In Frozen 2 or Game of Thrones, freezing is a threat and ice is a fearsome weapon. Yet in the garden, frozen soil is not all bad news for plants.
What freezes is the water between soil particles. Soil types that that hold more water, such as clay, are more likely to freeze than freely draining soil, such as sand.
Roots in the soil can freeze too, since they’re full of water, according to Sharon Yiesla, plant knowledge specialist at The Morton Arboretum in Lisle. When water in plant cells freezes, sharp-edged ice crystals can slash the cell walls.
But roots have evolved a number of ways of protecting themselves, said Luke McCormack, tree root biologist at the Arboretum.
Roots of cold-adapted plants are able to prepare themselves for winter by increasing the concentration of sugar in their cells, McCormack said. That lowers the freezing point so the liquid resists freezing at temperatures below 32 degrees. Many plants also develop antifreeze proteins.
For roots, being surrounded by icy soil has pluses and minuses. They can’t absorb water that has turned to ice, so winter is a dry time for plants. “It can be very similar to drought,” McCormack said.
On the other hand, a layer of frozen soil on the surface can be a shield against cold air above ground, which is often far colder than the soil.
If the frozen soil has cracks or gaps, the shield fails and lets frigid air reach deeper roots.
Snow is a great insulator and can fill gaps in the frozen layer, McCormack said, but Chicago gardeners can’t rely on snow cover alone.
One danger to plants in winter is frost heaving, which occurs because water expands when it freezes. Frozen soil may swell so much that there’s not enough room for it, so a chunk pops out of the soil surface like a pothole in asphalt. The gap then exposes roots to cold air.
The freeze-thaw cycle common in this climate — a series of warm days in late winter or spring, each followed by a deep freeze — often leads to frost heaving, according to Yiesla.
“Newly planted trees and shrubs are especially vulnerable,” she said. Digging and settling may have created gaps in the soil that let freezing air penetrate. New plants may not have had time to acclimate to cold soil, and haven’t spread out a network of roots that could anchor them against frost heaving.
The best way we can add protection, for new plants or old, is by spreading a layer of insulating mulch, Yiesla said. It will cover up treacherous gaps in the soil shield, keep the soil temperature steady, and protect the plant from the effects of the freeze-thaw cycle.
“On a sunny winter day, take a walk around your garden to make sure you have mulch everywhere you need it,” Yiesla said. Look for frost heaving. If you see a displaced plant, gently press the root ball back in place and cover it with mulch.
When spring is truly, safely here, the ice in the soil will melt and growth will begin.