Cool trees surviving cold temps

The first cold snap this fall found many of us appreciating the warmth inside our homes and offices, during the cold, damp weather. Many of us find ourselves waiting with “bated breath” and crossed fingers this time of year, hoping that our trees will be spared from an early snow or severe deep freeze. Perhaps you found yourself on the phone in a last-minute plea to have your irrigation system blown out ahead of freezing temps. Luckily, for now, we were spared that heavy branch breaking snow and seriously low temps. It’s a reminder that autumn can be a really tricky time for trees, especially along the front range of Colorado.

We weren’t so lucky in November of 2014, when temperatures suddenly dropped over 70 degrees along the front range and stayed well below zero, causing widespread damage to our trees that they are still recovering or dying from four years later. You can see it in the red-leafed crabapples, pears and Siberian elms, even now. These quick temperature changes prevent trees from reacting in an appropriate way. Low temperatures, extreme temperature fluctuations, wind stress and lack of water are major contributors to tree stress in the winter.

Since trees can’t go inside, how do they survive each winter? Trees have active mechanisms that help them improve their cold tolerance. Cold tolerance refers to a tree’s ability to react to cool and cold temps. A number of other factors help trees navigate cold temperatures. Thick bark can help insulate trees from sudden freezing temperatures. The large mass of tree trunks with moisture inside of them can act as a thermal mass, resisting fast temperature changes. The canopy of leaves on trees can also act as a temperature shield. Look under a tree after a cold night. The frost typically appears out in the open in the grass, not under the canopy.

“Hardening” is another process that trees utilize to survive the winter. Shoots are triggered to harden off by the falling temperatures and reduced light. Starches in trees are broken down into smaller sugars, which can help resist the formation of ice crystals similar to an antifreeze in your car’s radiator. These sugars are accumulated in the cells at the same time that the cells begin to dehydrate.

The cold days that come in fall set this process in motion, but it does not occur overnight. It takes some time for trees to “harden off.” Initially in winter, once trees have hardened off and are dormant, their cold tolerance is at its highest level. As winter progresses on and cold days are disrupted by higher temps, a tree’s cold tolerance decreases, and trees can even de-harden in the late winter, leaving them susceptible to damaging late-weather freezes.

Common damage that can be seen on the trunks of many young and thin-barked trees is something called “sunscald.” It’s also called “southwest Injury” because it appears on the southwest side of tree stems. This happens when direct sun rays warm up the temperature of the tree trunk causing the cells to lose their dormancy.

Once the sun goes down, the temperatures drop and those cells that were brought out of dormancy are damaged and killed when they quickly re-freeze. Often times this damage piles up year after year and bands of dead tissue are visible on the trunks of the trees.

What can you do to help your tree survive the winter you ask? There are a few good practices to follow. For young trees with thin bark it’s a good idea to wrap the trunk of the tree to protect from winter sun and sunscald. This is best done with a reusable piece of white corrugated plastic.

Giving your trees a long deep watering in the fall before they go dormant also helps prepare them for winter stress. Watering once per month on warmer winter days will also make sure they don’t dry out. Finally, applying a layer of wood chip mulch over the root zone can help conserve moisture and provide an extra layer of insulating material to help regulate the temperature of the root zone. Remember that stressful conditions in the dormant season can affect the health of a tree the following year.