Temperature swings, not cold, endanger plants. Solution: mulch.

November 27, 2018

Most gardeners know it’s a good idea to have mulch over the root zones of trees, shrubs, and other plants in winter. But why?

Many think the purpose of mulch is to insulate the soil and keep it from freezing. In fact, the opposite is true.

“We want the soil to be cold while trees and other plants are dormant,” said Sharon Yiesla, plant knowledge specialist in the Plant Clinic. “Cold is an important signal that tells them to stay in their safe, sleeping state until spring.”

By insulating soil to keep it cold, mulch guards against damage from the treacherous warm spells that are common in Chicago’s volatile winter weather. Even in January and February, temperatures can rise from the 20s to the 60s in a day and plunge back to freezing just as suddenly.

“Those temperature swings are a real danger to trees and other plants,” Yiesla said. “If plants mistake a warm spell for spring and start opening their buds or sending up sprouts, the tender new growth can be frozen when the temperature falls.”

Warm days may urge plants to start growing, but soil that is kept cool by a layer of mulch can slow the growth response. The soil doesn’t necessarily need to be frozen, Yiesla said, although many of our plants don’t mind frozen soil around their roots.

The insulation of mulch also can prevent other hazards of Chicago’s freeze-thaw cycles. For example, it can reduce the risk that smaller plants will be heaved up out of the ground as water in the soil freezes, expands, and then thaws. 

“If you don’t have mulch in your garden, wait to apply it until the ground is good and cold, or frozen,” Yiesla said. “December is a good time.”

If you already have a layer of mulch, your soil will cool more slowly in fall, because insulation works both ways. The difference won’t matter to plants. “As long as the soil eventually gets cold and your plants go dormant and stay dormant until spring, the mulch is doing its job,” Yiesla said.