How do I protect plants from frost tonight?
Short answer: Before sunset, drape frost cloth, sheets, or cardboard over tender plants so the cover touches the ground all around and traps soil heat. Water dry soil beforehand, move containers into a garage or shed, and mulch perennials. A light freeze (29-32°F) can kill tender plants. Uncover once it warms above freezing; remove plastic soon after daybreak.
How cold is a light frost vs a hard freeze?
Plant cell water freezes, expands and ruptures cell walls at 32°F or lower, according to UGA Extension. New Mexico State University (NMSU) Extension groups freezes by severity:
| Category | Temperature | Typical damage |
|---|---|---|
| Light freeze | 29°F to 32°F | Tender plants killed, little damage to other plants |
| Moderate freeze | 25°F to 28°F | Heavy damage to fruit blossoms, tender and semi-hardy plants |
| Severe freeze | 24°F and colder | Damage to most plants |
Why are clear, calm nights the riskiest?
At night the ground radiates the heat it collected during the day. UGA Extension warns that “radiational freezes” on clear, cold nights hit plants in open, sunny areas hardest. NMSU notes frost can form under clear skies once air drops to about 40°F.
So a forecast low in the mid-30s on a still, cloudless night may still frost your garden. Gardeners generally find low spots, where cold air settles, freeze first.
How do I cover plants for frost?
Covers trap the heat the soil releases overnight, which only works if the cover reaches the ground.
- Use what you have. Frost cloth, old sheets, blankets, or cardboard boxes all work. Missouri Botanical Garden also lists newspaper, straw and evergreen boughs.
- Drape it to the ground. UGA Extension says covers should contact the ground all around the plant.
- Support it. Stakes or tomato cages help keep heavy fabric from breaking stems.
- Cover before the frost. Missouri Botanical Garden advises covering before frost occurs; doing it before dusk holds in more of the day’s warmth.
- Uncover in the morning. Missouri Botanical Garden says to remove coverings, especially plastic, once temperatures rise above freezing. UGA Extension says plastic should come off soon after daybreak, since sun can overheat plants beneath it.
NMSU reports hot caps and cloches protect to about 26°F, but they must be vented or they can overheat.
Should I water plants before a frost?
Yes, if the soil is dry. UGA Extension says drought-stressed plants are weaker and more vulnerable, and moist soil helps plants resist freeze injury.
Missouri Botanical Garden describes a separate sprinkling technique for tender crops: apply about 1/8 inch of water per hour and keep going until the ice melts off the plants. Do not use it on trees, shrubs or other plants that the weight of ice could break.
What do I do with potted plants?
UGA Extension notes container plants cool faster because they lack the benefit of ground heat. Move them into a garage, shed or house for the night. If they must stay out, grouping them in a sheltered spot and wrapping them in blankets or burlap may help.
NMSU also points out that a cold-tolerant plant can survive while its container does not, so bring in fragile pots.
Does mulch protect perennials?
Yes. UGA Extension says mulch holds in heat the ground releases at night and keeps soil moist. NMSU lists materials like leaves, straw, and pine needles. Shredded fall leaves are free; see rake or mulch leaves.
Which vegetables need protection?
NMSU Extension sorts common vegetables like this:
| Group | Examples |
|---|---|
| Likely damaged by light frost (29-32°F) | Tomatoes, beans, corn, winter squash |
| Tolerate light frost | Lettuce, beets, carrots, arugula, peas |
| Tolerate hard frost (25-28°F) | Kale, cabbage, broccoli, spinach, onions, leeks |
Focus your covers on the first group.
Quick answers
What temperature should I cover my plants? Cover tender plants when lows near 32°F are forecast. On clear, calm nights, consider covering when lows near 40°F, when NMSU says frost can form.
Can I use a bed sheet to cover plants? Yes. Drape it to the ground on all sides and support it so it does not crush stems.
When should I take frost covers off? Once temperatures rise above freezing. Remove plastic soon after daybreak.
Will one frost kill tomatoes? A light freeze of 29-32°F can kill tomatoes and other tender crops, per NMSU. Covering or harvesting first is safer.
Freezing nights also threaten plumbing; see how to keep pipes from freezing.