Your Trees Aren't Actually Resting This Winter—Here's What's Really Going On

There's a popular idea that once the leaves drop, trees basically go into hibernation and don't need any attention until spring. It's a comforting thought—low maintenance, no worries. Unfortunately, it's also not quite accurate, especially here in Boulder where winter is anything but gentle or predictable.

Colorado winters have a personality all their own. We get seventy-degree days in January followed by a foot of snow two days later. The ground freezes and thaws repeatedly. The sun at our elevation is intense even in December, and our dry air pulls moisture out of everything—including your trees. Dormancy isn't a vacation for your trees. It's more like a long shift with some unusual hazards.

The Freeze-Thaw Problem Nobody Talks About

Boulder sits at the foot of the Rockies in a way that creates some genuinely weird weather patterns. Warm Chinook winds can push temperatures into the fifties or sixties mid-winter, and then a cold front rolls in and drops things back below freezing overnight. This happens repeatedly throughout the season.

For trees, this is stressful in ways that aren't always visible. The bark on the south and southwest sides of tree trunks absorbs warmth during sunny winter days, causing the cells underneath to become active. Then when temperatures plunge after sunset, those activated cells freeze. Over time, this repeated cycle causes what arborists call southwest injury or sunscald—a condition where the bark actually cracks or peels away, creating wounds that invite disease and insects.

Young trees and thin-barked species like maples, lindens, and fruit trees are especially susceptible. The fix is surprisingly simple: wrap the lower trunk with light-colored tree wrap from just below the soil line up to the first major branches. It reflects sunlight and keeps bark temperature more stable. Remove the wrap in spring so it doesn't trap moisture or create hiding spots for pests.

Snow: Friend and Enemy

Fresh snow is actually great for trees. It insulates roots from extreme cold and provides moisture as it melts slowly. But Boulder has a habit of delivering the other kind of snow—the heavy, wet, cement-like variety that piles up fast and tests the structural limits of every branch in your yard.

Evergreens are the most vulnerable because they hold their needles all winter, giving snow a surface to accumulate on. A single heavy storm can deposit enough weight to crack or completely break major limbs. If you're watching snow pile up on a blue spruce or a juniper, gently brushing it off with an upward sweeping motion (not shaking or beating the branches, which can cause more damage) can prevent a lot of heartache.

For deciduous trees, the concern is a little different. Ice storms—which we do get along the Front Range—coat every surface with a layer of ice that's incredibly heavy. In those situations, there's honestly not much you can safely do in the moment. The bigger preventive step is making sure your trees have been properly pruned before winter arrives so that weak, crossing, or dead branches aren't waiting to fail under load.

Winter Pruning: Yes, It's a Real Thing

Here's where a lot of homeowners get surprised: late winter, just before trees break dormancy, is actually one of the best times to prune many species. The tree's structure is fully visible without leaves, which makes it much easier to identify problem branches, crossing limbs, and structural weaknesses. Wounds also close faster once spring growth kicks in shortly after.

For oaks specifically, winter pruning is strongly preferred because it dramatically reduces the risk of oak wilt, a devastating fungal disease that spreads through fresh wounds during warmer months when beetles are active.

That said, timing matters. Pruning too early in deep winter can leave wounds vulnerable for an extended period. Pruning too late—once buds are swelling—stresses the tree right when it's putting energy into new growth. Late February through mid-March tends to be the sweet spot in Boulder, though it varies by species and the specific winter we're having.

What About Watering?

This is probably the most counterintuitive piece of winter tree care advice: your trees may need water even when everything looks frozen and dormant.

Boulder's winters are frequently dry. When we go weeks without precipitation and the ground isn't frozen solid, tree roots are still active and still need moisture. Evergreens are especially at risk because they continue to lose water through their needles even in cold weather—a process called desiccation. If the roots can't replace that moisture because the soil is dry, you get winter burn: brown, dead needles that won't recover.

On warmer winter days when temperatures are above forty degrees and the ground isn't frozen, watering deeply around the drip line of your trees can make a significant difference. It's not a daily task—once or twice a month during dry stretches is usually enough.

Mulch Is Your Secret Weapon

A four-inch layer of wood chip mulch around the base of your trees—kept a few inches away from the actual trunk—does a remarkable amount of work in winter. It insulates roots from temperature extremes, retains soil moisture, and prevents the kind of soil heaving that can damage shallow roots during freeze-thaw cycles.

If you don't have mulch down already, it's worth adding some before the next cold snap.

Getting a Head Start on Spring

At Boulder Tree Service, we're out working on trees year-round—because the trees need it year-round. If you're not sure how your trees are holding up through winter, or if you want to get pruning scheduled before the spring rush, we're happy to take a look. A quick assessment now can save you from dealing with a much bigger problem once the warm weather arrives.