How to Protect Plants from Frost Tonight (and Will Mums Survive?)

A quick, research-backed checklist for frost and freeze nights

When a frost advisory or freeze warning is in the forecast, a few minutes before sunset can save your tomatoes, flowers, and fall mums. Here’s how cold is too cold, which plants need help, how to cover them properly, and what to do the morning after.

Frost vs. freeze: what the forecast means

• Frost forms when leaves and the ground radiate heat to a clear sky and drop to freezing, even if the official air temperature reads a few degrees higher. Low spots and open lawns frost first.
• Freeze means the air itself, and the inside of the plant, drops below 32°F. A freeze does more damage than a frost.
• Hard (killing) freeze is about 28°F or colder. Iowa State Extension notes covers are usually no longer effective below about 28°F, and damage can happen above that if freezing lasts 5 hours or more.

Which plants are tender, and which are hardy?

Groupings from Michigan State, South Dakota State and NC State Extension vegetable guides and Virginia Tech, Iowa State and University of Missouri guides on mums. Varieties differ, and a long freeze does more damage than a brief dip.

How to cover plants the right way

1. Water the day before. Moist soil holds more daytime heat and gives it back overnight. Don’t dig or cultivate just before a frost; loose, dry soil lets cold air in.
2. Use sheets, blankets, or frost cloth. Lightweight fabric is best. Row cover can add a few degrees of protection, and sometimes more.
3. Drape it all the way to the ground. The cover works by trapping heat rising from the soil, not the plant’s own heat. Weigh the edges down with bricks or soil and don’t gather it around the stem.
4. Keep plastic off the leaves. Plastic touching foliage conducts cold, so the leaves freeze where it touches. If plastic is all you have, hold it up on stakes, buckets, or chairs.
5. Cover before sunset. That way you trap the day’s warmth in the soil.
6. Bring containers in. Move potted plants, including mums, into a garage, shed, or porch. Group pots you can’t move against the house and mulch around them.
7. Uncover in the morning. Remove covers once the air is back above freezing. Frost nights are usually followed by sunny days, and heat builds fast under a cover, especially plastic.

Wet covers lose their power: Colorado State Extension notes that blankets and sheets work only while dry. A soaked cover can chill the plant by evaporation.

Harvest tender crops before a freeze

• Tomatoes: pick every fruit that is at least “mature green” (pale, shiny, starting to lighten) and ripen it indoors at room temperature.
• Peppers, eggplant, beans, cucumbers, summer squash, okra: harvest everything usable. They won’t survive even a light frost.
• Winter squash and pumpkins: bring them in before a hard freeze. Fruit that has started to color will keep coloring off the vine.
• Sweet potatoes: dig them before frost, ideally before the soil drops below 50°F.
• Leave the hardy ones: kale, collards, Brussels sprouts, parsnips, and carrots can stay in the garden and often taste sweeter after cold weather.

Mums and frost: in the ground vs. in a pot

Garden mums (Chrysanthemum × morifolium, once sold as “hardy mums”) are perennial in roughly zones 5–9, but hardiness varies by cultivar. Virginia Tech Extension explains why they’re no longer called “hardy.” A light frost may brown open flowers, but the plant keeps going.

• In the ground: leave the stems and foliage standing through winter (Iowa State found mums survive better this way). After the ground freezes, add 3–4 inches of loose mulch such as straw, pine needles, or evergreen boughs to prevent frost heaving.
• In containers: potted roots freeze much more easily. Bring pots into an unheated garage, basement, or cold frame before a hard freeze, and keep the soil barely moist until spring. Mums planted late in fall, while in full bloom, often don’t root in time to survive winter.
• Florist mums: grown for indoor display and not winter hardy.

After frost damage: don’t rush to prune

• Wait. Cold injury often takes days or weeks to show fully, and damaged leaves protect the living tissue below from the next freeze.
• Prune woody plants in spring once new growth shows what is alive. Scratch a twig: green under the bark means alive, brown means dead. Cut back to living wood.
• Annuals and vegetables killed by frost can be pulled any time. Tender perennials, such as some salvias and lantana, may resprout from the roots.
• The only exception: remove broken limbs that could fall right away.

When does the first frost usually arrive?

How we calculated this table: for each ZIP code we take the nearest NOAA NCEI 1991–2020 Climate Normals station’s 50%-probability 32°F first-fall-frost date, then report the median across the zone and the range covering the middle half of its ZIP codes. A 50% date means frost comes earlier than this in half of years. Get the date for your own address in the ZIP planner.

Find planting dates for your zone: Zone 3 · Zone 4 · Zone 5 · Zone 6 · Zone 7 · Zone 8 · Zone 9 · Zone 10 · Zone 11. Or enter your ZIP for your nearest station’s frost dates.

Frequently asked questions

Sources & further reading

1. Iowa State University Extension and Outreach (Yard and Garden): How to protect plants from frost and freeze
2. Michigan State University Extension: Protecting trees and shrubs from frost damage
3. Michigan State University Extension: Identifying and preventing freeze damage in fall vegetables
4. South Dakota State University Extension: Fall frost tolerance of common vegetables
5. NC State Extension: Frost and freeze protection for vegetable crops
6. Colorado State University Extension: Frost protection and extending the growing season
7. University of Florida IFAS Extension: Winter plant protection
8. Virginia Cooperative Extension (Virginia Tech): Overwintering mums
9. Iowa State University Extension and Outreach (Yard and Garden): Growing chrysanthemums in Iowa
10. University of Missouri Integrated Pest Management: Winter care of hardy mums
11. UC Statewide Integrated Pest Management Program (UC ANR): Freezing and frost
12. N.C. Cooperative Extension, Wayne County Center: Care for cold damaged plants
13. Frost dates: NOAA NCEI U.S. Climate Normals 1991–2020; zones: USDA 2023 Plant Hardiness Zone Map ZIP data (PRISM Climate Group, Oregon State University).

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