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Predicting Frost in the Garden

Photos from Old Computer 6067
Frost protecting plant blankets
Frost protecting plant blankets

Frost in the garden happens when the surfaces of plant leaves or fruit—or other solid objects in the garden—are colder than freezing and moisture in the air condenses on the surfaces to form ice crystals. A light frost or white frost happens when the air temperature drops to around 32°F. If you know frost is coming, you can protect plants with plant blankets, floating row covers, plastic tunnels, or even a cardboard box set over the top of plants.

Three Signs Frost is Likely:

1. The night is clear and bright stars are easily visible. (Clouds act as a blanket of protection against frost keeping ground warmth from escaping into the night.) 2. The air is dry before midnight with no condensation on car windows or outdoor furniture. (Moisture in the air releases heat when it condenses to form dew; this will prevent frost from forming if the air temperature is just above 32°F–but will result in frost when the air temperature drops below freezing.) 3. The temperature is 45°F or colder by 10 p.m. (You can expect the overnight temperatures to continue to drop and be the coldest just before dawn.)

Protecting Plants from Frost

Plants under a solid surface such as cloth, plastic, or cardboard—or under the overhand of a building–are less likely to be damaged by a light frost. Cold air is dense; its molecules are tightly packed. Cold air will sink or flow to the lowest point it can, but it cannot move through a solid surfaces. (For tips on protecting plants from frost and freezing weather click here to read Season Extension articles.)

Reviving Plants Hit by Frost

Sprinkle plants touched by frost with water from the hose before the sun shines on them. Heat in the water will raise the air temperature around plants and the temperature of plant tissue. After a frost, don’t assume damaged plants are dead. Let plants sit for a week, then remove only tissue that is clearly dead. Leave perennials alone until their normal period of growth begins, then prune out dead plants. In many areas, the first frost is followed by days of warmer weather. It is worth the effort to get plants past the first cold snap; there may be more harvest to come before the first solid freeze. For a list of crops that can survive frost click her to read Vegetable Garden Plant Hardiness.

Written by Stephen Albert

Stephen Albert is a horticulturist, master gardener, and certified nurseryman who has taught at the University of California for more than 25 years. He holds graduate degrees from the University of California and the University of Iowa. His books include Vegetable Garden Grower’s Guide, Vegetable Garden Almanac & Planner, Tomato Grower’s Answer Book, and Kitchen Garden Grower’s Guide. His Vegetable Garden Grower’s Masterclass is available online. Harvesttotable.com has more than 10 million visitors each year.

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