One of my great pleasures in fall is watching the change that accompanies the seasonal return of frost and cooler nighttime temperatures. It is not uncommon on an autumn morning to see fog drifting along streams and laying over ponds. The same humid but cool conditions that produce fog will produce frost in low-lying areas. When the temperature of the ground and plants falls below the dew point, frost will begin to accumulate. This shows up first in low areas, because cool air is more dense than warm air and will flow to the lowest part of the landscape. The other morning I took these photographs of both fog and frost along our entrance drive at Winterthur.

A week of sunny weather in April can give a gardener amnesia. Suddenly you find yourself planting as though it is May, forgetting that the nights can still get cold and turn your hard work into mush. Because today has been cool and cloudy I’ve already had two calls from gardeners asking for advice on frost. Keep in mind that it is not typically cool cloudy days that bring frost; frost is much more common on clear, cool nights with little wind.

Frost damages tender plants by causing cell walls to rupture. You can simulate the effect by putting a leaf of iceburg lettuce in a freezer for a minute. As the lettuce demonstrates, the leaves of a frost damaged plant wilt as though they have been crushed and will not recover on thawing. To keep frost at bay you can use any number of simple strategies. Begin by watering your plants thoroughly during the day, as water and soil are effective thermal masses that will help keep the micro-climate around your plants warmer. If your plants are in pots or trays, move them into a garage or shed for the evening. Alternatively, cover plants with sheets/blankets, paper/newspaper, or sheets of plastic. You can use stakes distributed throughout your beds to hold the covers off of the foliage of your plants. Remember to remove covers as the sun comes out the next day to prevent overheating. If your plants are damaged, but not killed, you can clip out the frost-damage and they should be fine.