By Mike Hillstrom, DNR Forest Health Specialist, Fitchburg, Michael.Hillstrom@wisconsin.gov or 608-513-7690 and Brian Wahl, DNR Forestry Specialist, Fitchburg, Brian.Wahl@wisconsin.gov or 608-225-7943
During Wisconsin’s first week of 2021, a beautiful weather phenomenon occurred: rime ice. Rime ice forms when the tiny water droplets in fog freeze on trees and other objects. Hoarfrost, a similar phenomenon that occurs without fog, can form when water vapor in freezing air contacts a surface. The best news is that these winter conditions should not concern tree health when we emerge from the winter fog in spring.
We do not anticipate damage from the rime ice, but winter ice storms and wet, heavy snow can break branches or cause entire trees to fail. It’s also common to see frost damage on exposed tree parts and even roots if they are not insulated with snow in the spring. In most cases, trees recover from frost damage, but severe or repeated events may kill a tree or stress it enough that insects and fungi can invade.
Sunscald, frost cracks and winter desiccation may also damage trees. Sunscald occurs when warm, sunny winter days are followed by freezing temperatures at night, causing trees to crack open, typically on the south or west sides. Frost cracks are a very similar issue. Trees are often able to recover from sunscald and frost cracks.
Winter desiccation happens when conifers start photosynthesizing on warm, windy days in late winter or early spring. Conifers may dry out in these conditions if they use their stored water and cannot replace it because the ground is frozen. Conifers will survive minor or moderate damage but may die if the damage is severe.
Salt is another winter threat to trees. Salt spray from vehicles or salt that dissolves into the soil can result in bud death, twig dieback and root damage. Some plants are more sensitive to salt than others so avoid planting sensitive plants along roads or other surfaces where salt is used.
Although Wisconsin’s native trees have evolved to deal with winter weather, they can still be damaged. Watch for damage and take corrective action if needed. Trees should be pruned when they are dormant during the winter since the chances of infestation by insects and diseases are very low. A little winter tree care is an effective step towards healthy trees.
Learn more about environmental issues that impact trees on the DNR Forest Health environmental damage webpage.