How to Protect Plants from Frost: Covers, Timing, Temps
A frost cover, a watering can and the right timing protect most garden plants through a cold night. The two things to know are how cold the night will get, and how much cold each plant can take.
Published October 22, 2026 · Cite this page
Light frost and hard freeze are different events
Frost is ice on the surface of leaves. UC Master Gardeners in San Joaquin County note it can form when the air is 33–36 °F, and usually does only minor damage. A freeze is ice forming inside plant cells, at 32 °F or below; the same source says a short freeze may not do permanent harm even to tender plants, but a freeze lasting 24 hours or more is dangerous.
Oregon State University Extension sets the line between light and heavy frost at 28 °F: semi-hardy crops tolerate light frost of 28–32 °F, and hardy crops withstand heavy frost below 28 °F. UC Master Gardeners in Sacramento County add the weather behind each: a light frost comes on a clear, dry night and the day usually warms; a hard freeze is an Arctic front or inversion that can last days or weeks.
So the question on any cold evening is not only "will it frost?" but how far below 32 °F the night will go, and for how long.
Which vegetables survive what
| Group | Takes | Crops |
|---|---|---|
| Hardy | Heavy frost, below 28 °F | spinach, onions, garlic, leeks, rhubarb, rutabaga, broccoli, kohlrabi, kale, cabbage, collards, Brussels sprouts, corn salad, arugula, fava beans, radish, mustard greens, Austrian winter pea and turnip |
| Semi-hardy | Light frost, 28–32 °F | beets, carrots, parsnip, lettuce, chard, garden pea (before flowering), Chinese cabbage, endive, radicchio, cauliflower, parsley and celery |
| Tender | Killed by the first frost | bush beans, zucchini, summer squash, okra, cucumbers, southern peas, sweet corn, winter squash, pumpkins, tomatoes, peppers and sweet potato |
The hardy and semi-hardy lists are Oregon State University Extension's. Cold slows hardy crops rather than killing them, it says; in semi-hardy root crops like beets, carrots and parsnips the tops may die back while the roots survive, and headed crops like radicchio may lose outer leaves over a sound centre. The tender row is this site's own crop table, the same classification the What can I still plant? tool uses — a judgement, not a measurement. Varieties differ.
When to have your covers ready, by zone
The first fall freeze (32 °F) at the median NOAA station in each zone, and the earlier date that station passes in 9 years of 10.
| Zone | Covers ready by | First fall freeze, median | Middle half of the zone | Stations |
|---|---|---|---|---|
| Zone 3 | September 7 | September 22 | September 15 – September 26 | 98 |
| Zone 4 | September 15 | September 28 | September 23 – October 2 | 698 |
| Zone 5 | September 22 | October 5 | September 29 – October 11 | 1,307 |
| Zone 6 | October 2 | October 17 | October 8 – October 22 | 1,728 |
| Zone 7 | October 12 | October 26 | October 20 – November 1 | 1,428 |
| Zone 8 | October 26 | November 9 | November 3 – November 14 | 1,253 |
| Zone 9 | November 11 | December 1 | November 21 – December 13 | 736 |
| Zone 10 | December 9 | January 1 | December 23 – January 9 | 234 |
⚠️ "Covers ready by" is the median station's 10% date: NOAA's normals put the first 32 °F freeze after it in about 9 years of 10. Half the stations in each zone freeze earlier than the median, and these are zone-wide figures. For your own station, enter your ZIP code in What can I still plant? — it gives the date, the station and its distance.
How to protect plants on a frost night
- Watch the forecast low and how long it lasts. UC Master Gardeners in Sacramento County: note how low temperatures will go and for how long. Find your garden's cold spots with a thermometer; cold air settles in low ground.
- Water during the day. Moist soil holds heat better than dry soil, and dry plants are more easily damaged (Sacramento County).
- Move pots. Bring containers next to the house or under a patio cover, especially on the south side, and cluster them together.
- Cover before sunset. Frost cloth, sheets or blankets, reaching the ground so the heat the soil gives off at night is held in. Frost cloth gives 4° to 8° of protection depending on weight, lets some light and air through, and can stay on for a few days (Sacramento County). Stakes or hoops keep heavier covers off the leaves.
- Add heat for the worst nights. Sacramento County's Master Gardeners use old incandescent C7 or C9 holiday lights — not LEDs, which give off no heat — under the cover, for up to 4° of protection. Use outdoor-rated, grounded cords.
- Uncover in the morning once it is sunny and above freezing, so the soil can take up heat for the next night.
For a longer season, Oregon State suggests high or low tunnels of hoops and clear plastic, row covers or cloches, and notes that lasting snow cover insulates the soil.
Citrus and other subtropicals
UC Master Gardeners in Sacramento County publish the temperatures at which citrus trees are damaged: limes at 29 °F, lemons and grapefruit at 26 °F, Meyer lemon at 22 °F, oranges and mandarins at 21 °F and kumquat at 19 °F. Fruit can be damaged below 27 °F depending on ripeness. At their Fair Oaks Horticulture Center they turn on lights below 32 °F, cover young trees and lemons and limes below 29 °F, and cover everything below 27 °F.
San Joaquin County's Master Gardeners add that frost damage to tender citrus is likely if it stays at 29 °F or below for 30 minutes or longer, and that subtropicals should not be fertilized or pruned after early summer, so new growth has time to harden. Wrap the trunks of tender trees before a hard freeze (Sacramento County).
Your ZIP code
Your ZIP, your answer
Enter your ZIP code to see your USDA zone, when to have frost covers ready in that zone, read from the table on this page, and the freeze dates of the NOAA station nearest you.
Your answer appears here.
Full details for any ZIP code: ZIP code zone finder.
Common mistakes
- Plastic touching the leaves. It can trap moisture and damage the plant (San Joaquin County).
- A cover that stops short of the ground. San Joaquin County: it must reach the ground to keep cold air from seeping in.
- Covering late. Sacramento County says cover before sunset, to capture the heat the ground gives off at night.
- Leaving covers on through a sunny day. The soil cannot recharge for the next night.
- LED lights for heat. They give off none.
- Dry soil going into a freeze. Water the day before.
- Pruning frost damage straight away. San Joaquin County says wait several months to let the plant recover.
- Trusting your zone for frost timing. A hardiness zone is the coldest winter night, not when fall frost comes. The evidence is in your hardiness zone is not a planting date.
Related
- What can I still plant? — every crop judged against your own first fall freeze
- What survives my winter? — fruit trees, berries and perennial vegetables by zone
- USDA hardiness zone by ZIP code
- Frost dates by state
- Planting dates by zone for annual herbs and flowers: Basil, Cilantro, Parsley, Dill, Sunflower and Zinnia.
- Planting dates by zone for vegetables: Tomato, Pepper, Lettuce, Carrot, Potato, Onion, Cucumber, Zucchini and Squash, Bean, Pea, Spinach, Broccoli, Kale, Cabbage, Cauliflower, Beet, Radish and Sweet Corn.
- When to Plant Garlic by Zone: Fall Dates for Zones 3–10
- When to Plant Trees and Shrubs in Fall, by Zone
Questions
- What temperature is a hard freeze?
- Oregon State University Extension groups vegetables by it: crops that withstand heavy frost take temperatures below 28 °F, and semi-hardy crops tolerate light frost of 28–32 °F. UC Master Gardeners in Sacramento County describe a hard freeze as an Arctic front or temperature inversion that can last for days or weeks, against a local frost on a clear, dry night that usually warms during the day.
- Can there be frost above 32 °F?
- Yes. UC Master Gardeners in San Joaquin County say ice crystals can form on leaf surfaces when the air is in the 33–36 °F range. That frost usually causes only minor damage, though flowers and buds are more sensitive.
- Can I cover plants with plastic?
- Not touching the leaves. UC Master Gardeners in San Joaquin County do not recommend plastic covers because they can trap moisture and damage the plant. Use frost cloth, sheets or blankets; Sacramento County’s Master Gardeners say frost cloth gives 4° to 8° of protection depending on its weight and can lie directly on foliage.
- When should I put frost covers on and take them off?
- Cover before sunset, to hold in the heat the ground gives off at night, and make sure the cover reaches the ground. Remove it during the day if it is sunny and above freezing so the soil can absorb heat again. Both are UC Master Gardener guidance from Sacramento and San Joaquin counties.
- Should I water before a frost?
- Yes. UC Master Gardeners in Sacramento County say dry plants are more easily damaged and moist soil holds heat better than dry soil.
- Should I prune frost-damaged plants right away?
- No. UC Master Gardeners in San Joaquin County advise waiting several months to let the plant recover before pruning damaged parts.
Sources
Dates on this page are computed when the site is built from NOAA's 1991–2020 Climate Normals (first fall freeze at 32 °F, for every station the ZIP codes in each zone cite). Zones are the USDA Plant Hardiness Zone Map, 2023. Every rule — how long before or after a freeze to plant, how deep, how much — is quoted from the extension publications below, each of which writes for its own area.
- Oregon State University Extension Service, Plant cold-hardy vegetables now for a spring harvest (Published Sep 2025, reviewed 2025). Written for: Oregon (USDA zones 5b–9b).
- UC Master Gardeners of Sacramento County (University of California), Frost and Protection for Sensitive Plants . Written for: Sacramento County, California (USDA zone 9b).
- UC Master Gardeners, San Joaquin County (University of California), Protecting Plants from Extreme Cold , by Constance Starner . Written for: San Joaquin County, California (USDA zone 9b).
Nothing here replaces your own state's cooperative extension service, which knows your county. Where this site's numbers come from.
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