The deepest darkness of the year is a sign of the coming light, for plants as well as people. The winter solstice, which occurs in Chicago this year on Dec. 21, is the shortest day and longest night of the year in the Northern Hemisphere. It’s the day when the sun appears to travel its shortest path across the sky.
By this time, most plants have escaped light deprivation by entering their sleeping state of dormancy or by hiding away their next generation in the form of seeds. But the solstice also means that the days will start to increase in length toward the coming of spring when plants will begin to grow.
That’s why, for thousands of years, in many cultures and religions, people have greeted the solstice with light-filled celebrations. They defied the dark and trusted in the promise of sunlight, rebirth and new growth for the plants they depended on for food. They knew there still would still be gray months of cold, but spring was coming.
This winter, as every winter, the lengthening days — or shortening nights — will be an important signal to plants. Although they may be dormant, they are still alive and tuned in.
“Day length is one of several signals that tell plants when to sprout or put out their new leaves and flowers,” said Sharon Yiesla, plant knowledge specialist in the Plant Clinic at The Morton Arboretum in Lisle. “Different kinds of plants respond to different changes or combinations of changes in their environment.”
Some kinds of plants, such as most fruit trees, have a chilling requirement — a certain number of days their roots or seeds need to spend in the cold before they know it’s spring. “They’re counting down,” Yiesla said. “If the winter is too warm, they may not meet their chilling requirement.”
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For many plants, the soil temperature is also an important signal, and that is related to sunlight. “The soil is warmed by energy from the sun, so longer days will warm the soil more,” she said.
Ideally, the increasing sunlight would warm the soil just enough to signal plants to sprout at the right time — when days have become long enough to give them the energy they need to grow and the danger of frost is past.
Unfortunately, that doesn’t always work in our gardens. “In our vegetable plots and ornamental gardens, we’ve mixed up plants that evolved around the world, in different climates and conditions,” Yiesla said. “That tends to scramble their signals.”
Nonnative plants often aren’t built to handle the variations of a Chicago spring, which tends to seesaw between warm spells and hard freezes. They are easy to fool and often will sprout in a warm spell, only to be damaged or killed by the next cold snap.
The variations of the Chicago-area climate are now becoming even more volatile, so even native plants may be tricked. “They evolved for the local climate, but we’ve changed that climate,” Yiesla said. “Sunlight and warm soil may no longer be reliable indicators.”
Snow, which insulates soil and tends to moderate changes in soil temperature, is no longer a dependable help for plants. “We can still get the occasional big snowstorm in the Midwest, but in recent years we have not had a layer of snow on the ground all winter,” she said.
How can gardeners protect plants against this trickster climate? Spread mulch. “A mulch layer over the plants’ roots zones insulates the soil, so its temperature rise is slow and steady,” she said. “The soil will warm up gradually due to the sunlight from longer days, not crazy weather swings.”
It’s like the way we bundle up in puffy coats to maintain our body temperature while we enjoy winter celebrations. We keep ourselves warm, but not too warm, until the sun brings spring again.
For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or plantclinic@mortonarb.org). Beth Botts is a staff writer at the Arboretum.