Ever notice evergreen trees changing color during the winter? Probably not. Even for the most avid gardener, winter is a time for warm, indoor activities. We occasionally look out our windows and think, “It looks cold out there” then quickly retreat to warmer temperatures. Most people don’t think about their trees or landscaping during the cold winter months. After all, there’s not much going on out there. Right? Or, is there?
Actually, there is a lot going on in our landscapes. Winter is a time during which our trees and shrubs are tested. Survival of the fittest! (As Darwin would say.)
Best laid plans…
Secretly, quietly, our trees have been plotting and planning all year for winter. They have been methodically allocating energy towards growth, defense, reproduction, stability, and reserves.
Our deciduous trees now have to hope they have stored enough energy to continue to grow roots all winter (yes, many trees and shrubs continue to grow their roots during the colder months), then have enough reserves left over to produce foliage, flowers, seeds, etc., in the spring.
Ouch…
Meanwhile, our evergreens have a different strategy. As you know, they keep their energy-trapping foliage year-round (for a price). They must endure constant water loss from their foliage, coupled with dry air, and frozen 
soils.
Therein lies the potential problem. Not only do evergreens continue to grow their roots during the colder months (requiring chemical energy manufactured from sunlight, CO2, and WATER), they have to deal with dry conditions, frozen soils, and continuous water loss (demand) from their ever productive foliage.
As foliage continues to transpire (lose water vapor) and soil becomes drier, trunks, stems and branches can begin to shrink as the tiny water conducting vessels (or tracheids) inside are pulled taut from the forces involved. This process of pulling will continue until those tiny vessels begin to snap, never again able to conduct the precious water that drives the whole energy trapping system. Sound stressful? It is.
Spring attack!
Ever try leaving a car sit all winter, then try to start it in the spring? Now imagine you’re being chased by zombies, and you need to get away quickly. Sure, you could jump-start the car, but this will take time. Meanwhile, the zombies are gaining ground!
In essence, trees must play zombie attack every year. They use passive weapons of resistance, relying on making themselves distasteful or indigestible to pathogens (hungry little things). This method of defense does not stop attacks from occurring, it either slows them down, or discourages them from persisting.
When spring arrives, if too much winter damage has occurred, the trees are at a disadvantage. Instead of building defenses, they must first build new tissues to replace those that were lost during the winter. Meanwhile, pathogens are settling-in. It’s now a race. If the trees don’t get things moving quickly enough, the pathogens win, and damage occurs (or worse). When the cycle repeats annually, that’s when the real problems arise.
What can we do to help our trees?
First, remember why this happened? Lack of available water during winter. Don’t worry, you wont find me out there in the freezing cold watering my trees (building an ice-skating rink). There are far more practical and natural solutions. I will only talk about one of them here, as the others are rather complex and usually require hiring a service.
Think of a forest floor. Mulch! Organic mulch acts as insulation to help keep surface roots from freezing during those really cold periods. It also happens to hold up to five times its weight in water. Further, mulch increases microbial activity in the soil, increasing temperature, pore-space, and fertility. (More on soils in another article.)
Maintaining a layer of mulch within the root zone of trees is a great way to help them through stressful times. Two very important points to remember about mulching:
- DO NOT pile mulch against the base of trees (mulch-volcano with tree erupting from center). Doing this can cause a broad array of serious problems. A 2-4 inch gap between the mulch and the base of the tree should be sufficient.
- DO NOT pile mulch too deeply around trees and plants. A 3-4 inch deep layer of hardwood mulch is usually ample.
As always, an ounce of prevention is worth a pound of cure.
For more on mulching, Morton Arboretum has this great article:
Want to know more about how water is transported through a tree?
These guys did a great and entertaining job with this video. Two thumbs-up from this arborist!
Oh, in case you watched the video and are wondering, another major reason trees evaporate so much water: Cooling! During the summer months, foliage in direct sunlight can get very hot! Hot enough to be extremely damaging to living cells. The hotter the leaves get, the more water evaporates, cooling the leaves. Trees are true geniuses of nature.
