Frost, freeze and hard freeze: what the words mean

These get used loosely, but they mean different things for your plants.

UC Integrated Pest Management defines frost as what happens when plant tissue itself hits 32°F (0°C) while the air around it is a little warmer. A freeze is when the air temperature is at or below 32°F. Frost can happen with air readings in the mid-30s. That is why the National Weather Service issues a Frost Advisory for forecast lows of roughly 33 to 36°F on clear, calm nights, and a Freeze Warning when widespread freezing is expected.

A hard freeze is colder again. The NWS doesn’t use one number everywhere, because local offices set their own definitions. Around 28°F is common in the examples we found, and lower in some southern offices. What matters for you is that a light frost only bites the most tender plants, while a hard freeze kills a much wider range.

Weather stations measure air temperature a few feet off the ground, and the ground is often colder. So the reading on the forecast can be a couple of degrees kinder than what your lettuce sees.

When to act: reading the forecast

The average first-frost date for your area is a guide, not a promise. Purdue Extension describes the average date as a 50% chance of frost on or after that day, and notes that good growing weather often follows the first frost. Nebraska Extension says much the same. So you can’t relax on the day the calendar says it’s safe, and you don’t need to panic on the day it says it isn’t.

Iowa State Extension suggests keeping an eye on forecasts from mid to late September, and acting when lows are forecast in the mid-30s°F. That fits with the NWS frost-advisory range. Check the forecast each afternoon through the fall. Two things raise the risk on a given night:

• A clear sky and no wind. Heat escapes upward, and the coldest air settles.
• A low spot. University of California guidance notes that cold air pools in low areas, so the bottom of a slope or a hollow can be several degrees colder than higher ground nearby.

A second kind of cold night is different. UC Master Gardeners call it an advective freeze: a cold air mass moves in, and wind and cloud don’t help. Covers give little protection against this kind, so harvest and move pots.

An outdoor thermometer with a remote sensor makes this less of a guessing game. You can read the overnight low from the kitchen and learn how your own garden compares with the forecast.

Tender or hardy? Sort your garden first

You can’t cover everything, and you don’t have to. Delaware Cooperative Extension notes that light and moderate frosts do not harm cole crops, lettuce or spinach, and that peppers tolerate frost a bit better than tomatoes. Montana State University Extension lists tomatoes, peppers, cucumbers, snap beans and summer squash as having no frost tolerance at all, while carrots and lettuce are hardy.

Walk the garden and put each plant into one of these groups.

Harvest before the frost

For the vegetable garden, harvesting is often a better frost protection than covering. Virginia Cooperative Extension’s advice is to pick all the ripe tender crops when a frost warning is issued. If the warning is mild, with lows expected to stay no lower than about 30°F, you can cover tender plants that still hold immature fruit and try to buy them more time.

• Tomatoes: University of Nebraska–Lincoln Extension says green tomatoes picked before frost can be stored at 50 to 55°F and will ripen over several weeks. Fruit picked after a frost will not ripen or keep well.
• Peppers: frosted peppers blacken and turn mushy (UNL). Pick them before the frost, ripe or not.
• Winter squash and pumpkins: pick before the frost if you plan to store them. If a frost has already hit, UNL says to harvest within a day or two and cook, freeze or can them rather than store them.
• Hardy crops: leave carrots, kale, cabbage and spinach in the ground. Farmers’ Almanac advice matches the extension view here: don’t strip the whole vegetable garden.

How to cover plants properly

If you’re protecting plants that are still producing, or plants you can’t bring inside, a cover traps the heat the soil gave off during the day. Iowa State Extension lists sheets, blankets, towels, tarps, frost fabric and row cover as options. The Old Farmer’s Almanac also mentions cotton sheets as a reliable household choice.

1. Water in the afternoon.Moist soil holds and gives off more heat than dry soil, according to Delaware Extension. A deep drink the day before a frost is a low-effort head start.
2. Cover in the late afternoon or evening.You want to trap the day’s warmth before it drains away, not cover a garden that has already cooled.
3. Take the cover to the ground.Drape or tuck the edges into the soil, or weigh them with bricks or stones, so warm air stays in and cold air stays out.
4. Keep fabric off the foliage.Hobby Farms notes that damage tends to show up where cover touches the leaves. Use stakes, tomato cages or hoops to hold it up.
5. Uncover once it warms up.Iowa State says to remove covers the next day when it is above freezing. Commercial frost cloth lets light and air through, so it can stay on longer. Plastic must come off, because the plants will overheat under it.

Which cover?

For a raised bed or a row, a lightweight frost blanket over hoops is the cleanest fix, and you can reuse it for years. Sheets and blankets work in an emergency but get heavy when wet. Agfabric’s listing for its 0.9 oz frost blanket states protection down to 28°F. That is the manufacturer’s claim, not an independent test, and it lines up with the point above that a cover can’t do much below that.

Pots and containers

Plants in pots are more exposed than plants in the ground. The RHS names pot plants and evergreens as being at particular risk in winter, and pots cool from every side while the roots have little soil to buffer them.

• Bring the tender ones in. The Farmers’ Almanac lists tropicals, geraniums, begonias, coleus, tender succulents and citrus as plants to move indoors before frost. A porch, garage or unheated room that stays above freezing is often enough for a night or two.
• Group what you can’t move. Put pots together against a wall of the house, where stored heat helps, then cover the group.
• Wrap big pots. Fleece, hessian or bubble wrap around the pot protects the roots. A fleece cover over the top protects the growth.
• Lift pots off cold ground. A couple of pot feet or a board underneath keeps them off the coldest surface.

Water, mulch and cold frames

Three quieter tricks help without any fabric.

• Water the soil. Moist soil stores more heat than dry soil. Garden City Harvest, a Montana State-affiliated program, advises soaking the soil before a freeze.
• Time your mulch. For vegetable beds you’re still using, UC Master Gardeners suggest raking mulch back from the plants so the soil can warm during the day. For dormant perennial and bulb beds, University of Minnesota advice, as reported by the Farmers’ Almanac, is to mulch after the soil begins to freeze, not before.
• Use cold frames and tunnels. Iowa State says to open them by day and close them before a freeze. They can overheat on a sunny afternoon.

If you want to extend the season for months rather than weeks, a greenhouse or a proper tunnel is the next step. Our backyard greenhouse ideas cover the options.

A first-frost timeline

The order matters more than the details. Here is how one cold snap plays out for the average garden. The dates depend on where you live, and the NWS describes the sequence in Tallahassee as first frost in early November, first freeze in late November, and first hard freeze in late December or early January. Yours will differ.

The morning after

Check the plants before you rip anything out. Some of what looks dead isn’t.

• Uncover once temperatures rise above freezing. Don’t leave plastic on in the sun.
• Leave frost-damaged woody plants alone. The University of Arizona’s advice is to wait until spring growth starts before pruning. The damaged leaves also help trap heat around the plant, and it takes time to see how far the damage really goes.
• Don’t fertilize or prune late. UC Master Gardeners advise against both after August, because they push soft new growth that frost will kill.
• Expect more good weeks. A first frost often arrives well before the real end of the season. Nebraska and Purdue extension both note that mild weather often follows, so keep your covers handy.
• Lift tender bulbs and tubers. The Farmers’ Almanac suggests lifting dahlias, cannas and gladiolus where winters are too cold for them to overwinter in the ground.

Eight frost-protection mistakes

1. Waiting for the frost warning to start.By then you’re racing daylight. Watch the forecast from mid to late September so covers and hoops are ready.
2. Covering plants that don’t need it.Lettuce, spinach, kale and cabbage handle light frost. Save your effort for the tomatoes, peppers and basil.
3. Letting the cover touch the leaves.Damage tends to show where fabric rests on foliage. Hold it up on stakes or hoops.
4. Leaving gaps at the ground.Warm air escapes through the bottom. Tuck or weigh the edges.
5. Leaving plastic on in the morning.Plants overheat under it once the sun comes up. Take it off as soon as it is above freezing.
6. Trusting a cover in a hard freeze.Below about 28°F a cover is unlikely to save tender plants. Harvest instead.
7. Picking tomatoes after the frost.Fruit picked after a frost won’t ripen or keep. Pick green tomatoes before it.
8. Pruning frost-damaged shrubs straight away.Wait for spring growth so you can see what’s truly dead.

A note for UK gardens

The advice above holds in Britain, with a few differences in wording and units.

• Fleece is the standard cover. The RHS says heavier horticultural fleece gives roughly 2°C of protection, and two layers can be used together. Gardeners’ World says a 17 g fleece protects to just below freezing, 30 g to about −3°C, and a 70 g fleece jacket to about −5°C.
• Wrap at the first warning. The RHS advises wrapping vulnerable plants when a frost is first forecast. It describes a severe frost as below −5°C (23°F), and suggests chicken-wire cages packed with straw and covered in fleece for the most exposed plants. Unwrap by May.
• Exposed sites and pots are at most risk. The RHS says hardy plants may need protection in cold or exposed sites, and that potted plants and evergreens are at particular risk.
• Buying covers. The Amazon links in this article go to Amazon.com. UK readers can search Amazon.co.uk for horticultural fleece, fleece jackets and hoop tunnels in the grades above.

FAQ

Your frost plan

Get ready before the first cold night and it becomes a twenty-minute job. Sort what to harvest, cover and bring inside, keep your covers where you can find them, and check the forecast each afternoon. On the night itself, water, cover in the evening and uncover in the morning. If a hard freeze is coming, harvest and move pots instead of fighting it. If you want to prepare the rest of the garden for winter, see our winter garden ideas, our month-by-month lawn care schedule, and the frost-covered plants that look better in winter. For fresh food once the garden is done, try indoor growing kits.

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