Assessing Winter Damage in the Spring
“Winter damage” is a common term used when fruit trees are harmed by an extreme drop in temperature, but this damage can actually happen anytime from late fall to mid-winter. Damage happens in the fall, when trees haven’t had sufficient time to acclimate to colder temps and there is a drop of 20F or more from normal average temps. It can happen at night, when skies are clear and a cold front moves in. In mid-winter, the cold extremes usually come out of Canada or the Arctic, bringing sub-zero temperatures that can last for days.
Winter damage often doesn’t show up until late Spring, unfortunately, when it’s not possible to get any replacement trees.
It doesn’t matter what the “cause” of damage is, it’s the effect of the damage that we are most concerned about. No one likes to lose a crop or even a single tree, but it often can happen with severe extremes that challenge the definition of the Hardiness Zones (read “Micro-Climates and Hardiness Zones” for more). When your Hardiness Zone has severe temperatures approaching, its “defined” low average, that’s when you can often expect to see damage.
How can you tell the extent of the damage?
If fruit trees have achieved enough chilling hours, their branches will give you the hints you need to discover if the blossoms have been damaged enough to cause significant crop loss. Cutting healthy branches that have lots of blossom buds and placing them in a vase with water in a temperate room with sufficient sunlight will usually allow the buds to start swelling. Poor bud swelling may be an indication of bud damage. One can also take a razor blade or very sharp knife, and cut across a flower bud to determine its viability. If you do this and see a dark brown pistil, the female part of the flower is dead and no fruit will be able to set. This is also the case when the whole blossom looks brown. Blossom damage is one of the easier things to determine, typically blossoms will suffer before the wood is damaged.
Wood damage can come in many forms
Often, in less severe cases you will see dead tips of branches or branches that die back significantly. Lots of times in the spring the trees appear to blossom, start to leaf out well, and then when heat stress hits in late spring damage will occur. You will notice tips, limbs, whole branches, or whole trees will suddenly just collapse. Often you can “smell” winter damage by breaking a twig and smelling the wood. In cases of significant winter damage the wood will have a sour smell, very different from the smell of healthy wood.
Judging wood damage is not quite so easy. If you think there may be winter damage from extreme cold, then in early spring you can start to bring in branches and look for discolored wood. Sometimes the severity of the discoloration is an indication of how damaged the tree actually is. Typically, most trees will show a little damage on the extreme tips, but the rest of the tree will be fine. In extreme cases, you can cut into the scaffold branches or trunks and see dying brown wood. While snow acts as an insulator, often the damage is above the snow line. If you suspect severe damage to the tree, you might want to do a little surgery and cut into the bark on the trunk. You will want to discover a line where the damage ends and the good wood seems to be. Sometimes trees can re-sprout from below this line, but don’t expect to regenerate a good tree in every case.
Prognosis for recovery
When damage is not too severe, good fertilization is recommended to get the tree off to a vigorous start in the spring. This will help the tree to recover and get stronger. Trees that are already weak to begin with may not respond as well and will often be the first to die off. That is why maintaining a good level of vigor in fruit trees is important to their long-term survival and productivity.