Many vegetables and flowers keep producing until they freeze, while tropical houseplants that spend the summer outdoors are definitely not frost-hardy. So when can gardeners expect that first night when the temperature falls to 32 degrees?
“Planning for freezing weather has gotten more tricky as the climate is changing,” said Sharon Yiesla, plant knowledge specialist in the Plant Clinic at The Morton Arboretum in Lisle. “Our weather has become more volatile and unpredictable.”
Her best advice is to ignore the calendar, watch the garden, pay close attention to the weather forecast, and be ready to take action to protect vulnerable plants if you need to. “Don’t be lulled by fall days when you can still do without a jacket,” she said. “The weather can turn on a dime, and a single cold night can kill plants.”
The old rule of thumb was to expect the first frost in the Chicago area around Oct. 15. “That was never very reliable, because it was just an average,” Yiesla said. “An average can hide a lot of extremes.” Although the National Weather Service says the average first freeze date at O’Hare Airport is Oct. 19, a freeze has occurred there as early as Sept. 22.
An average also doesn’t account for the wide range of weather across the large Chicago metropolitan area. In general, areas closer to Lake Michigan and in the city are more likely to get a freeze later. For example, in 2024 the first freeze at The Morton Arboretum in DuPage County came on Oct. 16. The Loop didn’t feel a freeze until Nov. 20 — but within a week, nighttime temperatures there were down to 15.
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How can you prepare your garden for freezing in autumn?
Bring houseplants indoors soon: Although spending the warm months out in a shady garden or balcony may help them grow lush and lovely, most houseplants are native to tropical places and have no defenses against the cold. Cool weather can stress them even at temperatures well above freezing. “Get them indoors once nighttime temperatures reach the 50s,” Yiesla said.
Be ready to cover plants: You can help some annuals bloom a little longer or keep some vegetables yielding for a while if you’re ready with protection when the first few cold nights are predicted. Old sheets spread over plants will work, although a lightweight commercial product called floating row cover is sold for the purpose. Do not use plastic sheeting, which can trap harmful moisture, and remember to uncover the plants during the day.
Use leaves for insulation: Once they start to fall, keep a pile handy to spread over tender plants on cold nights.
Mulch to prolong the harvest: You can delay digging up cold-tolerant root vegetables such as carrots, turnips and beets for a few weeks if you spread mulch or leaves over their beds to slow down the soil’s cooling. “Just be sure you dig them up before the soil freezes solid,” she said. Spinach, Brussels sprouts, and cabbages can usually withstand a few degrees of frost above ground.
Let hardy perennials freeze: Perennials that are winter-hardy in the Chicago area will die back to the ground once it gets cold, but their roots will stay alive even in frozen soil. A layer of mulch over your perennials will help them by keeping the soil cold during warm spells in late winter and spring, so the plants stay in their protective dormant state longer. “Spring weather is just as tricky as fall weather,” Yiesla said.
What about trees and shrubs? “Trees are already preparing for cold weather in autumn by discarding their leaves,” she said. “They’re entering dormancy. They’ll be fine.”
For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or [email protected]). Beth Botts is a staff writer at the Arboretum.