University of Illinois Extension

Getting Ready for Frost

Even with concerns about global warming, frosts have been occurring a little earlier than normal since the 1970s in Illinois.

“In the Chicago area, the first frost usually occurs around mid-October as compared to around Oct. 7 in northern Illinois,” said Ron Wolford, U of I Extension horticulturist. “Downstate, the first frost hits central Illinois around Oct. 15 and southern Illinois around Oct. 20. These are average dates--frosts have occurred as early as September and as late as November.”

Frost, Wolford noted, is nature’s way of bringing the growing season to a close. First frosts are usually followed by a few weeks of good weather.

“The first frosts of the fall are usually radiation frosts,” said Wolford. “These occur under clear skies with little or no wind. Frosts kill plants when the internal temperature of the plant is cold enough to breakdown plant cells thereby causing the plant to wilt and die.”

In the vegetable garden, crops like lettuce, carrots, Brussels sprouts, cabbage, collards, kale, and spinach are frost-tolerant, meaning they can survive temperatures below 32 degrees F.

“The flavor and sweetness of some frost-tolerant vegetables like collards and Brussels sprouts are actually enhanced by frost,” Wolford noted.

Parsnips and carrots can be left in the ground and pulled when needed all winter if the area is heavily mulched. Pumpkins and winter squash should be harvested before a frost.

Wolford said green tomatoes should be harvested before a predicted frost. Be sure the tomato is firm and free of disease, nicks, or holes. Remove the stems and wash and dry the fruit. Wrap each tomato in newspaper and place them in a single layer on a shelf or in a box. Set the tomatoes in a cool, dark location like the basement. Check periodically for decaying tomatoes.

“Low areas where cold air settles will freeze earlier in the fall,” he said. “Flowers planted close to or alongside your home are offered some protection from frost because of the heat given off from your home. When temperatures drop below 28 degrees F, even plants with protection will probably be killed.”

To protect plants from frost, trap the heat from the soil around the plants, he added.

“Cover plants with blankets, newspaper, straw, sheets, tarps, boxes, or plastic sheeting,” he said. “Apply the covers later in the afternoon and remove them in the morning.”

Floating row covers can also protect plants. This spun polyester material will raise the temperature 2 to 5 degrees F around the plants.

Plus, you can leave the covers on the plants, since these allow sunlight to get through,” Wolford said. “Watering the garden before a frost will also offer some protection. Wet soil can hold four times more heat than dry soil. The air temperature above moist soil can be as much as 5 degrees F higher than the air temperature above dry soil.”

However, no matter what you do to protect your plants from frost, eventually the cold weather will end the growing season.

“Enjoy your down time by looking through seed catalogs and catching up on reading those gardening articles you didn’t have time to read during the growing season,” he said.