Frozen soil is fine for gardens in winter, but mulch needed
Frozen margaritas: yummy. Frozen toes: bad. Frozen soil: just fine.
Many gardeners are afraid their plants will be harmed if the soil freezes in winter. Often they see spreading mulch as away to prevent damage to their plants by insulating the soil.
“It’s true that insulation is one of the purposes of mulch, but it’s not to keep the soil from freezing in winter,” said Sharon Yiesla, plant knowledge specialist at TheMorton Arboretum in Lisle. “It’s more to keep the soil steadily cold until spring.”
Once plants have safely entered their winter resting state, called dormancy, having frozen soil around their roots does them no harm. “Most roots are fine until the soil gets down to 5 or 10 degrees belowzero,” said Julie Janoski, Plant Clinicmanager at the Arboretum.
The challenge is keeping the plants dormant through Chicago’s erratic winterweather. Especially in February and early March, it’s common for a few 50- or 60-degree days to be followed by a plunge back to freezing temperatures.
As long as plants stay dormant, they’ll be fine. But if that balmy spell warms the soil and tricks a plant intowaking up too early, itmay send up sprouts or begin to unfurl its tender green leaves. When the cold returns, those fresh green leaves and sprouts will be toast.
“A good layer of mulch over your soil will keep it cool during awarm spell, so the plants don’t get the wrong signal and break dormancy too soon,” Yiesla said. Instead, they’llwait for the gradualwarmup that means true springtime is arriving.
What about sudden winter cold spells? Mulch is the best protection against them, too. It will prevent a few days of subzero cold from seeping down to reach plants’ roots.
Mulch also helps hold soil in place. The winter cycle of freezing and thawing that splits concrete and creates potholes can also cause the soil in a garden to crack and heave, wrenching up roots. An insulating layer ofmulch will prevent that by keeping the soil temperature cool and steady.
Spread mulch in an even layer 1 to 2 inches deep over planting beds and 3 to 4 inches deep around trees and shrubs, being careful not to pile it against the bark.
Many gardeners spread mulch in fall as part of their end-of-season cleanup ritual. Others use fallen leaves in October orNovember. If youwaited until December, your timing is perfect. “The ground will be good and cold, and the plants will be solidly dormant,” Yiesla said.
But don’t stress too much about exact timing.
“Some mulch is always better than no mulch,” Janoski said.“We can’t all spread mulch at the perfect time, so do it when you can.”
For tree and plant advice, contact the Plant Clinic at TheMortonArboretum (mortonarb.org/plant advice or plantclinic@mortonarb.org). Beth Botts is a staff writer at theArboretum.