When oaks leaf out, and why two of them never agree

Every April someone writes to ask whether their oak is dead. The maples down the street are in full leaf, the neighbor’s oak has been green for a week, and theirs is still bare, or has just started to show a pink haze at the tips. Almost always the tree is fine. Oaks are late by nature, and within any oak species the individuals disagree by a surprising margin, for reasons that have little to do with health.

What sets the date

A tree does not leaf out because the weather turned warm; if it did, every January thaw would be a disaster. Three signals are involved. First the tree has to sit through enough winter cold to break its dormancy, a requirement called chilling. Then it has to accumulate enough spring warmth to expand the leaves that were already preformed in the buds last summer. For some species, day length is a third gate: beech seems to wait for long days regardless of how warm March gets, which is why it shifts its date less than most trees between a warm spring and a cold one.

Species differ in how much chilling and warmth they demand, and that sets the order. Individuals within a species differ too, and that sets the spread.

The usual order in an eastern woodland

This sequence holds across most of the Mid-Atlantic and Upper South, with the dates sliding by latitude and elevation.

• First: elms, red and silver maples, black cherry, serviceberry and flowering dogwood.
• Next: tuliptree and sweetgum, along with most of the understory shrubs.
• Then the oaks, and here the two groups part ways. In our observation the red oak group (northern red, southern red, scarlet, pin and black oak) usually breaks bud a week or so before the white oak group (white, chestnut, post and swamp white oak). The difference is not perfectly consistent.
• Last: hickories, black gum, persimmon and black walnut, and at the very end catalpa and Kentucky coffeetree, which are often still bare when everything else is in full leaf.

The late trees are not being lazy. Their new leaves are easily killed by frost, and a late start is insurance, bought with a shorter season. That trade is one reason the fast growers in the growth estimator are mostly early starters: a species that leafs out early and drops late has more weeks to grow.

Why two oaks side by side can be two weeks apart

Plant two northern red oaks ten feet apart and one is often in full leaf while the other is just cracking its buds. The gap tends to repeat, in the same direction, year after year. Several things produce it:

• Genetics. The chilling and warmth thresholds are inherited and vary a lot within a species. An early tree is early for life, and its acorns tend to make early seedlings.
• Seed source. A tree grown from seed collected three hundred miles south carries that climate’s schedule. In our experience southern seed moved north tends to wake earlier in a mild spring and, more reliably, keeps growing later into fall; both habits put it in the path of frost. Provenance trials do not all agree on the spring side, so treat that as a tendency rather than a law.
• Site. A tree on a south-facing slope, or beside a stone wall, a rock outcrop or a sunny driveway, sits in a warmer pocket and reaches its warmth total days earlier. A tree in a frost hollow does the opposite.
• Age and size. Seedlings and saplings usually leaf out ahead of mature trees of the same species. A young oak in the understory has every reason to open its leaves before the canopy closes over it.
• Last year. A tree that was defoliated, drought-stressed or transplanted may leaf out later, and more thinly, while it rebuilds its reserves.

The same spread shows up in fall. Two trees of one species color and drop a week or two apart, each on its own schedule, year after year: one scarlet oak on a street turns crimson in October while its neighbor stays green and then goes straight to brown. Drought and a hot, dry September push everything earlier.

Late is not the same as sick

A bare oak in mid-April, when the maples are green, is normal. A bare oak in late May is a different matter. Real trouble shows itself in ways that have nothing to do with the calendar:

• Twigs that are dry and brittle, brown rather than green under a scratch in the bark, with shriveled or missing buds.
• One side or the top of the crown still bare after the rest of the tree has leafed out.
• Leaves that emerge and then stop, staying small, pale or curled through June.
• A thin crown with sprouts breaking out along the trunk, the sign of a tree trying to replace a canopy it can no longer support.

A late frost that kills the first flush of leaves is not the end either. Oaks carry reserve buds and push a second set within a few weeks, at some cost to the year’s growth. Reddish new leaves on that second flush are the normal sunscreen, not frost damage.

What to watch in your own trees

Write down the date each tree breaks bud and drops its leaves, for a few years running. You will find that the order among your trees barely changes, that the whole schedule shifts with the spring, and that your earliest tree is the one most likely to be caught by a late freeze. That last fact is worth remembering when you plant: on a frosty site, a late-leafing individual or a local seed source is a safer choice than the fastest tree in the row, and a tree that is slow to wake up is often exactly the tree you want.