Mulch around a tree: how deep, how wide, how far from the trunk, and what rock, fabric and rings do

Lay 2 to 3 inches of mulch out to the drip line, or as far toward it as the yard allows. Keep it a hand’s width or more off a young trunk and 8 to 12 inches off an old one. A new tree needs a ring at least 3 feet across. The best mulch is the tree’s own fallen leaves, as the case for leaf litter over wood chips sets out, and wood chips or shredded bark come next. Keep the layer flat and clear of the bark, since a heap against the trunk rots it. Rock, rubber, landscape fabric and raised rings of soil all do harm, and none belongs around a tree.

What a layer of mulch does

Mulch cuts the water the soil loses to evaporation, keeps the soil cooler in summer and warmer in winter, and smothers weed seedlings. As it rots, it feeds the soil life. In a two-year test, Katrina Greenly and Donald Rakow mulched young pin oaks and white pines with 3, 6 and 10 inches of wood chips. After a week without rain, 3 inches had raised the moisture of the topsoil by 58 percent, and the deeper layers added little more. Deeper mulch smothered more weeds but held the soil cooler, and both trees grew best under 3 inches.

Mulch helps only where the roots are, and a tree’s roots can spread two to three times as wide as its crown:

“Thus, adding mulch only to the area near the base of a tree does not help to conserve much, if any, moisture for the tree to absorb.” (Rutgers Cooperative Extension, Problems With Over-Mulching Trees and Shrubs)

Widen the ring each year as the branches spread, out to the drip line or past it. A wide ring gives the roots more ground to grow in.

Why a mulch volcano kills slowly

A cone of mulch heaped against the trunk keeps the bark wet, and the bark at the base of a tree cannot live that way. Under the pile it softens and its inner layer dies, and fungi, borers and voles move in. Some trees grow roots straight out of the buried trunk into the pile, and roots in mulch die when it dries out in a drought. Others circle the trunk inside the mulch and can girdle it as they thicken.

None of it shows for years, and the decline usually gets blamed on something else:

“The side-effects of volcano mulching occur slowly because it takes time for roots to grow and girdle, or decay to set in, or rodents to make a comfy home.” (Purdue Landscape Report, Mounds upon Mounds of Mulch)

A trunk that goes into the mulch straight, like a post, has its root flare buried. Pull the mulch away by hand until the flare shows, working gently, since the bark under the pile is tender. Leave a well of open air around the trunk and spread the rest flat over the roots. Many trees improve in color and vigor within months.

Rock, fabric, rubber and raised rings

Four common alternatives each work against the tree:

• Rock and gravel. Stone reflects the summer sun into a hot spot, packs the soil and adds nothing to it. Limestone chips also raise the soil’s pH. Gravel suits only plants of dry, gritty ground: leadplant takes a thin gritty mulch or a scatter of gravel, kept off its crown.
• Landscape fabric and plastic. Soil and leaves collect on top of fabric, and weeds root in them and down through it. Unprotected from sunlight, fabric breaks down in as little as a year. Tree roots grow into it and tear when it is pulled up, and fabric near a trunk can lead to girdling roots. Black plastic shuts out rain and air altogether. Cut out any fabric already around a tree’s roots.
• Rubber mulch. Shredded tires control weeds less well than wood chips and add nothing to the soil. Rubber catches fire more easily than a dozen other mulches and is harder to put out. As it breaks down it leaches zinc, which makes up as much as 2 percent of a tire, and some plants take up zinc to the point of death.
• Tree rings and raised beds. A ring of stone or brick filled with soil buries the roots. Soil piled over roots shuts off the air they need, and soil against the trunk keeps the bark wet until it rots. To plant under a tree, set small plants by hand between its roots instead.

Grass to the trunk

Grass up to the trunk brings the mower and the string trimmer to the bark. The thin bark of shadblow serviceberry scars easily from both, and every bark wound on a flowering dogwood invites dogwood borer.

Grass also takes the water. Tree and grass roots feed in the same top 6 inches of soil, where grass roots fill more of it and take more water, above all around young trees. In a 1982 South Carolina study, Stephen Fraedrich and Donald Ham mulched silver maples on sandy soil with 4 inches of pruning chips and left paired maples in grass. In one year the mulched maples grew 26 inches taller on average, and the maples in grass grew 6 inches.

To turn grass into a ring without digging into the roots, lift the sod with a hand trowel, since a spade can nick roots and bark. Or mow it as low as the mower will go, cover it with mulch the same day, and pull what pushes through later. Skip cardboard and newspaper underneath, since voles nest beneath the sheets. Do not till, which tears roots in the top 4 to 6 inches of soil, and keep weedkiller off the ring, since lawn herbicides can harm a tree.

Mulch by situation, and the yearly top-up

The figures change with the tree’s age and the ground:

Check the depth once a year, and add only enough to bring the whole layer back to 2 or 3 inches. Rake the old layer loose first, since matted mulch sheds water. Add new mulch in early spring once the soil has warmed, or in fall once it has cooled. Hardwood chips last about a year, and pine bark and pine needles two to four.

Mulched soil stays moist longer, so test it before each watering. The watering calculator has you push a trowel about 6 inches into the root ball and skip a day while it is still moist. The care calendar for a new tree lays out the rest of its first five years. Blue Whale Trees grows every tree and shrub it sells from seed, and the shop narrows them by size, sun and soil moisture.

Common questions

How many bags of mulch do I need for one tree?

A ring 6 feet across and 3 inches deep takes about 7 cubic feet, or three and a half 2-cubic-foot bags. A cubic yard covers 324 square feet an inch deep, or about 108 square feet at 3 inches.

Does mulch around a tree attract termites?

Termites prefer cardboard to wood chips, but keep any mulch 6 to 12 inches from the walls of a house. Termite treatments go into the soil along the walls, and mulch there can bridge them.

Can you put rocks around the base of a tree?

Not over its roots: stone reflects summer sun into a hot spot, packs the soil and adds nothing to it. Gravel suits only plants of dry, gritty ground, kept off their crowns.

Can I cover exposed tree roots with soil?

No: soil over the roots cuts off the air the fine roots below them need. Spread 2 to 3 inches of mulch over them instead, which also limits the frost heaving and erosion that expose them.

Sources

• Problems With Over-Mulching Trees and Shrubs: Rutgers Cooperative Extension (Bruce Crawford and Raul Cabrera, FS099, 2021): a total depth of 3 inches with last year’s layer counted and 2 on packed or poorly drained soil, roots past the drip line, bark that cannot stay wet, roots in mulch dying in drought, disease and borers in wet bark, the flare dug out with the well left open and recovery within months, the gaps of 3 to 5 inches and 8 to 12, stone that packs and heats soil, limestone raising pH, and waterlogged soil needing no mulch.
• Mounds upon Mounds of Mulch: Purdue Landscape Report (John Bonkowski, 2020), with two articles by Ben McCallister, Re-mulch Your Trees (2023) and Simple Steps to Care for Your Tree on mulching (2025): roots grown from a buried trunk, roots circling and girdling in the pile, softened bark and rodents, the slow decline blamed on other causes, a volcano dug back to the flare, and the old layer raked before more goes on.
• Proper Mulching Techniques: International Society of Arboriculture (2021), with the brochure Trees and Turf: mulch out to the drip line and at least 3 feet from the trunk, no more than 2 inches on wet soil, girdling roots from mulch against the trunk, stone and shredded rubber adding nothing to the soil, tree and grass roots in the top 6 inches, mower and trimmer damage, fill soil suffocating roots, lawn work tearing the top 4 to 6 inches, and lawn herbicides harming trees.
• How to Mulch Trees and Shrubs: The Morton Arboretum, with the page Tree Root Problems: soil cooler in summer and warmer in winter, less water lost to evaporation, nutrients and soil life as mulch rots, fallen leaves as free mulch each autumn, a yearly check of depth, mulching once the soil warms in spring or cools in fall, hardwood chips lasting a year, and exposed roots mulched rather than buried in soil against frost heaving and erosion.
• Mulch: Clemson Cooperative Extension (HGIC 1604): weeds smothered, a ring 3 to 4 feet across for a new tree and widened as it grows, roots two to three times as wide as the crown, 2 to 3 inches from stems, pine needles and shredded hardwood on slopes, gravel that heats the landscape, black plastic, matted mulch, early spring, bark and needles lasting 2 to 4 years, a cubic yard to 324 square feet an inch deep, and mulch kept 6 to 12 inches from walls against termites.
• Mulching Trees and Shrubs: University of Maryland Extension: mulch out to the spread of the branches, weeds rooting in soil on top of fabric, and fabric near a tree cut away because it can lead to girdling roots.
• Underplanting Trees - Respect the Roots: Penn State Extension (Elise Ford, 2025): sod lifted with a hand trowel because a spade nicks roots and bark, no herbicide for clearing, no raised bed and no soil over exposed roots, soil that shuts off air to roots and keeps bark wet until it rots, and small plants set by hand.
• The Myth of Landscape Fabric: Washington State University (Linda Chalker-Scott), with her sheets on rubber mulch, paper sheet mulch and arborist wood chips: fabric breaking down in a year of sun, weeds and roots in fabric, rubber’s weak weed control, fire, zinc and breakdown, voles under sheets of cardboard and newspaper, termites preferring cardboard to wood chips, and weeds mowed low and mulched at once.
• Wood Chip Mulching Around Maples: Effect on Tree Growth and Soil Characteristics: S.W. Fraedrich and D.L. Ham, Journal of Arboriculture 8 (1982): silver maples on sandy soil under 4 inches of pruning chips against paired trees in grass, their height growth in one year, soil kept moister between rains, and a wide mulched area as a larger zone for roots.
• The Effect of Wood Mulch Type and Depth on Weed and Tree Growth and Certain Soil Parameters: K.M. Greenly and D.A. Rakow, Journal of Arboriculture 21 (1995): pin oaks and white pines under 3, 6 and 10 inches of wood mulch for two years, topsoil moisture 58 percent higher under 3 inches after a dry week, fewer weeds and cooler soil under deeper mulch, and the best growth under 3 inches.