Frost Date Calculator
First & last frost dates for your hardiness zone.
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Frost Risk Calculator
Read a forecast temperature against your crop's kill point for a damage verdict, frost severity class and cover recommendations.
Frost Risk Calculator
Chlorova · updated for 2026
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Kill temperatures describe roughly a quarter-hour of exposure, so a two-hour plunge at 30 does far more damage than a five-minute brush at 31, and wind, humidity and how fast the morning warps up all rewrite the outcome. Forecasts report screen height, while cold air drains and pools several feet lower, so the frost hollow at the bottom of your slope sees numbers colder than this page ever shows. Uncovered plants against a still, clear-sky radiation frost behave worst of all, and the verdict bands are triage guidance, not a guarantee either way.
A frost forecast is only useful once you translate it into your plants' currency, and that currency is degrees past a kill point nobody quite agrees on. This calculator takes the predicted overnight low, subtracts the threshold for the crop you are actually standing in front of, and returns the margin, a damage verdict, the severity class of the night and a cover strategy that fits the number. It is the four-second decision between running inside with relief and losing the whole back tomato bed before breakfast.
Reading the Night in Steps
- Choose the crop class you are standing out next to tonight.
- Enter the overnight forecast low in plain Fahrenheit.
- Note the kill point printed for that specific crop class.
- Read the margin: degrees above or below that kill point.
- Check the crop risk verdict band as your first answer.
- Compare it against the frost severity class of the air.
- Follow the cover strategy line for tonight's actual job.
- Walk the beds at dawn and confirm what the margin predicted.
- Re-run with the updated forecast next afternoon if the warning holds.
Margins, Kill Points and Frost Classes
The core line is a subtraction: forecast low minus the crop's kill point. Tomatoes and peppers die near 32, squash vines a degree lower, hardy greens down around 26, and most fruit tree blossom candles out near 28 in full bloom. A positive margin buys degrees of safety, a zero margin is a cover-everything night, and three degrees below means the plants are already lost wherever fabric never touched them.
The second band ignores your crop entirely and classifies the air itself, following the standard light, moderate and severe frost cutoffs at 32, 28 and 24. The two bands disagree sometimes on purpose, because a 29-degree night is a light frost in the weather book yet a death sentence for an uncovered pepper, and reading both is exactly how gardeners learn which fights to cover for.
Worked Example: October Warning on Tomatoes
- The overnight forecast reads a 29-degree bottom.
- Pick tomato from the picker, whose kill point sits at 32 degrees.
- The margin is 29 minus 32, so three degrees below the kill.
- The crop band reads below the kill point tonight.
- The severity class calls the same air a light frost.
- That disagreement is the page's whole teaching moment.
- Uncovered fruit is lost; fabric alone buys minutes at best.
- A harvest rescue beats heroics at this three-degree margin.
- Greens in the next row see plus four degrees of safety.
- Cover them lightly and skip the tomatoes entirely tonight.
- Same cold night, two completely different evening chores.
Kill Points and Frost Classes
Approximate quarter-hour lethal temperatures for the crop classes here, alongside the meteorological frost severity cutoffs the second band applies. Blossom tolerance tightens as flowers open: tight buds ride several degrees lower than full bloom or small fruit.
| Reading | Temperature | What it means |
|---|---|---|
| Tender crops kill | 32°F | Tomato, pepper, squash done if naked |
| Squash vines kill | 31°F | One degree lower, same obituary |
| Blossom damage | 28°F | Full bloom fruit sets lost |
| Hardy greens kill | 26°F | Lettuce and spinach covered survive lower |
| Light frost | 30 to 32°F | Tender plants in danger |
| Moderate frost | 26 to 29°F | Half-hardy crops killed |
| Severe freeze | under 26°F | Perennial roots in shallow pots at risk |
Key Terms
- Kill point
- The temperature at sustained exposure ends a crop, quoted here per class.
- Advection frost
- A freeze brought in by moving cold air, wide-area and hard to fight.
- Radiation frost
- A clear still night when ground heat escapes skyward, the one covers can beat.
- Frost hollow
- A cold-air drainage pocket where nights run many degrees colder than the forecast.
- Harvest rescue
- Picking everything mature and maturing before a loss-margin night, tomatoes ripen off the vine.
How Growers Actually Fight Frost
- Cover matters more than forecast: one screen-height number means a survivable brush in a warm breezy valley and a kill in a still, drained hollow, so growers read sky and wind before the thermometer.
- Wet soil is a battery, because moist ground stores daytime heat and releases it slowly through a clear night, which is why extension says to irrigate the beds before a frost warning, not after.
- Fabric must not touch the foliage in a hard frost: a row cover laid on tomato leaves freezes to them and does the kill itself, so give covers hoops or a taut drape.
- The morning after is a second decision, since frosted leaves cut off water uptake while the sun refreezes and thaws them in cycles, so shade recovery beats instant full sun.
- Sprinklers can protect blossom through the physics of latent heat, but only run continuously from below freezing upward, and one stop mid-freeze glazes flowers worse than doing nothing.
Common Mistakes & Fixes
- Covering plastic-direct onto foliage and adding a contact freeze to the weather damage.
- Believing the forecast number over your own slope, because cold air lies at the bottom of a grade.
- Uncovering at first light and re-killing recovering leaves in a refreeze.
- Treating every 32-degree reading as a kill for greens that could take 26.
- Wiring up sprinklers intermittently on a night you should have just covered.
- Skipping the harvest rescue three degrees below the kill point to save the plants that are already gone.
When to Use This (and When Not To)
Open it the afternoon of any frost or freeze warning during shoulder season, when one subtraction decides the evening's chores. It handles single nights, not seasons, so for fall and spring date planning use the frost-date calculator, and let the hoop-structure sizing tool make next year's cover permanent rather than a nightly scramble.
Frost Risk Calculator: Frequently Asked Questions
Is 35 degrees too cold for tomatoes?
Not fatally, but it is the last comfortable night, because sustained time under 32 kills the plant and 35 with clear still air can still mean a frost on the foliage itself. Cover or harvest-rescue when the low lands in the 30s.
Does a frost always kill plants at the kill point?
No, exposure time rewrites everything: five minutes at 30 may brush a tomato while two hours at 31 finishes it. Wind direction, humidity and how fast morning thaw arrives all move the effective kill point a degree or three.
Will row covers actually work?
Floating row covers buy roughly two to four degrees on a clear still radiation night, which is precisely why a plus-three margin band reads cutting it fine rather than safe. Against a wind-driven advection freeze they help far less, since they only slow the air exchange.
Can I save tomatoes that frosted black?
The fruit you harvest green will ripen indoors, so cut and rescue mature-green tomatoes the morning you find blackened foliage. The plants themselves rarely recover a true kill, and leaving spent vines only feeds next spring's disease.
Do frost blankets work better than plastic?
Woven fabric blankets breathe, hold a dead-air layer and can touch soil without glazing the plant, while plastic sheeting transfers cold directly and needs hoops. Fabric plus a mulched, watered bed outperforms bare plastic in almost every garden scenario.
Why is my low spot always colder?
Because chilled night air is dense and drains downhill like water, pooling several degrees below the surrounding grade in hollows and behind fences. The forecast samples air at four feet in an open shed; your bottom of the slope is a different thermometer.
Sources & Further Reading
- Frost Protection for Vegetables — University of Minnesota Extension. Basis for kill temperatures and row-cover degree gains.
- Frost, Freeze and Fruit Tree Blossom — Michigan State University Extension. Source of blossom critical temperatures and bloom-stage tolerance.
- Understanding Radiation and Advection Frosts — National Weather Service. Framework behind the severity bands and microclimate cautions.
Chlorova Gardening Team
Builds, checks and documents every tool we publish
Chlorova editorial review
Checked against extension-service publications
Formula verified against published references · Updated