Now is the time I have to steel myself against the autumn gardener’s worst enemy: complacency.
We’re having lovely weather at the beginning of October, with clear blue skies, low humidity and days in the upper 70s or low 80s. Though most flowers except the toad lilies, the last anemones, turtlehead and a few unwary annuals have done their blooming, everything else in the garden is perking up. There’s a little more rain, after a hot, dry summer that betrayed the promise of a wet spring. Now almost everything, including the tropical houseplants that spend their summers outdoors, is sleek and green. It’s as though we’re having the summer we wish we’d had.
But that’s an illusion. Many nights are getting down into the 40s. A frost could, in fact, happen any time; the average first frost date where I live is October 15. I’d better hustle to get those tropicals indoors before Chicago weather has one of its cruel whims and the temperature drops like a rock one night. I’ve already gathered all the green tomatoes and let the plants go.
Forecasters draw careful distinctions between a frost and a freeze. The National Weather Service issues a frost advisory when the nighttime temperature is expected to fall below 36 degrees. Lots of plants can make it through a frost, especially if you cover them overnight with a cardboard box or a bed sheet. (Never plastic. And get that cover off first thing in the morning.) Or, if you have a greenhouse, you can move the more portable plants inside overnight and back out again if it seems the next few days and nights will be warm enough.
A freeze warning is far more dire. It means the temperature is expected to fall below 32 degrees. A fall to 28 degrees is a hard freeze. Either one means the gardening season is over: It’s time to move those tender plants under glass or inside by a window for the winter and start the garden cleanup.
Frost is part of the normal progression of fall. With shorter days and less sunlight, the soil is no longer storing enough warmth to keep the air above it warm at night. The soil is gradually cooling toward freezing.
A freeze is usually a sharp blast of seriously cold air blown in on a wicked wind from Canada or the Arctic. We should be used to those too — they’re a normal part of our crazy mid-continental climate — but somehow the first hard freeze never fails to shock.
Every place has known dates for first frost in fall and last frost in spring. It’s important to remember, though, that those dates are averages that can mask wide variation. Around here, we’ve had frosts in September and other years when the first frost didn’t come until after Halloween.
The dates also vary quite a bit even between nearby places. If you live near Lake Michigan, which is a huge heat sink, your frost date might be a month later than if you garden just 20 or 25 miles inland. There are similar differences in many places in the country, but they tend to be obscured in large metropolitan areas where TV and radio weather forecasts are generalizations covering several counties. So it behooves a gardener to research the situation in his own zip code.
Frost isn’t the end of the gardening season, of course. You can still plant trees and shrubs (although earlier is better), since their roots will continue to grow after their top growth is dormant. I’ve planted several substantial shrubs this fall, and I need to keep watering them (and everything else I planted this year, including a hefty musclewood tree) until the ground freezes.
There are perennial debris and annual containers to dump into the compost, as well as that abundance of leaves that I will have to rake and shred for compost and mulch.
Best of all, though, the time after a frost— when I know the soil is finally cool enough — is the right time to start planting bulbs. Which means it’s time to dream of spring.