What do trees do in winter?

Once the leaves are down, a maple or an oak looks like it’s simply waiting for spring. It isn’t. All winter it’s guarding next year’s buds, living off food it stored in summer, keeping ice out of its living cells, and in some cases still growing a little underground.

Getting ready in fall

Winter preparation starts before the leaves change. Shortening days in early autumn, along with cool nights that stay above freezing, start a tree’s hardening for winter. A stretch of freezing weather finishes the job (Iowa State University Extension). The timing matters: a tree caught by a hard freeze before it has hardened can be badly hurt. In Michigan, an unusually warm fall in 1950 left peach trees unprepared, and a Thanksgiving freeze killed many of them (Michigan State University).

Before the leaves drop, the tree salvages what it can from them. Worldwide, leaves return on average about 62 percent of their nitrogen and 65 percent of their phosphorus to the tree before they fall (Vergutz and others, Stanford). Then a layer of weak-walled cells at the base of each leaf stem breaks apart, and a second layer, toughened with waterproof suberin and lignin, seals the scar against water loss and disease (University of Missouri). Some oaks and beeches never quite finish that layer and hang on to their dead leaves all winter; see why oaks and beeches keep their leaves.

Two kinds of rest

Botanists split winter dormancy into two parts. The first, sometimes called true dormancy or endodormancy, is a locked rest: during it, buds won’t grow even during a warm spell. The lock comes off only after the tree has logged enough hours of cold, usually counted as time between about 32°F and 45°F. Temperatures below freezing count for less, and warm spells can disrupt the count (University of Minnesota Extension).

Once the chilling need is met, the tree moves into ecodormancy. It is capable of growing again, but environmental conditions still hold it back. For many trees, enough warmth will then start the process toward bud break. This two-step system keeps a January thaw from fooling a tree into leafing out in midwinter. The rest of that story, from swelling bud to full leaf, is in when do trees leaf out?

Next spring is already packed

The buds on a winter twig aren’t new. They formed the summer before, while the leaves were still working. Inside each one are the beginnings of leaves, a shoot, or flowers, wrapped in protective bud scales, which are modified leaves and are often coated with a gummy substance (Nebraska Extension). Trees also draw water out of their buds before winter (Northern Woodlands). Bitternut hickory looks different: its yellow bud scales meet edge-to-edge rather than overlapping like shingles (Woody Plants of Ohio).

A bare twig also keeps a record. Each spring, when the end bud opens, its scales fall off and leave a ring of scars. Count the rings back from the tip and you can see how much the branch grew each year. The leaf scars below each bud, and the dots inside them where the veins once ran, have shapes distinctive enough to identify many trees in winter (University of Illinois Extension, College of St. Benedict and St. John’s University). See the parts of a leaf for the summer version.

Living on stored food

With no leaves, a deciduous tree can no longer rely on its leaves to make food, so it runs largely on savings. Through the growing season it stores sugar as starch in living cells inside the wood of its branches, trunk, and roots, especially in the bands of cells called rays that run outward through the wood (Furze and others, 2018). In late fall and winter, some of that starch is turned back into sugar, which helps protect the tissues from freezing (Penn State Extension). Dissolved sugars lower the freezing point of the fluid in the cells, much as antifreeze does (Iowa State University Extension).

Maple syrup comes from this winter sugar. On late-winter days that swing from below freezing at night to above freezing in the day, pressure builds and falls inside a sugar maple’s wood. Thawing pushes sap out of a tap hole, and freezing draws water in from the soil. The flow stops when the buds break (Penn State Extension). The pressure can top 25 pounds per square inch and sometimes reach 40 (Massachusetts Maple Producers Association).

How trees keep from freezing

Ice inside a living cell kills it, so trees have two main ways to keep it out. Many familiar hardwoods, including oak, elm, maple, beech, ash, walnut, and hickory, protect some of their most freeze-sensitive living cells through deep supercooling, keeping water in those cells liquid well below 32°F. That trick has a floor. At about −40°F the water freezes all at once and the cells die. Iowa State says this limit helps explain why mountains have timberlines (Iowa State University Extension).

The hardiest northern trees, including paper birch, quaking aspen, and willow, can rely more heavily on another strategy. As it gets colder, water moves out of their cells and freezes harmlessly in the spaces between them, leaving the cells shrunken and their remaining fluid thick with sugars that act like antifreeze (Iowa State University Extension). Trees built this way can take astonishing cold. In laboratory tests, black and white spruce survived −112°F (−80°C), the coldest temperature tested, and after slow pre-cooling, paper birch buds and bark and balsam fir buds and needles survived liquid nitrogen at −321°F (Strimbeck and others, Frontiers in Plant Science).

Evergreens in winter

Conifers keep their needles, but that doesn’t mean they’re busy. Needles have a waxy coating, and their pores are recessed in small chambers out of the wind (College of St. Benedict and St. John’s University). In a study of a Rocky Mountain subalpine forest, needles sometimes warmed to temperatures good for photosynthesis on sunny winter days, but the trees gained no carbon overall through most of the winter. They loaded up on protective pigments instead and started gaining again only after their trunks thawed in spring (Bowling and others, 2018).

One of an evergreen’s biggest winter risks is drying out. When the ground is frozen, roots can’t take up water, while sun and wind keep pulling it from the leaves and needles. The result, browned foliage called winter burn, is common on exposed evergreens. Rhododendrons react by curling their leaves when it drops below about 20°F, which cuts their water loss (Penn State Extension). More in why evergreens stay green.

Aspen’s green bark

Look closely at a quaking aspen and its smooth bark has a greenish tint. There’s chlorophyll in a thin layer just beneath it, and on sunny days a sun-warmed trunk can photosynthesize even when the air is near freezing (Northern Woodlands). Research on aspen stems found this doesn’t produce a net gain of food. Instead, it recaptures some of the carbon dioxide the living trunk gives off, so the tree loses less of its stored sugar (Foote and Schaedle, Plant Physiology). See why aspen groves turn together.

Roots in winter

Roots are the tender part of a tree. They harden later and less than the trunk and branches; University of Minnesota Extension says the roots of most of the state’s trees are killed somewhere between 0°F and 10°F (University of Minnesota Extension). They’re usually spared that cold because soil and snow insulate them. Fluffy snow is an especially good blanket, and the deeper it is, the better it insulates (Nebraska Extension).

Under that blanket, roots may not stop at all. Roots of deciduous trees keep growing until the soil drops below about 40°F, according to Nebraska Extension. A 2025 study of 330 trees found the woody roots of beech, birch, oak, and aspen kept growing slowly through the winter as long as the soil didn’t freeze, even when it was near freezing, long after the trunks had stopped (Swiss Federal Research Institute WSL).

What winter can still do

• Sunscald. On a bright winter afternoon, the sun warms bark on the south and southwest side of a trunk and wakes its cells, which lose their hardiness and are injured in the night’s freeze. Young trees with thin bark are most at risk (Utah State University Extension). On sunny days, wood can be up to 18°F warmer than the air (UMass Extension).
• Frost cracks. When that sun-warmed side cools suddenly at sunset, the bark and wood shrink at different rates and the trunk can split, sometimes with a sound like a rifle shot (Purdue Extension). Purdue suggests white tree guards on trunks under about 4 inches across (Purdue University).
• Ice and snow. Ice can make branches 30 or more times heavier. A coating of a quarter to half an inch breaks small limbs, and half an inch to an inch breaks large ones (University of Illinois, via Connecticut DEEP). Trees with several main trunks, clump birches, upright evergreens, and branches with narrow angles take the most damage from snow and ice (University of Illinois Extension).

Food and shelter for wildlife

A winter tree still feeds the woods. Ruffed grouse depend on aspen and birch buds through the winter, and deer, moose, hares, squirrels, wild turkeys, and finches all eat buds (Northern Woodlands). Hollow trees and old cavities give birds and small mammals shelter from the cold (Penn State). On the ground, last fall’s leaves shelter insects and other small animals; see should you rake your leaves?

A tree’s winter at a glance