When extreme weather hits, one of the first questions I’m asked is, “How will this weather affect plants next year?” To which I astound people with my very insightful answer: “I don’t know.”
Many factors determine whether a plant will come through the winter unscathed, making it very hard to predict the outcome. Some factors include how low of a temperature a plant can tolerate, snow or ice cover, wind exposure, previous plant stress and spring weather conditions. This list explains why even in mild winters, we may see winter damage, or plants may come through uninjured.
Basic plant genetics dictate how cold of a temperature a plant can tolerate. To aid farmers, landscapers and gardeners in determining which perennials to plant, the United States Department of Agriculture publishes a Plant Hardiness Zone Map. Zones range from 1a in northern Alaska to 13b in Puerto Rico.
Wisconsin Plant Hardiness Zones range from 3b in the northwest to 5b in the southeast. Due to our proximity to Lake Michigan, zone 5a and 5b have a strip that extends from Kenosha all the way up through Door County. I generally say that we are in zone 5a, but to be safe, I recommend choosing plants that can survive in the colder zone 4b.
Because there is a greater variety of plants that grow well in warmer climates, gardeners can be tempted to stretch this recommendation by selecting plants that aren’t quite winter-hardy enough. In many years that might be perfectly fine, but then we get a cold snap like we had this winter … Some marginally winter-hardy plants that could have significant damage include peaches, apricots, sweet cherries and ornamentals such as Japanese maple, among others. Again, depending on other factors, these plants may survive, or plants that are definitely adapted to our climate may die.
A second reason why a plant survives or not is the amount of snow or ice cover. Snow is an excellent insulator and can greatly protect plants from injury. Leaving plant stubble on low-lying plants can help trap snow and provide some insulation. Despite variable snow cover the last two months, soil probes at six weather stations around the county show that soil temperatures at both two and four inches deep have stayed very consistent in the lower 30s, with a few drops into the upper 20s. Obviously, temperatures at individual sites vary. There is also evidence that snowflakes trap atmospheric nitrogen, which can fertilize plants when the snow melts.
Snow and ice can have negative effects when excessive amounts cause tree and shrub branches to break. Snow cover also allows voles to roam undisturbed, and they may chew on tree bark, make tunnels through lawns and pull out bulbs.
Wind affects plants as well. Strong winds can cause branches to snap, but mostly they dry out plant tissue. This type of damage, often called winter burn, is most common in evergreen trees. We saw quite a bit of winter burn last year, so it will likely be an issue this year as well.
Previous plant stress will compound any potential impact of cold weather. A lack – or overabundance – of water throughout the growing season will often take its toll on plants during the winter. Diseased or insect-damaged plants will be weaker entering the winter and will more likely succumb to winter injury. Additionally, plants that are located in non-ideal sites will likely sustain more damage than that same plant in a more suitable location.
Spring temperatures are just as variable as winter temperatures and can cause problems of their own. Freeze-and-thaw patterns can cause ice heaving, which essentially pops the plant out of the ground. A late frost can kill all emerging buds. On the other hand, moderate temperatures and the melting snow cover may provide the plant with the exact amount of water it needs to begin to recover and thrive.
Winter injury may not be immediately visible in the spring. Some plants may be clearly dead, while others may leaf out and then die. Root or bud injury may occur in a section of the plant while the rest looks fine. Cracks in tree bark might not kill the tree outright, but they could be entry points for a plant pathogen that ultimately kills the tree. Slow plant decline over the next few years may also occur. We are still seeing winter injury from 2014, so there may be a prolonged effect from this most recent polar vortex.
For more information, visit the Door County Extension office at 421 Nebraska St. in Sturgeon Bay, call us at 920.746.2260 or email me at [email protected].