Frost can form on a night the forecast low is a few degrees above freezing.
As recorded: On a night whose forecast low is 36 F (2 C) or colder, tender and very tender crops can be frosted, because the forecast is the air's temperature a couple of meters up and, on a clear, still night, the plants run colder. Half-hardy and hardy crops take a light freeze, and can be hurt at 28 F (-2 C) or colder.
Promising A person has checked it against its sources. If you skip this, the planting fails. The planner warns you.
How it works
On a clear, still night leaves lose heat to the open sky, so a leaf can fall to freezing and ice can form in it while the thermometer, a couple of meters up, still reads a few degrees above.
What to do about it
The evening before, cover the crops at risk with a sheet or frost cloth, never plastic, reaching the ground and loose at the bottom so the soil's warmth can rise under it, and take it off in the morning. A cover helps least in a long, hard freeze.
The evidence
Evidence: checked against 7 sources in the research-based literature.
The 7 sources
Issued up to 24 hours when temperature is forecast to be between 32° and 36° on nights with calm winds
… temperatures below 28°F for an extended period of time can kill most types of commercial crops and residential plants.
Frost occurs when air is warmer than 32°F but plant tissues drop to 32°F or below because plants radiate (lose) heat into the atmosphere, especially during cool, clear nights.
Tender crops will be damaged or destroyed by a light frost
Crops from more-temperate regions can withstand light freezes (28 to 32 degrees Fahrenheit):
A light frost (32°F) can damage tender plants, while a hard frost (28°F or colder) will kill most warm-season crops.
Radiation frosts occur on calm clear nights that lack cloud cover to hold in heat.
Where this rule stops
It reads the forecast low for a spot about 10 km across, so a garden in low ground, where cold air pools on still nights, can frost on a night the forecast says it won't: look to your low ground first. A forecast can be off by a few degrees, and it cannot say whether the night will be clear and still, so from 33 to 36 °F it says frost is possible, not certain. Guides sort crops into tiers differently: the tiers are Colorado State's, and South Dakota State puts some of that hardy tier (lettuce, spinach, peas) among the crops that take only a light freeze, so hardy crops share half-hardy's 28 °F line, though the cabbage family usually takes a few degrees more.
Crops in the ground, in open or roofed beds; a bed indoors is left out. The lines are the weather service's: a frost advisory for forecast lows of 33 to 36 °F, a freeze at 32 °F or under, and 28 °F, below which, its safety page says, a long freeze "can kill most types of commercial crops and residential plants". 28 °F is also where UMN's hard frost begins and South Dakota State's light freeze ends. It sets no date: the Calendar's frost dates stay on the normals and a forecast never moves them. The lines ride the trigger in Fahrenheit, as the sources give them, and are compared with the forecast's Celsius converted.
Plants it applies to
- Amaranth
- American persimmon
- Anise hyssop
- Apple
- Apricot
- Artichoke
- Arugula
- Asparagus
- Autumn olive
- Avocado
- Banana
- Basil
- Beet
- Black chokeberry
- Blackberry
- Blueberry
- Borage
- Buckwheat
- Butternut
- Cabbage
- Calendula
- Carrot
- Celery
- Cereal rye
- Chamomile
- Chives
- Cilantro
- Common bean
- Corn
- Cosmos
- Cowpea
- Crimson clover
- Crocus
- Cucumber
- Currant
- Daffodil
- Dill
- Edamame
- Eggplant
- Elderberry
- Endive
- False indigo bush
- Fava bean
- Feijoa
- Fennel
- Fig
- French marigold
- French tarragon
- Garlic
- Grape
- Hairy vetch
- Hardy kiwi
- Honeyberry
- Horseradish
- Kiwifruit
- Lacy phacelia
- Lavender
- Leek
- Lemon
- Lemon balm
… and 68 more.
How this rule has changed
Every change is dated, newest first. The full record, with each reason, is in the data download.
- A person read the sources and confirmed it
- Details updated
- Added to the rules
Cite this page
Milpa Gardens. "Frost can form on a night the forecast low is a few degrees above freezing." Evidence grade: Promising. https://milpa.garden/rules/night-whose-forecast-low-36-f-2/
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How we grade evidence
- Well established
- The rule names its mechanism and is settled, by sources a person has read and checked or by plain arithmetic and geometry.
- Promising
- The mechanism is named and the literature backs it. A few questions are still open.
- A good hunch
- Plausible by its mechanism, but field studies are thin, so treat it as a lean.
- Contested
- The evidence points both ways, or no study has tested it, so the planner does not enforce it.
- Refuted
- The claim did not survive its sources. It is kept so the folklore can be answered.