Gardening Columns

When Frost Threatens

First published on garden.org on October 9, 2008, by Suzanne DeJohn

If you haven't been hit by your first fall frost, it's surely right around the corner. Fortunately, the first cold snap is often followed by an extended warm spell, so if you can protect tender plants you'll probably get a few more weeks of growth. So begin gathering row covers, tarps, and old sheets so they'll be handy to drape over plants.

Check weather forecasts daily. When a frost is predicted, go out in late afternoon to cover tender plants. The idea is to capture the warmed air that's radiating from the soil at night, so make sure the covers reach all the way to the ground, and anchor the bottoms with rocks or boards. If possible, prop up the protectors with tall stakes to prevent them from touching the foliage. Move tender plants growing in containers to a sheltered spot. Once the air temperatures warm up to the 50s the next morning, remove the covers and move containers back into their normal positions. Repeat each time a frost is predicted until you get an extended cold spell -- or you get tired of covering and uncovering plants.

Understanding Frost
You can modify your landscape or take advantage of natural features to protect plants, too. For example, areas near the house or a stone wall or patio tend to stay warmer because the structures absorb heat during the day and radiate it at night. Similarly, humidity in the air can help prevent frost damage because as the water vapor condenses and forms dew, some heat is released. Without that moisture, the temperature drop is more rapid. So to keep the odds in your favor, water your plants when a frost is predicted.

Avoid the Low Spots
Cold air is heavier than warm air, so it flows downhill and settles in valleys and low spots. Therefore, the best place to locate a garden of tender annuals or borderline-hardy perennials is midway down a south-facing slope. Hilltops and mountaintops also tend to be colder because of the altitude and wind exposure. So plants growing partway down a slope have the best chance of surviving frost. If the slope faces south, even better, because there the soil receives more of the sun's warmth during the day and can radiate this heat at night, warming the air around plants.

There's something melancholy about waking up to find basil plants blackened by a chilly night. I dream of a greenhouse extension or sunroom, where I can grow tender plants year-round. Maybe Santa will find room in his sleigh this year.