Will plants freeze at 40 degrees?

At 40°F the air alone cannot freeze plant tissue. Frost can still form on the leaves themselves. On a clear, still night the leaves radiate heat to the sky and cool below freezing while the official reading sits near 40°F. Tender crops can show damage on such a morning, so check your yard's real low.

Can frost occur at 40 degrees?

Frost can form at 40°F. The official low is read at about 5 feet in a shaded screen, while ground-level leaves lose heat to the open sky and cool faster. Frost forms on those surfaces while the screen still reads 38°F to 40°F.

At what temperature should I cover my plants for frost?

Cover tender crops when your yard's adjusted low approaches freezing. The forecast is read miles away and your yard runs colder or warmer by a measurable amount, so compare the plant's tolerance to your own low rather than the regional number.

Can plants survive one night of frost?

Hardy crops like kale and spinach survive many frosts; tender crops like tomatoes and basil may not survive one. The outcome depends on how low the leaves got and how long they stayed there. We cover this in more detail in our guide on one night of frost.

October 8, 2026

What's the frost temperature for plants in your yard

Frost forms at or below 32°F at the thermometer, and tender plants can take damage within a few degrees of that. The temperature that decides whether your plants make it is the low in your own yard, which runs colder or warmer than the regional forecast by a measurable amount. Match the plant's tolerance to that yard low, then cover the tender crops.

Can frost form at 40°F

Frost can form at 40°F. The official low is read at about 5 feet in a shaded screen. On a still, clear night the ground and your plant leaves radiate heat to the open sky and cool faster than the air that screen measures. Leaf surfaces can drop below freezing while the reading holds at 36°F to 40°F. So a morning the forecast called 38°F can still leave frost on your beds. The frost on the leaf is what damages the tissue.

Freeze thresholds by plant type

Plants fall into rough groups by how much cold their tissue tolerates. The boundaries blur, because a plant that has hardened off through cool weeks takes more cold than the same plant moved straight from a warm windowsill.

Tender crops. Tomatoes, peppers, cucumbers, squash, and basil take damage at the first light frost near 32°F, and a hard freeze several degrees lower usually kills them.
Half-hardy crops. Lettuce, celery, and strawberries shrug off a light frost and fail in a hard freeze.
Hardy crops. Kale, spinach, carrots, and garlic survive temperatures well below freezing and often improve in flavor after a frost.

A single degree number cannot settle the question on its own. How long the cold lasts, whether the night is windy or still, and how wet the soil is all shift the outcome for the same crop. Treat the thresholds as a starting point, not a guarantee.

Your yard's real low

The forecast low comes from a model or a station that can sit miles away, and your yard rarely matches it. On a still night the denser cold air sinks, flowing downhill until it collects in hollows and against walls. A sheltered bed stays warmer; a hollow runs colder. In our own garden the station learned a -7°F offset against the forecast, which pushed our published last frost date of April 28 out to an effective May 20. The forecast would have told us to plant tender crops three weeks too early.

Siltwell shifts both your frost alerts and your frost dates by this microclimate offset. On a free account you enter the offset yourself in settings, a negative number for a colder yard. With a connected station on Pro, Siltwell learns it from about 14 days of your own overlapping outdoor readings, so you do not have to guess the number. Either way, the low you act on is your yard's adjusted low. For how the same offset moves your planting dates, see why your last frost date is probably wrong.

Frost alerts run on every Siltwell garden, free and Pro: a Today banner, a push, and an email the afternoon before a frost is forecast. Pro adds the station-learned offset; on free you set the offset yourself.

Tonight, compare your yard's adjusted low to the tolerance of what is in the ground. If a tender bed is heading for freezing, cover it before sunset while the soil still holds its heat. Our guide on whether one night of frost kills your plants walks through which crops to save first.

Common questions.

Will plants freeze at 40 degrees?: At 40°F the air alone cannot freeze plant tissue. Frost can still form on the leaves themselves. On a clear, still night the leaves radiate heat to the sky and cool below freezing while the official reading sits near 40°F. Tender crops can show damage on such a morning, so check your yard's real low.
Can frost occur at 40 degrees?: Frost can form at 40°F. The official low is read at about 5 feet in a shaded screen, while ground-level leaves lose heat to the open sky and cool faster. Frost forms on those surfaces while the screen still reads 38°F to 40°F.
At what temperature should I cover my plants for frost?: Cover tender crops when your yard's adjusted low approaches freezing. The forecast is read miles away and your yard runs colder or warmer by a measurable amount, so compare the plant's tolerance to your own low rather than the regional number.
Can plants survive one night of frost?: Hardy crops like kale and spinach survive many frosts; tender crops like tomatoes and basil may not survive one. The outcome depends on how low the leaves got and how long they stayed there. We cover this in more detail in our guide on one night of frost.

Related guides

Free planting tools

When to plant shows the last date to sow each crop before your fall frost.
Microclimate checker shows how much colder or warmer gardens near you ran than the forecast low.
GDD estimator adds up a typical season's heat and shows which crops can ripen where you are.