One of the greatest risks for plants in the garden in late winter is not cold but warmth. “We often get short warm spells in February, right up into the 50s or 60s,” said Sharon Yiesla, plant knowledge specialist at The Morton Arboretum in Lisle.

What’s wrong with that? “It doesn’t last,” she said. “Before you know it, the temperature will have dropped back down below freezing.” The problem for plants is that the warm days may prompt them to sprout or even bloom, exposing tender new growth that will be killed by the cold that follows. Days in the 40s or above are often followed by nights that are well below freezing. While we sleep snugly inside, our plants must survive the cold nights outdoors.

“Plants will be better off if they wait to grow until it’s consistently warm enough for them to be safe,” she said.

You can lower the risk for your garden plants if you choose them carefully and prepare to protect them.

If plants sprout in late-winter warm spells, they may be killed or damaged when the cold returns. (Beth Botts)

Mulch: A layer of mulch helps by insulating the soil so it warms up more slowly when temperatures rise. “Plants can tell it’s spring through a combination of signals, including the longer days, warmer air temperatures and warmer soil,” Yiesla said. “If you can keep the soil cool, it will help block the false signal sent by a short spell of warm air temperatures.” This works best if you think ahead and spread mulch over perennial beds and around trees and shrubs in autumn.

Protect: If plants have sprouted and a below-freezing night is predicted, cover them for protection. Fallen leaves saved from autumn are fine fluffy insulation and can be left in place as mulch. You can also cover a bed of sprouts with an old sheet or purchased floating row cover fabric if you will remember to remove it in the morning. “Never use plastic bags or plastic sheets,” Yiesla said. “They trap moisture and encourage disease.”

Choose hardy plants: Make sure the plants you choose for your garden are labeled as hardy for our region. The Chicago area is generally in USDA Winter Hardiness Zone 5, which means we can use plants labeled for zones 5, 4, and 3. “You may think that because our climate is changing, we can get away with Zone 6 plants that are a little less winter-hardy,” Yiesla said. “In fact, the weather is becoming more variable and more risky, so it is still safer to choose plants labeled for Zone 5 or the colder zones 4 or 3.”

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.