"For plants that are fully dormant, there shouldn't be any problem," said Sharon Yiesla, plant knowledge specialist at The Morton Arboretum in Lisle. Dormancy is the resting state that plants enter to protect themselves against winter cold and drought.
The aboveground parts of trees and shrubs have bark to protect them. Buds at the tips of branches are shielded by bud scales. Perennials have allowed their stalks and leaves to die back already. Plants' roots are still alive, but they are safe in the ground. "As long as the roots are underground, they are protected," Yiesla said.
The main risk she sees is for evergreens, which may not have fully entered their dormant state because of warm weather in the fall. Evergreen needles retain some water all winter and could be dried out by bitter cold and wind.
"Depending on how long the cold goes on and how low the temperature falls, we may see a little more winter damage on evergreens than we sometimes do," she said. "But it won't be apparent until spring, and there's nothing a homeowner can do about it now."
The greatest danger isn't from cold, but from warm weather that can follow. Chicago often swings between cold and warm spells, and plants can be harmed by the quick shifts. When soil warms, dormant plants may start to wake up and then be hit hard by the next cold snap.
Plants with shallow roots, such as hydrangeas and hellebores, are especially vulnerable, along with species that are not quite hardy here, such as oakleaf hydrangea and butterfly bush.
"The best way to prevent damage from freeze-thaw cycles is to make sure there's a layer of mulch over the roots," Yiesla said. The mulch will insulate the soil, so it stays evenly cold.
You can use wood chips or fallen leaves, but another source of mulch is handy at this time of year: evergreen boughs. "Cut up your Christmas tree and lay the branches on the soil around vulnerable plants," she said. "They will help insulate the soil."
