare and a container ball whose roots wrapped around the pot both produce them, and the tree can look healthy for a decade first. See The habitat around a tree is part of the tree.

Heartwood and sapwood

A living tree is a thin sheath of cambium, phloem and outer sapwood wrapped around a column of heartwood that is chemically defended but no longer alive. Heartwood stiffens the trunk and neither conducts water nor stores food, so decay working in it is eating tissue the tree already wrote off. See Fungus on a tree is not a death sentence.

Hypertrophied lenticel

A bark pore on a flooded stem that has swollen far past its normal size. It helps aerate the stem and the roots and gives toxic compounds a way out. Along with adventitious roots, it is one of the two signs that a tree is acclimating to standing water rather than dying in it. See How floodplain trees survive high water.

Included bark

Bark squeezed into a seam between two stems that started a few inches apart and widened until they pressed together. It cannot become wood, so no fibers run from one stem to the other and the union is held by pressure alone, under bark that grows over the seam and hides it. See Multistem or single stem? Sometimes the trunk is hiding the answer.

Lenticel

A pore in the bark that lets gas pass through an otherwise sealed outer skin. They show on a young twig as small raised dots or dashes, gray on sassafras, and on a flooded stem they swell into the hypertrophied lenticel above. See Photosynthesis without leaves.

Mycorrhiza

A partnership between a fungus and a root, in which the fungus trades water and nutrients, phosphorus above all, for the plant’s sugar. About 80 percent of flowering plants and all conifers form one. Ectomycorrhizae, which sheathe the root tips, occur mainly on forest trees. A high supply of soluble phosphate holds back the commonest kind, arbuscular mycorrhizae. See Organic or synthetic fertilizer: how they differ and what a tree needs.

Nitrogen fixation

Turning nitrogen from the air into a form plants can use, done by bacteria living in nodules on the roots of some plants. Northern bayberry and leadplant carry them. On poor dune sand, bayberry adds an estimated 14 to 29 pounds of nitrogen an acre a year. See Native trees and shrubs for dry, sandy soil: the plants of barrens and dunes, and how to grow them.

Periderm: phellem, phellogen and phelloderm

The three layers that make up a stem’s outer skin. The phellogen, better known as the cork cambium, is a sheet of dividing cells. It lays down phellem, the dead corky layer people mean when they say bark, on its outer side, and phelloderm on its inner side. The chlorophyll is in the phelloderm and the cortex under it, never in the cork, which is why thick furrowed bark shuts the light out and a smooth thin-barked twig does not. See Photosynthesis without leaves.

Pith

The small soft core at the center of a stem, laid down by the growing tip in the shoot’s first season, with every later ring of wood wrapped around it. A trunk grown from one bud has one pith from base to top, so a cross section showing two was never one tree. See Multistem or single stem? Sometimes the trunk is hiding the answer.

Pneumatophore

A root specialized for gas exchange in wet soil. Bald cypress knees are the famous eastern candidate and the contested one. They lack the anatomy that defines a pneumatophore, and sealed-knee tests measured less oxygen uptake than other plants manage. Whether the knees work as one is an open question. See How floodplain trees survive high water.

Rhizosphere

The thin zone of soil around a root that the root itself changes, with no fixed edge. Roots release 10 to 40 percent of the carbon their leaves fix into it, as sugars, organic acids and mucilage, and it holds 1,000 to 2,000 times the microbes of bare soil. See Organic or synthetic fertilizer: how they differ and what a tree needs.

Root flare

The widening at the base of the trunk where the first structural roots leave it. It belongs in open air, at or slightly above the finished grade; buried, the bark stays wet, softens and rots. A trunk that enters the ground like a fence post has had something piled on it. See Ten planting mistakes that quietly kill trees.

Root plate

The thick, woody roots that spread from the base of the trunk through the shallow soil and hold a tree upright. On a large tree the plate reaches about 10 feet from the trunk. A trench or footing cut through it takes the roots that keep the tree standing, and the tree can fall in the next storm. See Tree roots run shallow and wide, not deep: what that means for watering, mulch and the house.

Sinker root

A root that grows straight down from a side root rather than from the base of the trunk. In most oaks the taproot dies back as the tree grows, and sinker roots from the laterals take over its downward work. See Tree roots run shallow and wide, not deep: what that means for watering, mulch and the house.

Taproot

The root a seed drives straight down ahead of its side roots; a chestnut sends one down before it sends a shoot up. Taprooted species resent being moved, which is why sassafras is seldom sold and why taprooted oaks transplant better in early spring than in fall. See Transplanting young trees: timing, water and protection.

Tylosis

One of the growths that plug the pores of wood from the inside; several are tyloses. The white oak group fills its pores with them and the red oak group does not. That is why a short length of red oak can be blown through like a straw and why white oak makes watertight barrels. Swamp chestnut oak is one of ours in that group. See Red oaks and white oaks, and where each belongs.

Buds, leaves and growth

Abscission layer

A zone of weak cells the tree builds across the base of a leaf stalk in late summer, sealed on the twig side with a corky plate. Once it is complete the leaf breaks away at the slightest pull, which is why leaves come down in a rush on a windy October day. See Marcescence: why young beeches and oaks keep their dead leaves all winter.

Adventitious bud

A bud that forms late and in an unusual place: in the callus around a wound, in the cambium, or on a root, with no thread of tissue running back to the pith. It is what a stump or a root sends up when it has no dormant buds left. See Epicormic buds: the sprouts that appear after damage or new sunlight.

Anthocyanin

The water-soluble red pigment that colors a new spring leaf bronze, pink or wine-red, the same one in blueberries and autumn leaves. It sits in the outer cell layers and cuts the light reaching the chloroplasts still being assembled underneath, and the tree takes it apart once they are ready. In fall the leaf builds it new from trapped sugar, most likely to shade the leaf while the tree recovers its nitrogen. See Why new leaves come out red and Why leaves turn red and yellow in fall, and which native trees give each color.

Bud scale scar

The ring of small scars a twig’s end bud leaves when its scales fall in spring. Each ring marks where one year’s growth began, so the stretch from the tip back to the first ring is last year’s growth. The rings farther down count the years before. See A year-by-year care calendar for a newly planted tree, from the day it arrives to its fifth spring.

Carotenoid

One of the yellow and orange pigments, carotenes and xanthophylls, that give a carrot its color. They sit in a leaf beside the green chlorophyll all summer, masked by it. Once the tree takes the chlorophyll apart in fall they show through, as the yellow of hickory, birch and witch hazel. Unlike anthocyanin, they are not made new for the season. See Why leaves turn red and yellow in fall, and which native trees give each color.

Chilling requirement

The winter cold a tree has to sit through before its dormancy will break; only then does spring warmth expand the leaves preformed in the buds last summer. The thresholds are inherited, so two oaks of one kind can be two weeks apart in April. See When oaks leaf out, and why two of them never agree.

Cladoptosis

The shedding of twigs, and now and then a larger branch, through an abscission layer at their base, the way a leaf is shed. Oaks, bald cypress, poplars, willows and maples do it, many every fall, and old trees and stressed ones shed more. A shed twig ends smooth and slightly rounded, where one cut by a squirrel shows a slant and one cut by a twig girdler a ring. See Twigs all over the lawn: squirrels, twig girdlers, or a tree shedding its own twigs.

Deciduous conifer

A cone-bearing tree that drops every needle in fall, as a hardwood drops its leaves. The East has three natives: bald cypress, pond cypress and tamarack. Bald cypress sheds its needles still attached to small side twigs, and tamarack turns gold from early September and is bare by mid-October. A bare one in winter is judged by a scratched twig, not by the calendar. See Is my bald cypress dead? The conifers that drop every needle in fall, and when brown means trouble.

Delayed greening

The habit of opening new leaves that are limp and red or white and only green up later. It is the botanists’ name for a pattern common across the world’s tropics, and the bronze flush on eastern oaks and maples in April is the same thing on a shorter timetable. See Why new leaves come out red.

Dioecious

Bearing male and female flowers on separate plants, so only the females fruit. Persimmon, sassafras, staghorn sumac and the hollies work this way, which is why they are planted several at a time. See Why a tree flowers but bears no fruit: it may be a male, lack a partner nearby, or be too young.

Dormant bud

A bud that formed in a leaf axil, never grew into a shoot and did not die. It stays joined to the pith by a thin thread of tissue. Each year it grows just enough to keep its tip at the surface as the twig thickens into a trunk. Hormones from the growing tips hold it asleep. See Epicormic buds: the sprouts that appear after damage or new sunlight.

End bud

The bud at the tip of a twig, also called the terminal bud, larger than the side buds and pointing straight ahead. Some twigs have none, and the last side bud stands in beside a small scar where the tip fell away, as on basswood. Oaks end each twig in a cluster of side buds crowded at the tip. See Winter identification: telling native trees apart by bark, buds and twigs.

Epicormic shoot

A shoot from a dormant or adventitious bud under the bark, breaking out when the balance between roots and crown is upset by a lost top, a hard cut, sudden sun or drought. Water sprouts run straight up from a branch, suckers from the base or the roots; a clump of basswood is a ring of them on one stump. See Epicormic buds: the sprouts that appear after damage or new sunlight.

Grass stage

The seedling stage of longleaf pine: a tuft of long needles at ground level with no visible stem, while the taproot drives down. It lasts from 2 to 15 years, and the needles around the bud shield it from the heat of a surface fire. See Native trees and shrubs for dry, sandy soil: the plants of barrens and dunes, and how to grow them and How fast native trees grow: a year’s growth by species, soil and sun.

Growth rate

The height a young tree adds to its leader in a year. Our growth estimator sorts species into three classes by it: 12 inches or less a year is slow, 12 to 24 inches medium, and 24 or more fast. A class describes a young tree in full sun on decent ground. Shade, grass over the roots, poor soil and deer can hold any species to a few inches a year. Good care can lift a slow one into the fast band. See How fast native trees grow: a year’s growth by species, soil and sun.

Leaf scar

The mark left on a twig where a leaf fell, just below the bud that formed in its axil. The dots inside it, the bundle scars, are where the leaf’s veins joined the twig, and their shape and number help name a tree in winter. On a black walnut the whole scar looks like a monkey’s face. See Winter identification: telling native trees apart by bark, buds and twigs.

Marcescence

Holding dead leaves on the twig through winter instead of dropping them in fall. The leaf dies on schedule and turns brown, but the abscission layer never finishes, so it hangs on until the buds swell in spring. Beech is the champion, the oaks come next, and American hornbeam does it too. See Marcescence: why young beeches and oaks keep their dead leaves all winter.

Naked bud

A bud with no scales, protected only by a pair of miniature leaves. Most buds are wrapped in scales that overlap like shingles or meet edge to edge in a pair. Witch hazel is one of the few species whose buds are naked. See Winter identification: telling native trees apart by bark, buds and twigs.

Palisade cells

The column-shaped cells packed under a leaf’s upper surface, where the leaf absorbs light most strongly. A sun leaf has longer ones, often stacked in more layers, and that is much of why it is thicker than a shade leaf. See Sun leaves and shade leaves: why one tree grows two kinds of leaf, and what it means for a seedling moved into shade.

Photoinhibition

Damage to a leaf’s light-harvesting proteins when it takes in more light energy than it can use. A brand-new leaf is the most exposed to it: thin, its cuticle unfinished, its chloroplasts still being assembled. The red sunshade of anthocyanin is the answer. See Why new leaves come out red.

Photosynthetic capacity

The fastest rate at which a leaf can fix carbon in strong light, set by the enzyme that captures carbon dioxide and by the chain that carries the light’s energy. In 23 tree species it peaked within days of the June solstice and fell as the days shortened, while the leaves stayed green and the summer grew warmer. See How a tree’s photosynthesis changes through the year, from leaf-out to leaf fall.

Resorption

The salvage of nutrients from a leaf before the tree drops it. In fall the tree takes the leaf apart and moves its nitrogen and phosphorus into the parts that live through winter. That salvage can supply most of both nutrients in an overwintering tree. Fall red most likely shades the leaf while it goes on. In a 2003 test, shrubs with a mutation that left them almost no red recovered far less nitrogen than normal plants. See Why leaves turn red and yellow in fall, and which native trees give each color.

Seasonal needle drop

The fall shedding of an evergreen conifer’s oldest needles, the innermost on each branch, which turn yellow or brown while the current season’s growth stays green. White pine shows it most, dropping its two- and three-year-old needles each autumn, and a dry year makes the drop heavier. Browning on new growth, on one side or rising from the lower branches has another cause. See Is my bald cypress dead? The conifers that drop every needle in fall, and when brown means trouble.

Stomata

The pores in a leaf’s skin, each opened and closed by a pair of guard cells, that let carbon dioxide in for photosynthesis and water vapor out. Most of the water a plant takes up leaves through them. In drought or high heat they close, and photosynthesis slows or stops until they open again. See How a tree’s photosynthesis changes through the year, from leaf-out to leaf fall.

Sun leaf and shade leaf

The two kinds of leaf one tree grows, each built for the light it gets. A sun leaf, at the top and outer edge of the crown, is smaller and thicker, with a thicker cuticle and longer palisade cells. A shade leaf, inside the crown or under taller trees, is larger and thinner, with more chlorophyll for its weight. A grown leaf can barely change from one kind to the other, so a tree moved into new light answers with new leaves. See Sun leaves and shade leaves: why one tree grows two kinds of leaf, and what it means for a seedling moved into shade.

Damage and decay

Beech snap

A beech trunk that breaks off in heavy wind before the tree has died, because beech bark disease has killed the wood under the bark. A scale-infested beech with a healthy-looking crown can still snap, so it counts as a hazard. See Beech leaf disease: the striped leaves, how far it has spread, and whether to plant beech now.

Buck rub

Bark shredded from a young trunk when a buck rubs the velvet off its antlers in the fall rut. It can kill a young tree, and it strikes species deer seldom eat, such as sweet birch. A guard goes on each year from August through December until the trunk is 3 to 4 inches thick. See Native shrubs and trees deer usually leave alone, and why the lists disagree.

Canker

A patch of bark killed by a fungus or bacterium, which often gets in through a wound. The bark over it sinks, darkens or splits from the living bark around it, and it may ooze. Over months or years a canker can girdle a branch or a trunk, and chestnut blight kills American chestnut stems this way. The bark is the test against a burl, whose bark stays alive. See Burls: what makes the big knobs on a trunk, and do they harm the tree?.

Compartmentalization (CODIT)

Walling damage off instead of healing it. Wounded wood does not regenerate the way skin does; the tree builds chemical and structural barriers around the damaged zone and grows new wood over the outside, so the decay stays behind the walls. Alex Shigo, the plant pathologist who described it, gave the model the initials CODIT. See Fungus on a tree is not a death sentence.

Frost crack

A split that opens lengthwise up a trunk in a sudden deep cold, sometimes with a sound like a gunshot, most often from an old wound or branch stub. New wood seals it the next summer, but it often reopens in later winters, and the growth along the seam builds a raised ridge called a frost rib. See Frost cracks and sunscald: why young bark splits in winter, and what a trunk wrap can and cannot do.

Gall

Plant tissue grown into a shape the plant would never make on its own, around an insect. A gall wasp lays an egg in a bud, leaf, twig or root. Chemical cues from the egg and then the larva make the tree build a chamber, line it with nutritious tissue and wall it in. The oaks we grow will each host their share. See Wasp galls on oaks.

Iron chlorosis

Yellowing between the green veins of young leaves, which may turn white. It is a symptom of soil pH rather than of a soil short of iron: above pH 7.0, iron in the soil becomes less available to roots. Chelated iron greens the leaves for a while, but the yellowing returns until the pH is corrected. See Organic or synthetic fertilizer: how they differ and what a tree needs.

Leaf scorch

Browning that starts at a leaf’s tips and edges, the parts last in line for water, when sun, heat and wind draw water from the leaves faster than the roots can replace it. Newly planted trees are more prone to it, because their roots have not yet grown out, and a shade leaf moved into full sun is at risk too. Scorch alone does not kill an otherwise healthy tree. See Sun leaves and shade leaves: why one tree grows two kinds of leaf, and what it means for a seedling moved into shade.

Nematode

A microscopic roundworm. Most nematodes are free-living, and most of those that feed on plants live in the soil and eat roots, but the one behind beech leaf disease lives in the buds and leaves of beech. It winters in the buds and damages the leaves before they open. See Beech leaf disease: the striped leaves, how far it has spread, and whether to plant beech now.

Phytophthora root rot

A root rot caused by Phytophthora, a water mold that thrives in poorly drained or wet ground. It is among the commonest diseases of azaleas and rhododendrons, and on clay and low ground it kills American chestnut, mainly in the South. See Native trees and shrubs for heavy clay: which take it wet, which take it dry, and how to plant them.

Retrenchment

The stage in which an old tree sheds: leaders snap out, big limbs tear away, the crown lowers and thickens and the trunk hollows. Ancient-tree specialists in Britain named it, and trees managed through it rather than removed live for centuries in that state. It is the middle of a long life, not the end. See Fungus on a tree is not a death sentence.

Sapsucker well

One of the shallow holes a yellow-bellied sapsucker drills through bark into the living tissue and returns to through the day to lap the sap. They are about a quarter inch across, in neat horizontal rows, often banded around a trunk, and hummingbirds, wasps and sap-feeding butterflies drink at them too. See Sapsucker holes and the wasps that follow them.

Sunscald

Death of the living tissue under the bark on the south or southwest side of a thin-barked trunk in winter. Afternoon sun wakes the tissue from dormancy, and a fast drop in temperature at sunset or under a cloud freezes it. The bark over the patch later sinks, cracks and peels away. A white wrap for a young tree’s first two or three winters guards against it. See Frost cracks and sunscald: why young bark splits in winter, and what a trunk wrap can and cannot do.

Winter injury

Damage a woody plant takes between fall and spring: stem dieback, frost cracks, sunscald, browned evergreens. The Morton Arboretum ties it more often to swings in temperature, such as a hard freeze after a mild fall or a thaw followed by a sharp drop, than to deep cold. See What a hardiness zone tells you about a tree, and what it leaves out.

Planting and care

Age class

The two numbers a forest nursery gives a bare-root seedling, such as 1-0, 2-0 or 2-1. The first counts the growing seasons in the bed where the seed was sown, and the second the seasons in a transplant bed after the seedling was lifted and replanted in wider rows. The age is the sum, so a 2-1 is three years old. See Reading a nursery listing: seedling, sapling, caliper, bare-root and potted, and what a tree’s age tells you.

Assisted migration

Moving seed on purpose from warmer places to cooler ones, ahead of a warming climate. It comes in three scales: within a species’ range, just past its edge, or far beyond it. For most eastern trees the current guidelines treat a move of about 200 miles north of the seed’s source as safe, and sources disagree on going further. See Seed source and provenance: why where the parent tree grew decides how yours will do.

Balled and burlapped

Dug from a nursery field with a ball of soil around the roots, wrapped in burlap and often set in a wire basket. The ball keeps only a small share of the roots, since a tree spade can cut away up to 98 percent of them. A ball over 2 feet across generally takes a machine to move. See Why a small sapling catches up with a big transplanted tree, and often passes it.

Bare-root

Moved or sold with no soil on the roots, which is how most small trees are sold; each plant page says which form that species ships in. The fine roots dry out in minutes, so they stay wrapped and out of sun and wind and go back in the ground the same day. See Bare-root trees on arrival: the first hour, the first week and the first spring and Transplanting young trees: timing, water and protection.

Caliper

Trunk diameter, the number most planting rules run on. It is taken at the root collar on a seedling and 6 inches above the root flare on a larger nursery tree, moving to 12 inches once the trunk reaches 4.5 inches there. A root ball is about a foot wide for each inch of it. A new transplant gets 1 to 1.5 gallons of water per inch at each watering. A protected root zone is a foot of radius per inch. Our watering calculator does the arithmetic. See Transplanting young trees: timing, water and protection and Reading a nursery listing: seedling, sapling, caliper, bare-root and potted, and what a tree’s age tells you.

Cation exchange capacity

How much positively charged nutrient a soil can hold against leaching. Clay and organic matter carry negative charges that hold calcium, magnesium, potassium and ammonium, while nitrate, a negative ion, washes through. A soil test reports it in milliequivalents per 100 grams: 1 to 5 for sand, over 30 for clay and 200 to 400 for organic matter. Sand, holding so little, is fed little and often. See Organic or synthetic fertilizer: how they differ and what a tree needs.

Central leader

The topmost upright stem of a young tree, the one that carries the trunk upward, with the side branches spaced along it. Where two stems of equal size fork, the weaker is cut out while both are small, before bark is trapped between them. Shrubs and trees that grow in clumps by nature keep several stems. See A year-by-year care calendar for a newly planted tree, from the day it arrives to its fifth spring.

Claypan

A dense layer of clay under the surface soil that water passes through slowly, so the ground above it stays wet long after rain. On the level glacial till plains of southwestern Ohio and southern Indiana and Illinois, a claypan keeps the pin oak flats wet through winter and spring. See Native trees for the Great Lakes and Midwest: Michigan, Ohio, Indiana, Illinois and Wisconsin.

Double dormancy

A seed that needs a warm spell to grow its root and then a winter to release its shoot, so in the wild it comes up the second spring after it falls. White fringetree, witch hazel and gray dogwood behave this way. See Planting acorns, nuts and seed: collecting, storing and the first year.

Drip line

The circle on the ground under the outer tips of a tree’s branches, where rain drips off the crown. The roots spread two to three times as wide as the crown, so an established tree is watered past it, and a black walnut’s juglone reaches far beyond it. See Tree roots run shallow and wide, not deep: what that means for watering, mulch and the house and Juglone: which plants a black walnut poisons, how far its reach goes, and what to plant beneath it.

Dry shade

Shade where the soil is dry as well as dark. A dense crown keeps the ground dry through the first minutes of every rain, and the tree’s roots take most of what gets through. Maples and beech make the hardest of it, and plants chosen for it cut the watering without ending it. See What to plant under a big tree: native shrubs for shade, and how to plant without hurting the tree.

Establishment period

The time a newly planted tree takes to grow its roots out into the soil around it, until its shoots grow as fast as before the move. After it, the tree needs water only in drought. It runs by the inch of trunk and by climate: about 3 to 4 months per inch in Florida, 6 in zones 7 and 8, 12 in zone 5 and 18 in Minnesota. See Why a small sapling catches up with a big transplanted tree, and often passes it and A year-by-year care calendar for a newly planted tree, from the day it arrives to its fifth spring.

Float test

Sorting acorns or nuts in water on the day they are gathered: sound ones sink, and the floaters, weeviled ones among them, are unsound. After dry weather some good acorns float until they have soaked overnight, and after rain some bad ones sink until they have dried a few hours. Overcup acorns float even when sound. See Planting acorns, nuts and seed: collecting, storing and the first year.

Frost heaving

The lifting of a young plant out of the ground as soil water freezes into layers of ice and thaws, again and again, through a winter. Fall-planted seedlings on heavy clay are the most exposed, and late-fall watering and an organic mulch reduce it. See Bare-root trees on arrival: the first hour, the first week and the first spring.

Frost pocket

A low spot where cold air pools, so frost comes there first. The USDA names frost pockets among the microclimates too small for its hardiness map, which is drawn in cells about half a mile across. A tree at its cold limit, or one that flowers early, belongs on higher ground. See What a hardiness zone tells you about a tree, and what it leaves out.

Guaranteed analysis

The three numbers on a fertilizer bag, such as 10-10-10: the percentages by weight of nitrogen, phosphate and potash, always in that order. Phosphorus is counted as phosphate, P2O5, and potassium as potash, K2O. The label guarantees only what it lists. See Organic or synthetic fertilizer: how they differ and what a tree needs.

Hardiness zone

The USDA zones, 1 coldest to 13 warmest, that say how cold a winter a plant survives; the tree finder and the site check both filter on it, and the ZIP code lookup gives yours. Each is a 10-degree band of the coldest night of the year, averaged over 1991 to 2020, split into a colder half, a, and a warmer half, b. It is one input among several, because soil, drainage and exposure decide as much as cold does. See What a hardiness zone tells you about a tree, and what it leaves out and Why the range maps are wrong.

Heat zone

One of the 12 zones of the American Horticultural Society’s Plant Heat Zone Map of 1997, set by the average number of days a year above 86 degrees Fahrenheit. That is the point at which plants begin to suffer heat damage. The heat zone is the summer counterpart of the hardiness zone, and few nursery labels give it. See What a hardiness zone tells you about a tree, and what it leaves out.

Heeling in

Planting bare-root stock temporarily to keep the roots moist when the final planting has to wait more than a few days. The stock goes in a shaded trench or a bed of damp sand, sawdust, peat moss or soil. The plants lie at an angle with the roots covered, and they have to move to their final spot before the buds swell. See Bare-root trees on arrival: the first hour, the first week and the first spring.

Juglone

The compound black walnut makes in all its parts, most of all in its buds, nut hulls and roots. It stunts or kills sensitive plants such as tomatoes, azaleas and rhododendrons. A mature tree’s roots carry it 50 to 80 feet from the trunk, and roots left after the tree is cut release it for several years. See Juglone: which plants a black walnut poisons, how far its reach goes, and what to plant beneath it.

Mature spread

The width of a tree’s crown when it is grown, which on many small trees equals the height and on eastern redbud exceeds it. Half of it is the reach of the branches on each side of the trunk, so it, not the height, sets how far a tree goes from a wall, a walk or a property line. See Small native trees and large shrubs for a small yard, and how to place them.

Microclimate

Local climate on a scale the hardiness map cannot show, since the map is drawn in cells about half a mile across. The ground in front of a south-facing wall runs warmer than the zone, and a low spot where cold air pools runs colder. At a larger scale, a city holds heat in its concrete and blacktop. See What a hardiness zone tells you about a tree, and what it leaves out.

Mulch volcano

A cone of mulch piled against the trunk. It holds the bark wet, invites rot and rodents, gives voles a shelter with food inside, and sends roots up into the pile and around the stem. Mulch goes on flat and wide instead, pulled back so the flare stays in open air. See Ten planting mistakes that quietly kill trees.

Multistem

Grown as several trunks from one base, the natural form of serviceberry, redbud, hazelnut and witch hazel. A clump spreads from the ground, and stems that press together trap bark in a weak seam, so a clump is thinned young to a few stems that do not touch. See Multistem or single stem? Sometimes the trunk is hiding the answer.

Percolation test

A check of how fast water leaves a soil. Dig a hole a foot deep and a foot across, fill it and leave it overnight, then refill it and time the drop. Water gone within eight hours means soil that drains, and less than an inch an hour is too slow. See Native trees and shrubs for heavy clay: which take it wet, which take it dry, and how to plant them.

Recalcitrant seed

Seed that dies if it dries out. Acorns and chestnuts are recalcitrant and pawpaw seed nearly so, while hickory nuts, walnuts and persimmon seed survive drying. See Planting acorns, nuts and seed: collecting, storing and the first year.

Right-of-way

Land along a street held for public use, which usually takes in the strip between the sidewalk and the curb. Planting a tree there usually takes the town’s permit, even where the owner next to it must look after the trees, and some towns accept only species from their own list. See Native street trees: which ones take pavement, salt and drought, and how to fit one to the strip and the wires.

Root ball

The mass of roots and soil that comes with a tree. Container and balled stock are often already too deep in their own soil, so scrape the top until the first structural root shows and measure from there. Take off every bit of wire basket and burlap before the ball goes into the hole, and shave the sides to cut the circling roots. See Ten planting mistakes that quietly kill trees.

Root collar

The forester’s name for the point where a seedling’s stem meets its roots, the place the root flare forms on an older tree. Bare-root seedlings are planted with it just under the soil surface, and a hardwood seedling whose top dies back can resprout from it. See Bare-root trees on arrival: the first hour, the first week and the first spring.

Scarification

Breaking or softening a seed coat that water cannot get through, with hot water, a file or acid, so the seed can take up water. Eastern redbud, black locust and Kentucky coffeetree need it. See Planting acorns, nuts and seed: collecting, storing and the first year.

Scratch test

Gently scratching the bark of a small twig to see whether it is alive. Green tissue beneath means the branch is alive, and brown, dry tissue suggests dieback. A tree that has not leafed out is not judged dead before June 1, since a newly planted tree and a late species such as bald cypress both bud late. See Bare-root trees on arrival: the first hour, the first week and the first spring and Is my bald cypress dead? The conifers that drop every needle in fall, and when brown means trouble.

Seed zone

An area of similar climate within which seed of a species can be moved with little risk of falling out of step with the seasons. In 2014 forest geneticists drew 64 provisional zones for the lower 48 states. Each is a band of winter cold 5 degrees Fahrenheit wide, the width of a half hardiness zone, crossed with one of six classes of dryness. See Seed source and provenance: why where the parent tree grew decides how yours will do.

Seedling and sapling

A seedling is a young tree grown from seed rather than from a sprout, a cutting or a graft, and in nursery use one that has never been transplanted. Foresters call a tree a sapling once its trunk is about 2 to 4 inches thick at breast height, 4.5 feet up. Our pages use both words for size: a pencil-thick seedling has about a quarter inch of caliper, a thumb-thick sapling about three quarters. See Reading a nursery listing: seedling, sapling, caliper, bare-root and potted, and what a tree’s age tells you.

Soil texture

The shares of sand, silt and clay in a soil. Sand grains are the largest, up to 2 millimeters across, and clay particles are a thousand times smaller, so sand drains fast and holds little water or food while clay holds both. Wet and knead a handful: sand feels gritty and breaks apart, loam holds a ball, and clay pushes out a ribbon over 2 inches long. See Native trees and shrubs for dry, sandy soil: the plants of barrens and dunes, and how to grow them.

Stratification

Weeks of cold, damp storage that release a seed’s dormancy, given in a refrigerator or by a winter in the ground. Red-oak-group acorns want 4 to 12 weeks of it and black walnut 90 to 120 days; white-oak-group acorns need none and sprout in the bag if kept. The seed sowing planner gives each species’ days. See Planting acorns, nuts and seed: collecting, storing and the first year.

Transplant shock

The late leaf-out, scorch, thin crown and dieback of a newly planted tree whose roots have not yet grown out into the surrounding soil. Watering is the main defense, and a woody plant can take three to five years to establish. See Bare-root trees on arrival: the first hour, the first week and the first spring.

Tree lawn

The strip of ground between a sidewalk and the curb, also called a parkway, and usually public right-of-way. A tree that matures at 40 feet or less wants a strip 4 to 8 feet wide, and a taller one more than 8. See Native street trees: which ones take pavement, salt and drought, and how to fit one to the strip and the wires.

Tree shelter

A tube of solid plastic set over a seedling and held by a stake. It keeps deer and rabbits off the leader, and in a Connecticut trial seedlings in shelters grew taller than those in mesh sleeves or with no guard at all. It comes off once the tree has grown out of its top. See How fast native trees grow: a year’s growth by species, soil and sun and Protecting young trees from voles, rabbits, deer and squirrels.

Underplanting

Planting shrubs, small trees or ground covers under an established tree. It is done by hand, in small pockets between the roots, since a spade, a tiller or a raised bed harms the tree. Add no more than 2 inches of soil, and none within a foot of the trunk. See What to plant under a big tree: native shrubs for shade, and how to plant without hurting the tree.

Forests and habitat

Aftermath forest

A beech stand after the first wave of beech bark disease has passed. Many of the big trees are dead, the gaps are filled with thickets of root sprouts, and the scale and fungus are still at work in them. See Beech leaf disease: the striped leaves, how far it has spread, and whether to plant beech now.

Bottomland

Low ground along a river or creek: land that floods in winter, clay that stays wet into spring, backwater flats and rich floodplain soil. It carries its own oaks, overcup, swamp chestnut and cherrybark among them, and the largest intact tract of old-growth bottomland hardwood forest left in the Southeast, Congaree, stands on it. See Red oaks and white oaks, and where each belongs.

Browse

The twigs, buds and leaves of woody plants that deer and other animals eat. Deer prefer forbs when they can find them, but browse is there in every month. They reach it up to about five feet off the ground, which is the height a cage has to beat. See Native trees and shrubs for deer and wild turkey.

Crop tree release

Cutting or girdling the trees that crowd a chosen tree so its crown can spread. On an oak, hickory or wild fruit tree it is the quickest way to more mast, because a bigger crown in more light flowers and fruits more. See Native trees and shrubs for deer and wild turkey.

Disjunct

Separated from the rest of its species by a wide gap. Seaside alder grows in three disjunct areas hundreds of miles apart: Delaware and adjacent Maryland, southern Oklahoma, and one area of Georgia. A range map drawn as one outline often drops stands like these. See Why native trees become rare, and ten rare eastern species worth growing.

Endemic

Growing wild in one region and nowhere else. Ashe’s magnolia is endemic to the Florida panhandle, between the Ochlockonee River and Santa Rosa County. Oklahoma alder, a subspecies of seaside alder, is endemic to two counties of that state. See Why native trees become rare, and ten rare eastern species worth growing.

Fall Line

The zone where the hard bedrock of the Piedmont gives way to the softer sediments of the Coastal Plain, marked in northern Virginia by falls and rapids. Seaward of it, streams cut easily through sand, gravel and clay and rivers widen as the land flattens. See Native trees for the Mid-Atlantic: what to plant on the Coastal Plain, the Piedmont and the mountains.

Forb

A broad-leaved herbaceous plant that is not a grass, sedge or rush. Deer prefer forbs to browse because they are more digestible, and the herbaceous openings where they grow hold the insects turkey poults live on.

Hedgerow

A wide, mixed row of shrubs and small trees along a field edge or property line, planted for fruit, cover and nesting rather than kept to a clipped line. It works best for wildlife past 20 feet wide, as a row of trees between two rows of shrubs. A mix of species also outlasts a row of one, which a single pest can take. See Native hedges, screens and windbreaks: which trees to use and how far apart to set them.

Host plant

The plant a caterpillar can actually eat. Most leaf eaters are confined to the lineages they evolved with, usually one genus or family, so the adult butterfly drinks at almost any flower while its young starve on the wrong leaf. The zebra swallowtail uses only pawpaw. See Native trees for butterflies and moths.

Invasive plant

A plant brought from elsewhere that spreads on its own into woods, fields and wetlands and crowds out the plants native there. Birds carry the fruit of burning bush, Japanese barberry, privet and Callery pear, which is how they reach woods far from the yards they were planted in. Several eastern states now ban their sale. See Native replacements for burning bush, barberry, privet, Bradford pear and boxwood.

Keystone genus

One of the small number of plant genera that carry most of a place’s caterpillars: about 14 percent of the local genera support more than 90 percent of the butterfly and moth species. In the Eastern Temperate Forests the oaks lead with 436 caterpillar species, then cherries and plums at 340 and willows at 289. See Native trees for butterflies and moths.

Mast

The seed crop a woodland feeds on. Hard mast is the fat-rich seed that keeps, acorns and hickory nuts, walnuts, beechnuts, hazelnuts and chestnuts; soft mast is fleshy fruit, from serviceberry in June to persimmon after frost. Oaks do not bear evenly, and a mast year floods the seed eaters with more than they can eat. See Native trees for wildlife: mast, browse, cover, cavities.

Mast year

A fall when the oaks, hickories or beeches across a region bear a heavy crop together, after years of little or none. Most oaks have one every three or four years. A red oak’s acorns take two growing seasons, so a spring freeze that empties the white oaks can leave that fall’s red oak crop standing. See Mast years: why oaks, hickories and beech bear a huge crop one fall and almost none the next.

Oak savanna

Grassland with oaks scattered across it, where the eastern forest met the western prairie and fire kept the understory open, so prairie plants grew among the trunks. Oak savannas once covered roughly fifty million acres from Michigan to Nebraska, and about 30,000 acres remain. Some of the best are the black oak savannas on the dunes of Indiana Dunes National Park. See Native trees for the Great Lakes and Midwest: Michigan, Ohio, Indiana, Illinois and Wisconsin.

Old-field succession

What happens to a field in the eastern United States when the mowing stops. Annual weeds the first summer, then goldenrod, asters and broomsedge, then pioneer trees carried in on the wind and in bird droppings: redcedar, sweetgum, tuliptree, black cherry, sassafras, persimmon and black locust. A century on it is an oak and hickory woods. See What grows back when a field is left alone.

Physiographic province

A region set apart from its neighbors by its rock, landforms and geologic history. In the Mid-Atlantic the provinces run from the Coastal Plain through the Piedmont to the Blue Ridge, the Ridge and Valley and the Appalachian Plateaus. Each differs in soil pH, soil depth, elevation and drainage, which is why a tree at home in one can fail in the next county. See Native trees for the Mid-Atlantic: what to plant on the Coastal Plain, the Piedmont and the mountains.

Predator satiation

The leading explanation for mast years. Lean years keep the seed eaters few, and the next heavy crop is more than the survivors can eat, so some seed is left to sprout. A 2022 analysis of 48 studies found the effect in insects and other invertebrates but not in the fewer studies of vertebrate seed eaters. See Mast years: why oaks, hickories and beech bear a huge crop one fall and almost none the next.

Relict

A population left behind when a species’ wider range shrank, often far from the rest of its kind. Flora of North America reads the scattered groups of seaside alder as probable remnants of ranges held during and after the ice ages. NatureServe cites the same idea for Oglethorpe oak. See Why native trees become rare, and ten rare eastern species worth growing.

Snag and log

The dead wood a forest leaves standing and lying. A snag is a standing dead tree; the logs are what comes down. Woodpeckers excavate snags and their old holes house owls, bluebirds, flying squirrels and bats; beetles tunnel the wood and mason bees nest in the tunnels; ground beetles and salamanders work the fallen logs. See The habitat around a tree is part of the tree.

Specialist bee

A native bee whose larvae eat the pollen of one plant family or a few related genera and nothing else. About a quarter of the roughly 770 bee species native to the eastern United States are pollen specialists, and nearly 60 percent of those on the wider eastern list are rare. Willow carries 14, the blueberries ten and maleberry four. See Native trees for bees: pollen, nectar and nesting.

Stream buffer

The trees, shrubs and other plants along a stream or river, also called a riparian forest buffer. They trap sediment, take up nutrient pollution, hold the bank and shade the water. The Chesapeake Bay Program counts a stream as protected only once at least 70 percent of its edges are buffered. See Native trees for the Mid-Atlantic: what to plant on the Coastal Plain, the Piedmont and the mountains.

Understory

The layer of a woodland below the canopy: shrubs such as spicebush, small trees, and the young of the big ones, all in shade. A forest tree can spend decades suppressed there, adding a few rings you can barely see, and an invasive shrub layer at head height is dense enough that tree seedlings do not survive under it. See What old growth actually looks like.

Windbreak

Rows of trees and shrubs set across the prevailing winter wind, usually on the north and west sides of a house or farmstead. It shelters ground for about 10 times its height downwind. Sheltering buildings through winter takes at least three rows, two of them evergreen, standing 75 to 100 feet or more from the house. See Native hedges, screens and windbreaks: which trees to use and how far apart to set them.

Measuring and naming

Common garden

A test planting in which seed of one species, gathered from many places, is raised side by side on one site. Foresters also call it a provenance trial. A difference that lasts there is inherited, while one seen only in the wild may have been the site’s doing. Decades of such trials lie behind the limits on how far seed can be moved. See Seed source and provenance: why where the parent tree grew decides how yours will do.

Container class

The number on a nursery pot, such as #1, #3 or #5, which names a range of volumes rather than a size in gallons. A #1 pot holds anywhere from about two thirds of a gallon to just over one. The nursery stock standard calls a container class given without a plant size, such as height or caliper, an incomplete description of the plant. See Reading a nursery listing: seedling, sapling, caliper, bare-root and potted, and what a tree’s age tells you.

Crossdating

Matching the ring pattern of one tree against a regional chronology so that every ring carries a calendar year rather than a count. It is what separates a dated age from a guess: the oldest trees in the East, a bald cypress over 2,600 years old and white oaks over 450, were dated this way. See How old is that tree, really?.

Cultivar

A named selection of a plant. A tree cultivar is usually kept true by grafting, budding, cuttings or tissue culture rather than by seed. Every plant sold under the name is then a copy of one original tree, with that tree’s genes and its provenance. A planting of one cultivar lacks the variation a batch of seedlings carries. See Seed source and provenance: why where the parent tree grew decides how yours will do.

Deer-resistant

A rating list’s word for a plant deer seldom damage badly in the region where the list was made. No plant is deer-proof. The ratings shift with deer numbers, the season, a hard winter and what else there is to eat, so the lists often contradict each other. A young plant within reach still needs a cage. See Native shrubs and trees deer usually leave alone, and why the lists disagree.

Ecotype and provenance

Provenance is where the seed came from; an ecotype is the locally adapted strain a piece of ground has shaped. The trees in a wild stand are that strain, proven against every drought and winter on the site. That is why our cherrybark oak comes from a parent tree at the northern edge of the range. See When oaks leaf out, and why two of them never agree and Seed source and provenance: why where the parent tree grew decides how yours will do.

G-rank

NatureServe’s global conservation status rank for a species, from G1, critically imperiled, through G3, vulnerable, to G5, secure. Each state’s natural heritage program ranks the same species for its own ground, from S1 to S5. A tree can be G5 overall and S1 at the edge of its range, as chalk maple is in Mississippi. A rank is a judgment of risk, not a legal listing. See Why native trees become rare, and ten rare eastern species worth growing.

Increment core

A cylinder of wood bored out of a living trunk so its rings can be counted and dated without felling the tree. Cores from big old trees rarely reach the center, so the count one gives is a minimum. See How old is that tree, really?.

Sine method and tangent method

Two ways of measuring the height of a tree. The tangent method takes a level distance from the trunk and an angle to the top. It assumes the twig you sighted is directly above the point you measured to. At a 45 degree sighting angle, every foot the top leans toward you adds a foot of false height. The sine method turns a rangefinder’s straight-line distances to the top and the base into vertical components, so it cannot be fooled that way and can only underestimate. See Why almost every tree height you read is too tall.

Sources

• Fire Effects Information System glossary: USDA Forest Service: definitions of mast, snag, understory, ecotype and epicormic sprouting.
• Extension Gardener Handbook glossary: NC State Extension: definitions of branch collar, included bark, heartwood, sapwood, bare root, adventitious bud and abscission.
• Crossdating, the basic principle of dendrochronology: Laboratory of Tree-Ring Research, University of Arizona: matching wide and narrow rings between cores so each ring gets its exact year.