When the weather gets cold, it nips our noses, numbs our toes and makes us shiver. What does it do to our plants?
“Cold damages plants because it freezes water into ice,” said Spencer Campbell, Plant Clinic manager at The Morton Arboretum in Lisle. Plants consist largely of water and depend on liquid water for all their life processes, so ice is bad news.
Sharp ice crystals that form inside plant cells slice open the delicate cell walls. When water in the soil is frozen into ice, plants’ roots can’t absorb it. “Winter is basically a season of drought for plants, because they can’t take in frozen water and can’t transport it through their tissues,” Campbell said.
Perennials, shrubs and trees that are winter-hardy in the Midwest have adapted to handle the cold season in a simple way: They shut down to wait it out, in a state called dormancy. “They’re sleeping,” he said. “When the weather warms enough for them to operate, they’ll wake up.”
Most trees and shrubs abandon their flimsy leaves as they go dormant, keeping only branches and twigs that are protected from the cold by bark. They cover their buds to shield them until spring. Meanwhile, they concentrate sugar in the sap inside their tissues, reducing its water content and lowering the temperature at which it will freeze.
Perennials give up on all the green growth above ground, keeping only roots that are protected by the soil. “A peony plant’s root system can live for a century, but to survive cold winters it has to grow a whole new set of stems, leaves, and flower buds every year,” Campbell said.
Ice crystals can form inside plants at any air temperature below 32 degrees. This freezing damage is often worst in younger plants and new twigs that haven’t yet toughened up their bark.
Plants are most vulnerable to freezes in late fall, when some may not yet be fully dormant, or in early spring, when tender new sprouts can be killed by a freeze that follows a warm spell. Temperatures are lowest at night. Even a sunny, pleasant winter day can be followed by a hard freeze.
In gardening terms, a “light freeze” or “light frost” refers to temperatures that fall just a few degrees below freezing for a few hours. A “hard freeze” or “killing frost” comes when the temperature drops below 28 degrees for a longer time. Some hardy plants can withstand a light freeze, but a hard freeze will kill any top growth.
Although it’s often used synonymously with “freeze,” “frost,” technically refers to the silvery coating of ice crystals that forms on plants if a freeze occurs when there’s lots of water vapor in the air.
Even after leaves and stems are killed, plants may not be done for the year. “The soil takes a while to freeze even after the air is cold,” Campbell said. “Roots can keep growing for several weeks after top growth has died back.”
This extra time to establish roots can be crucial for trees, shrubs and perennials that you planted this year. Unless there has been plenty of rain, Campbell advises watering these new plants until the soil freezes solid.
To keep the soil from freezing longer, spread a layer of mulch, especially around new plants. “Mulch acts as insulation,” he said. “It also fills in gaps and cracks that let cold air into the soil, like the insulation and caulking in your house.”
Snow is another good form of insulation, since it’s mostly air. “Unfortunately, we don’t get snow cover as often as we once did because our climate is becoming more variable,” Campbell said. Since we can’t rely on snow, he said, “the best thing we can do for all our plants is to spread mulch over their roots.”
The most important way gardeners can help plants survive the cold is by choosing the right plants in the first place. “Plants that come from cold places are adapted to handle winter,” he said.
If you have chosen plants that are truly winter-hardy and have watered and mulched them, they will settle into their winter dormancy and wait for the warmth and moisture of spring.
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.