By Jeff Rugg-
Q: Apparently the polar vortex last winter killed several of my trees and shrubs. I had carefully selected the plants based on the hardiness zone maps in plant catalogs and on plant tags.
I know it was cold, but shouldn’t the plants have done better? Some of the trees were planted five to 10 years ago, but some of the shrubs were planted last summer and fall.
A: There are a lot of factors to look at when trying to determine why plants die over the winter. These include the coldest temperature; the change in temperatures over a short time; the genetics of the plant; the growth phase of the plant; wind; microclimate in the landscape; soil moisture; soil depth; plant cell anti-freeze; timing of fall pruning; and more.
Let’s consider the hardiness map first. The maps are based on and illustrate the AVERAGE lowest winter temperatures during the few years in which the temperature data was recorded. The coldest temperatures from each year in the data set are recorded and then averaged. If you want to base your plant choices (especially long-term plants such as trees) on the map, you need to choose plants rated for a hardiness zone or two colder than where you live.
A bigger problem for many plants wasn’t the record cold of the polar vortex but the fluctuation in temperatures between the fall and early spring. A plant that is exposed to a warm day or two will start to break out of dormancy. Then when the cold temperatures return, they don’t have to be extremely cold for it to cause damage.
Plants are very diverse organisms, but in our landscapes, we often have only a few cultivars.
Cultivars are clones, so they are genetically identical. If one plant is going to be harmed by the cold, then so are all the rest. Even with the lack of diversity, some plants or branches on an otherwise dead plant are still alive. They survived the record cold and could be used to make a new clone that is more tolerant of the cold than the old cultivar.
The growth phase of plants is often overlooked. We all know that plants grow leaves, flowers and seeds in the spring and summer. Many plants then go into a resting phase before growing again, if the weather lets them. If the weather gets too cool, the plant will go into a dormant stage. Plants in the resting stage are not as resistant to the cold and can be harmed in the fall and early winter — long before the damage is noticed in the spring.
Plants coming out of dormancy too early in the spring can be harmed because they are losing their resistance to cold.
Frozen ground will not let plants retrieve water. Wind can dry out the leaves on evergreens and the buds and stems of deciduous plants. With no water available, the leaves, buds and stems die. Deeper topsoil holds more water below the frost line and can help plants to get water until the soil freezes too deep.
Plants growing in the shelter of buildings, fences and evergreen trees may experience warmer temperatures, less wind and more water. Unfortunately, these seemingly protected plants may not go dormant when they should and can be harmed by fluctuating temperatures.
Dormant plants have chemicals that they use as anti-freeze to keep the water inside and between the cells from freezing. Plants that are exposed to warm temperatures will begin to break dormancy. The sunny side of a plant can warm up a lot, even on cold winter days. Hold your hand up toward the sun and you will feel the warmth.
Then, nighttime temperatures drop too low for the plant, and the warm plant areas get too cold and freeze. You will often see winter damage on the southwest side of the plant, as that area faces the warm winter sun and experiences the most temperature fluctuation.
Damage first occurs on flower buds, leaf buds and small branches, followed by trunks next. Damage will occur on the plant parts most exposed to the cold, wind and winter sun. Branches near the ground that are protected by snow may not show any damage at all. Roots are the hardiest and may survive and send out new growth in the spring.
Prune out all dead wood once it is clear that it is dead and no buds will be able to grow. Many established shrubs will come back from the roots if the whole top is pruned off. New shrubs may die because they don’t have a large enough root system.