We are sometimes surprised and bewildered when spring arrives and we find that one of our deciduous trees shows serious signs of dieback — when its new, tender growth at the tips of branches have died. Even worse, we may discover that a favorite tree has died during winter. In either case, we may assume that the cause was disease. But that usually isn’t the case. More likely the culprit was the effects of winter itself or things we failed to do.
To better understand why deciduous trees — recently planted ones as well as mature trees — don’t always get safely though our frigid winters, we need to understand how trees go about preparing for winter.
How trees prepare
With the onset of longer nights beginning in late summer, trees begin producing hormones in their leaves, which make all parts of the tree hardier to winter weather. These hormones are then moved throughout the tree and they become increasingly hardier as the weather in fall turns colder.
Acclimating trees to withstand subfreezing temperatures is called “hardening.” During this process, most of the moisture is removed from all the tree’s cells. Otherwise, the cells could freeze and burst, causing limbs or even the entire tree to die.
Ideally, but the time subfreezing weather arrives, deciduous trees will have become completely dormant and are able to get through winter safely — if the soil around the roots contains adequate moisture. But the genetic hardiness varies depending on the tree species, so be sure you select trees hardy to our USDA Zone 6.
When the following spring arrives, the hardening process begins to gradually reverse itself. Cells slowly begin to refill with moisture and deciduous trees gradually come out of dormancy. Leaf and flower buds begin to grow, and before we realize it, they have put on their new summer dresses of green.
But Mother Nature doesn’t always perform the way we expect. Sometimes subfreezing temperatures strike in late fall before trees have completely gone into dormancy. And sometimes temperatures tumble during late spring after our trees have started losing their dormancy. In either case, serious dieback or death can occur. We aren’t prepared to deal with these conditions as are orchardists with their wind machines. Fortunately, even if emerging leaf buds die, deciduous trees are able to send out a second set of leaf buds. This is not true with flower buds. However, there are other potential tree problems that we do have some control over that will help our trees get safely through winter.
Water your trees deeply
Because of the low annual precipitation in the Yakima Valley, we need to make sure that our deciduous trees have moist soil at root level throughout winter. With sufficient moisture, tree roots won’t become dehydrated and will begin to grow and be able to carry out their life processes when they come out of dormancy in spring.
If you have not already given your trees a long, deep drink, it’s not too late. Using soaker hoses or watering with a bubbler attached to your hose, water down to root level, which is about 12 to 18 inches down. Should the winter be particularly dry, you can water again as long as the temperature is above 40 degrees and the soil hasn’t frozen.
Young trees and sunscald
Trees planted during the past several years are susceptible to sunscald. During winter days, the western or southwestern side of the trunk is often exposed to bright sunlight. Even when day temperatures are below freezing, the exposed sides of the trunk can heat up and cause the sap to begin to refill the cells. Then when night comes and temperatures can drop, these cells can freeze and burst, This can cause vertical cracking and splitting on the west or southwest side, destroying the exposed cambium level. The tree will be weakened and becomes open to disease and insects.
You can prevent your young trees from sunscald damage by wrapping the trunk with a special trunk wrap available at local nurseries. These white wraps reflect bright sunlight and keep sunscald from occurring. Be sure to remove the wrap during spring when all danger of freezing has passed.
If you have a tree that does suffer sunscald, you may be able to repair the damage. Using a sharp knife, cut back the damaged bark to undamaged tissue. Then round off the corners at the top and bottom of the wound. In time, your tree will wall off the damaged area and should be fine.
Snow and ice damage
Severe ice storms, though infrequent in our Valley, can wreak havoc with trees. Weight of ice buildup on limbs can cause them to crack and break.
If we have an ice storm this winter, the best thing to do initially is nothing. Attempting to remove ice can cause even more damage because frozen limbs become extremely brittle and break easily. Wait until the ice melts on its own. Then if there is serious damage, your best move would be to call a certified arborist to try to save the tree.
Heavy, wet snow also becomes a weighty problem if the leaves haven’t yet fallen. The best thing to do during a heavy snowstorm is to go out and brush off the snow or gently shake off the snow before it accumulates. (This advice isn’t of any value if snow falls during the night or you have trees whose crowns can’t be reached from the ground.) Usually when snow accumulates on branches with leaves, it will melt off on its own during daytime as temperatures rise above freezing.
Many municipalities spread rock salt on streets to improve traction for vehicles. Many of us also use rock salt on our sidewalks and driveways to prevent accidents. But after rock salt melts ice, it can run off into our soil and eventually damage nearby tree roots. When salt is spread on the street, some of it will invariably be thrown onto trees by passing vehicles.
Of course, the best way to remove snow on our property is by shoveling it frequently, which will prevent ice from forming when the snow melts during the day and freezes at night. Sometimes after a big snow or ice storm it is hard to remove. Fortunately, there are other melting materials besides rock salt, and while not perfect, they are not as damaging to trees as rock salt. Look for ice melt products that contain potassium chloride, calcium chloride or calcium magnesium acetate.
Some trees are more salt tolerant than others. In the future, if you decide to plant trees at street side, consider choosing one of these that is salt-tolerant, such as red oak, junipers or honey locust.
• Freelance gardening columnist Jim McLain can be reached at 509-697-6112 or firstname.lastname@example.org.