Frost Temperature and Outdoor Plants: What You Need to Know
Protect your outdoor plants from cold temperatures by learning to predict when conditions for frost may hit your area and what precautions to take.
Quick Reference
- Frost: Ice crystals forming on leaves when air dips below 32°F. Damage depends on duration.
- Light freeze: 29 to 32°F. Kills tender plants.
- Moderate freeze: 25 to 28°F. Damages fruit blossoms and semi-hardy plants.
- Severe freeze: 24°F or lower. Damages most plants.
- Best protection: Cover before sunset, water the soil that morning, pick a sheltered south-facing site.
Frost is a gardener’s most frequent winter visitor and one of the most misunderstood. Whether a frost wipes out your tomatoes or barely touches them depends on temperature, duration, wind, humidity, and the location of your garden. Here is the Almanac’s guide to frost temperature for outdoor plants, with the freeze categories used by the National Weather Service and seven practical ways to protect your garden.
Cold Temperatures’ Effects on Plants and Vegetation
| Event | Temperature | Damage |
|---|---|---|
| Frost | Below 32°F at plant level | Ice crystals on leaves; damage depends on how long it lasts. |
| Light freeze | 29 to 32°F | Tender plants killed. Hardy plants survive. |
| Moderate freeze | 25 to 28°F | Wide destruction. Heavy damage to fruit blossoms and semi-hardy plants. |
| Severe freeze | 24°F or lower | Heavy damage to most plants. Only the most cold-tolerant species survive. |
Tips to Help Your Garden Survive Frost
Check the Sky Conditions
Frost (sometimes called white frost or hoarfrost) forms when air temperature dips below 32°F and ice crystals form directly on plant leaves. Clear, calm skies and falling afternoon temperatures are the classic setup. If temperatures are falling fast under clear, windy skies, especially with the wind out of the northwest, a polar air mass is on the way and a hard freeze is likely. A hard or killing frost is driven by large cold air masses, not just local cooling.
Cloudy Skies: You May Be in Luck
Cool nights with cloud cover often spare gardens. During the day, the sun’s radiant heat warms the earth. After sunset, the heat radiates upward. Clouds trap that warmth close to the ground. On clear nights, the warmth escapes to space and the plants chill below ambient air temperature.
Wind
Wind matters two ways. If the air is still, the coldest air settles to the ground. Plant level may be freezing while eye level is well above. A gentle breeze prevents that settling and keeps temperatures more uniform. If the wind itself is below freezing, frost damage is much worse than calm-night frost of the same temperature.
Moisture
Humidity and moisture are protective. When water vapor condenses out of humid air, it releases heat that can sometimes save your plants. Dry air does the opposite. Soil moisture evaporates, the evaporation cools the surrounding air, and the plants chill faster.
Location, Location, Location
Your garden’s location can decide whether an early frost wipes out your tomatoes or leaves your neighbor’s alone. Temperature drops 3 to 5°F for every 1,000 feet of elevation gain. Higher gardens chill faster. But lower is not always safer either. Cold air is heavier and sinks to the lowest spot in the yard. Frost pockets form in dips and hollows.
The best site for an annual vegetable garden is a gentle, south-facing slope that takes late-afternoon sun and sits protected from north winds. A garden surrounded by buildings or a treeline (which slows wind and traps daytime warmth) or one near a body of water (which buffers temperature swings) is much less likely to frost early.
Soil
Soil type affects moisture retention and how the soil releases warmth at night. Deep, loose, heavy, fertile soil holds more moisture and releases it into the surrounding air, raising the dew point and reducing frost risk. Thin, sandy, or nutrient-poor soil does the opposite. Heavily mulched plants are actually slightly more frost-prone, because mulch traps soil heat below ground and prevents it from warming the air around the leaves.
Know Your Plants
The plant itself determines how it will handle frost. Immature plants still pushing new growth into fall are most susceptible, especially the new tips. Plants with maroon or bronze leaves absorb and retain more heat. Downy or hairy leaves retain warmth. Compact plants expose less surface area to drying winds. Closely spaced plants protect each other by trapping warm air between leaves.
Frost on Its Way?
If a frost is predicted, cover plants before sunset to retain soil heat and protect against drying winds. Newspapers, baskets, tarps, straw, old sheets, and dedicated horticultural fleece all work. Anchor lightweight coverings so they do not blow off. Water the soil that morning. Moist soil holds and releases more heat than dry soil. Commercial fruit and vegetable growers actually run sprinklers all night during a hard freeze. As the water freezes on the plants, it releases heat that protects them. Sprinklers must run continuously as long as temperatures stay below freezing, which is why this method is rare for home gardens.
Cold-Hardiness Quick Guide
| Hardiness | Tolerance | Examples |
|---|---|---|
| Hardy | Down to 24°F | Kale, spinach, Brussels sprouts, garlic, leeks, parsnips |
| Semi-hardy | Down to 28°F | Carrots, beets, lettuce, broccoli, cabbage |
| Tender | Down to 32°F | Cucumbers, peas, potatoes, snap beans |
| Warm-season | Damaged below 50°F | Tomatoes, peppers, eggplant, squash, melons, basil |
Check your average frost dates by region to know when your first fall frost is likely.
Frequently Asked Questions
At what temperature does frost damage outdoor plants?
Tender plants are damaged at 32°F. A light freeze of 29 to 32°F kills tender plants. A moderate freeze of 25 to 28°F damages fruit blossoms and semi-hardy plants. A severe freeze below 24°F damages most plants except the most cold-tolerant.
Should I water plants before a frost?
Yes. Water the soil in the morning before a predicted frost. Moist soil holds and releases more heat overnight than dry soil. The released heat raises the dew point, which reduces frost severity.
What is the best way to cover plants for frost?
Cover plants before sunset to trap any remaining soil heat. Use old sheets, blankets, newspaper, baskets, tarps, straw, or horticultural fleece. Anchor lightweight covers against wind. Remove covers in the morning once temperatures climb above freezing.
Why does mulch sometimes make frost worse?
Mulch traps soil heat below ground and prevents it from radiating up to warm the air around plant leaves. Mulched plants benefit from steady soil temperature but lose some of the overnight warmth boost that bare soil provides on a frost night.
What is a frost pocket?
A low spot in a yard or field where cold air settles. Frost pockets can be 5 to 10 degrees colder than the surrounding land on a still night. Avoid planting tender annuals or fruit trees in known frost pockets.
Which vegetables tolerate frost best?
Kale, spinach, Brussels sprouts, garlic, leeks, parsnips, and most herbs in the mint family. Many of these actually taste sweeter after a frost because the cold concentrates sugars.
This article was published by the Staff at FarmersAlmanac.com. Any questions? Contact us at questions@farmersalmananac.com.
- Farmers' Almanac Staff
- Farmers' Almanac Staff
- Farmers' Almanac Staff
- Farmers' Almanac Staff
Golden Isles Ga rarely drops below 75% humidity. When temps drop below 32 it’s only 1 hour then temp will quickly climb back above 40. I always take all of my 36 succulent plants inside. I’m wondering if this is necessary?
Or perhaps place on porch up against house?
Enjoyed article , I learned a lot?
Thank you for that very helpful information! This is my first year for gardening, and of course my first fall garden. My pole beans aren’t looking good, so even though the low is forecasted at 36, think I’ll give them a little TLC and throw a cover over them. Do you think this is a waste of time?
Janice, zone 9b, Visalia, CA
You should be fine but maybe look into why your beans aren’t happy.