Many trees and shrubs are hardier than the coldest climate where they actually survive winter dormancy. The reason? They simply can’t effectively combat the cold temperatures combined with repetitive damage from ice and snow or wind desiccation. What this means to the homeowner is that a particular tree or shrub may survive winter temperatures in your area if that were all the hardship they had to withstand. But they can’t survive low temperatures AND other debilitating environmental factors.
Trees and shrubs must be able to outgrow breakage and dieback each year if they are to thrive. Some, like Japanese Maples, are notorious for failing to adequately harden off late summer growth at the tips of their twigs. Consequently, the entire tree may be covered with dead tips where the last internode didn’t have enough growing season left to harden off sufficiently. Frost damage is fairly common in colder areas where semi-hardy species of trees and shrubs grow. That’s why it’s important to “deadwood” each year, removing dead or failing twigs to allow more light and air circulation throughout the canopy, as well as cleaning up habitat for pathogens to overwinter and proliferate. Nearly all pathogens, fungi, bacteria and insects included, favor dead plant matter, so it makes sense to minimize such substances throughout your landscape.
For example, roses are known to have lower recurrence of black spot and powdery mildew if affected leaves are removed and weak or damaged twigs are pruned to encourage strong vigorous growth. Most thuja family members (arborvitae, cedars, etc.) naturally “flag” each year, shedding tips that brown out and die. Incomplete as it may be, this is nature’s way of conserving nutrients, water and resources by only routing it to branches that get enough light to photosynthesize effectively. Arborvitae back-bud and thicken their canopy appreciably in response to shaking these dead branch tips out from the centers of these shrubs. By helping nature get rid of waste, and thinning canopies to allow more light and air circulation, it is possible to promote healthier growth that is hardier to winter conditions.
Some trees suffer breakage from heavy snow and ice accumulation. If that’s a rare occurrence in your area, they can heal over the following year and replace the lost photosynthetic surface area with new growth the following growing season. Some areas, liker here in Anchorage, get prodigious amounts of snow accumulation and even more damaging ice storms that laden tree branches and twigs with additional weight that may crack wood fibers or break them off entirely.
A great way to combat this phenomenon is to bind up the branches or wrap them with burlap to prevent snow from spreading the upward growth and accumulating in quantities that exceed the branch strength. Baling twine, burlap with string wrapped to hold it in place, or even cheap tarps will adequately protect shrubs and small trees. Medium to Large landscape trees (>12 ft. in height) are more difficult to protect, so it is often better to thin their canopies rather than mount a tremendous effort that requires a small army and large quantities of rope and burlap.
Perhaps the best thing about using burlap is that although it is porous and somewhat translucent (letting some light through to the branches it covers), it also helps reduce water loss due to wind desiccation. Many trees suffer from lack of water during winter. As crazy as that sounds, it’s easy to understand the mechanics behind it when you consider frozen water isn’t easily accessible by roots at low temperatures. Even at warm temperatures, wind has a strong drying effect, but during winter, it pulls water out of small branches and needles as it does blows around them. Without being able to replace lost moisture by taking up more water through the roots, plant can gradually succumb to drought even when they have no leaves! If trees and shrubs can’t maintain water in their twig tissue to counteract the drying action of wind constantly blowing on them, the needles and small twigs begin to die. By spring, entire sections of a tree or shrub can fail to bud out because of “winter dieback.”
Sunscald and bark damage are additional problems faced by woody plants during winter. When the air temperature is cold and the sun is shining, the difference in temperature between the sun and shade sides of a tree trunk can be as much as 60 degrees, simply because of radiant heat from the sun hitting bark. By covering the stems/trunks, you can keep the sap from starting to flow due to the warmth and then freezing in the plant tissue at night when the temperature drops drastically back to the ambient air temperature. Now you understand why Granddad painted the orchard trunks white! White reflects the sun and kept the sap from flowing early!
Retailers now sell pre-formed cages with light material attached to the wires to cover roses and small shrubs, so now it’s even easier to cover winter-tender shrubs. Burlap and baling twine are still my standbys to protect shrubs and trees from winter conditions, but it’s nice to see manufacturers responding to the needs of folks in “The Know.” Now that you can count yourself amongst that number, get out in your landscape and really look at your trees and shrubs to see if you notice a need for protecting some of them. A side effect of wrapping your plants is that they are also less likely to suffer from deer or moose browsing, which can be pretty severe here in Anchorage!