Are you still planning your autumn gardening based on the first frost date? It may be time to rethink that, says an expert at The Morton Arboretum in Lisle.
“The way our climate has shifted, the frost date is no longer a very useful guideline,” said Sharon Yiesla, plant knowledge specialist in the Arboretum’s Plant Clinic. “Our weather has become so variable and prone to extremes that the first frost and the weather that follows it are just too unpredictable.”
Traditionally, gardeners have been told to think of any locality’s growing season as extending between two dates: the average date of the last frost in spring and the average date of the first frost in autumn. For the Chicago area, these dates were roughly May 15 and October 15. “They were always just averages, so they could be off by two or three weeks on either end of the season,” Yiesla said. “But 30 or 40 years ago, when our climate was more consistent, those dates worked for general guidance.”
Now, the weather is much more volatile. “The first frost may come a month or six weeks earlier or later from one year to the next,” she said. “It may be followed by weeks of warm weather, and plants may keep growing. You can no longer assume that the first frost means the end of the growing season.”
On average, spring now comes a couple of weeks earlier than it did decades ago and winter is not as long, as detected by monitoring when plants open their buds in spring and drop their leaves in fall. But that average masks much greater variability from year to year and from week to week than in the past. Droughts are more frequent, storms are bigger, and even with shorter winters, harsh freezes are still entirely possible.
What’s a gardener to do? “Watch the weather, not the calendar,” Yiesla said. “Pay close attention to the 10-day forecast, and let that guide your garden planning. But watch the 24-hour forecast too, because the 10-day forecast is no guarantee.”
Keep actively gardening. “That means actively watering,” she said. “As long as your plants are growing, they need water.”
Tender plants, such as basil, tomatoes, impatiens, petunias, and nearly all houseplants, will be killed by a ny frost, no matter when it comes. Other plants may grow or even resprout after a frost, if the temperature rebounds. Plants such as trees and shrubs that should be entering dormancy — the sleeping state that protects them over the winter—may struggle to go dormant if autumn weather is still too warm.
For tender plants, it’s best to play it safe. “Bring your houseplants indoors when nighttime temperatures fall into the 50s,” she said.
If you want to try to keep tender plants going for a while outdoors, be prepared to protect them against a sudden freeze. Draping them with fabric, such as old sheets or commercial floating row cover, is usually enough to keep plants two or three degrees warmer than the surrounding air temperature. You can also rake fallen leaves over tender plants to insulate them. Do not use plastic sheeting, which traps both heat and moisture that can harm plants.
“The coldest temperatures come after midnight, so set up your protection before evening,” Yiesla said. “The next day, remove any fabric so the plants can get sunlight and won’t overheat.”
A layer of mulch over the roots of all your plants will insulate the soil and moderate temperature changes in both fall and spring, although it won’t protect the plants’ leaves or fruit from a freeze. For edible crops, it’s wisest to harvest all tender crops such as tomatoes well before any frost is predicted. Some plants, such as spinach and parsley, can withstand a few degrees below freezing, and some, such as Brussels sprouts, actually taste better after a frost. Root crops such as carrots, beets and turnips are quite frost-hardy but need to be dug up before the soil freezes.
“All your plants have different needs and tolerances, so it’s important to know about each of them so you can help them handle the weather changes,” Yiesla said. “The most important things gardeners need to do in our volatile new climate are to learn more about our plants, and to pay attention to them and the weather every day.”
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.