The recent warm spell has made it even harder to accept winter is here. We have already had our first snow and the holidays are upon us. Some may wish for a continued warm winter trend, but the plants are in desperate need of a good cold spell in Illinois.
If you are like my friend, who had a columbine blooming after Thanksgiving, or if you had a magnolia blooms popping up in the wrong season, you may have noticed that unseasonably warm spells cause plants to become downright confused. The flowering of these plants in the correct season is compromised and they will likely not flower spring when they are supposed to.
Why do plants need a long sustained cold winter?
Most flowering trees require 500-2,000 chilling hours where the temperatures are below 45 degrees to have a good flower set. Fruit trees such as apple trees require more chilling requirements that a peach. If the trees have reached their chilling hours, warmer temperatures along with longer day length may cause buds to break and growth to occur earlier than usual. If we were to get another hard frost, opening buds and young fruit could be extremely susceptible to damage.
Our perennials are adapted to cold winters and lured out of dormancy by a combination of cool weather and longer days. An early warming period is described by University of Illinois Research Dokyoung Lee using native prairie grasses as an example.
“These plants come out of dormancy when the soil warms, carbohydrates in the plants' rhizomes are converted to fuel for new growth. If there is a cold snap, these shoots die, leaving the rhizomes depleted." He also describes that the warming temperature wakes the plants up but even without a cold snap, the short days leave them “stagnant.”
Another concern is pollinators. We see that late cold spells and lack of sustained cold winters affect when plants flower. Will this confuse the pollinators that depend on these plants if the flowers are either not producing flowers or flowering at times when the pollinators are not active?
Another concern is weedy plants, as pointed out by Lee. If perennials in nature grow less because of an early warm spell, would aggressive annual weeds take over? Heavy biennial seeders like teasel and garlic mustard are one of the greatest threats to our natural areas.
The next time you revel in the early or late warm spell, consider what this is doing to the plants. Researchers have a long way to go to understand these seasonal changes and their effects on plant communities.