Frost alerts · FrostCall
What temperature should you cover plants for frost?
Cover tender plants such as tomatoes, peppers and beans when the forecast low is about 36°F (2°C) or colder. Frost can coat leaves at that air temperature on clear, calm nights, and these crops are damaged below 32°F (0°C). Below about 28°F (−2°C), covers often fail, so harvest or bring plants indoors.
Why plants can get frost when the forecast says 35°F
Frost and freezing air are not the same thing. The US National Weather Service defines frost as thin ice crystals that form on the ground and other surfaces when the temperature of those surfaces falls below 32°F. Because frost is mostly the result of radiational cooling, the Weather Service notes, it frequently occurs when the thermometer reads in the mid-30s.
The gap comes from where temperature is measured. Forecast services such as Open-Meteo report air temperature at 2 metres above the ground, while leaves and soil lose heat straight to a clear sky. Iowa State University Extension puts it plainly: frost can form even when air temperatures are slightly above freezing, because the plant surface itself cools to the freezing point.
Some nights are worse than others. In an Iowa State Extension article, horticulturist Sherry Rindels lists cool air, clear skies and light or calm winds as the conditions frost needs. Cold air is heavier than warm air, so it settles at the bottom of slopes and forms frost pockets in low spots, which is why one corner of a garden can be white while another stays green.
What frost advisories, freeze warnings and hard freezes mean
The National Weather Service issues a frost advisory during the growing season when widespread frost is expected over a large area, with surface temperatures usually in the mid-30s Fahrenheit. Iowa State University Extension gives the working range as forecast lows between 33 and 36°F.
A freeze means the surface air temperature is expected to be 32°F or below over a wide area for a meaningful length of time. A freeze watch says significant freezing temperatures are possible but the timing or location is still uncertain. A freeze warning is issued during the growing season when temperatures are expected to drop below freezing over a large area for an extended period, whether or not frost forms.
A hard freeze, in the Weather Service's definition, is surface air at 28°F or lower for an extended period, killing most types of vegetation. All of these products describe a whole forecast area. Utah State University Extension points out that urban and suburban yards run slightly warmer than the surrounding land, while low ground where cold air drains has a shorter season, so your garden can run warmer or colder than the headline number.
Which plants need covering, and at what temperature
Tender, warm-season crops have no frost tolerance. South Dakota State University Extension lists beans, sweet corn, potatoes, peppers, eggplant, cucumbers, summer squash, melons and tomatoes among the vegetables that should be protected or harvested before even a light frost. Iowa State Extension says tomatoes, peppers and green beans do not tolerate temperatures below 32°F and can be damaged by frost below 36°F. Basil is more sensitive still: SDSU notes its leaves can turn black when temperatures dip into the 40s.
Crops that take a light frost can usually ride out a night just under freezing. SDSU puts beets, carrots, lettuce, onions, garlic, chives, parsley, peas, radishes, rhubarb, spinach and Swiss chard in the group that withstands light freezes of 28 to 32°F. Iowa State adds that radishes, lettuce, chard and onions may show minor damage below 32°F but usually recover within 7 to 14 days.
Hardy crops go lower. SDSU lists cabbage, Chinese cabbage, kale, kohlrabi, turnips and broccoli as tolerating moderate freezes of 24 to 28°F, with cauliflower at 26°F, and notes that Brussels sprouts are often best after a light freeze. Iowa State says the hardiest cool-season vegetables, such as spinach, kale and cabbage, typically take only minor damage when temperatures drop below 28 to 26°F.
When covering stops working
Covers work by holding in the heat the soil gives off at night. Iowa State University Extension says sheets, blankets, towels, tarps, frost fabric and row cover all help keep frost from forming on leaves, but once temperatures drop below about 28°F, covering is typically no longer effective. Damage can also appear above 28°F when below-freezing temperatures last five hours or more.
Purpose-made row cover buys a few degrees. The University of Maryland Extension rates lightweight row cover at about 2°F of frost protection, medium weight at 4 to 6°F and heavyweight at up to 8°F. That is enough for a light frost, not for a hard freeze.
When the forecast heads into the mid-20s, move what you can and pick what you can't. Bringing plants indoors is the easiest protection of all, according to Iowa State. For tomatoes, SDSU says fruit picked at the mature green stage, when it has turned from deep green toward red, will keep ripening off the vine.
Common mistakes with frost and covers
Waiting for the forecast to say 32°F. Frost damage on tender crops starts above freezing air temperature. Iowa State advises taking action once forecast lows dip to the mid-30s.
Trusting the regional number for a low spot. Frost pockets at the bottom of a slope get colder than the forecast for the area. If your vegetable bed sits in a dip, treat a 36°F forecast as a frost night.
Covering plants that don't need it. Iowa State says protecting perennials, woody shrubs, roses and small trees in autumn is neither necessary nor beneficial, since they need cold to go dormant and most handle temperatures at or just below freezing. Save the sheets for vegetables, annuals and tropical plants.
Forgetting to look. Iowa State notes that a below-freezing night in late September or early October is often followed by a week or more of warmer weather, which makes the one cold night easy to miss, and suggests watching forecast lows from mid to late September onward. If you would rather not check every evening, FrostCall compares tonight's forecast low with the threshold for each crop you grow and sends one line at 8 PM only when a crop is at risk, such as "Tonight −2°C — cover the tomatoes."
Frequently asked questions
Will plants freeze at 35 degrees?
They can get frost. The National Weather Service notes that frost often forms when the thermometer reads in the mid-30s, because leaves and soil cool below the air. Iowa State University Extension says tender crops like tomatoes and peppers can be damaged by frost below about 36°F.
Do I need to cover plants at 40 degrees?
Not for frost in most cases. Some warm-weather plants still suffer, though: SDSU Extension notes that crops from warmer climates may begin to struggle below 50°F, and basil leaves can turn black in the 40s.
What is the difference between a frost advisory and a freeze warning?
A frost advisory means widespread frost is expected with temperatures usually in the mid-30s. A freeze warning means temperatures are expected to fall below 32°F over a large area for an extended time, whether or not frost forms. Both come from the National Weather Service.
Can lettuce and kale survive a frost?
Usually. SDSU Extension lists lettuce among crops that withstand light freezes of 28 to 32°F, and kale among those that tolerate 24 to 28°F. Iowa State says lettuce with minor damage usually recovers within one to two weeks.
Is it worth covering plants when it will be 28 degrees?
Covers may not be enough. Iowa State University Extension says covering is typically no longer effective once temperatures fall below about 28°F, which the National Weather Service calls a hard freeze. Bring pots indoors and harvest tender crops instead.