Frost Protection: When and How to Cover Your Plants

Frost is the hard boundary of the growing season, and it rarely arrives politely on your zone's average date. A clear, calm night in late September can drop 10 degrees below forecast and take out every tomato, pepper, and basil plant you own. The good news: with an hour of warning and some fabric, most frost events are completely survivable.

Reading the Risk: When to Actually Worry

• Forecast low of 38°F or below: pay attention. Official temperatures are measured at 5 feet; ground level can be 4-5 degrees colder.
• Clear skies and calm wind: the classic radiative frost setup. Clouds act as a blanket; wind mixes warm air down. A clear, still 38°F night is more dangerous than a windy, cloudy 34°F one.
• Low dew point: dry air cools faster. If the dew point is well below the forecast low, frost risk rises.

Rule of thumb: forecast under 40°F, clear and calm, in your shoulder season? Cover tender plants. The cost of covering unnecessarily is 20 minutes; the cost of not covering is the season.

Know Your Plants' Actual Tolerance

Killed by the first light frost (cover or harvest)

Tomatoes, peppers, eggplant, basil, cucumbers, squash, beans, melons.

Tolerates light frost (28-32°F), often improves

Kale, Brussels sprouts, cabbage, carrots, beets, lettuce (briefly), chard, broccoli. Frost converts starches to sugars in many brassicas — post-frost kale is genuinely sweeter.

Shrugs off hard freezes

Garlic, spinach (established), mâche, parsnips. Plant and stop worrying.

What Actually Works as a Cover

1. Floating row cover (best): purpose-made spun fabric, gives 2-8°F of protection depending on weight, breathes, and can stay on for days. Worth owning if you garden in a frost-prone area.
2. Old bedsheets and light blankets (good): 2-4°F of protection. Must come off in the morning.
3. Cardboard boxes and buckets (good for individual plants): trap ground heat well; weigh them down.
4. Cold frames or cloches (excellent, more setup): effectively move the plant a zone south.

What doesn't work: plastic sheeting touching foliage. Plastic transfers cold directly to any leaf it touches, freezing exactly the tissue you're protecting. If plastic is all you have, tent it over stakes so it touches nothing, and remove it at sunrise before the greenhouse effect cooks the plant.

The Technique: It's About Ground Heat, Not Wrapping

A frost cover doesn't warm the plant — it traps heat radiating up from the soil. This changes how you cover:

• Cover in late afternoon, while the ground still holds daytime warmth, not at 10pm after the heat has escaped.
• Drape covers all the way to the ground on every side and pin or weight the edges. A cover with open sides loses the heat pocket entirely.
• Don't cinch fabric around the stem like a lollipop — you're cutting the plant off from the ground heat that's supposed to save it.
• Water the soil lightly in the morning before a frost night: moist soil holds significantly more heat than dry soil.

The Morning After

Remove or vent covers once temperatures climb above freezing — trapped heat and humidity under a cover on a sunny morning stresses plants and invites fungal problems. If a plant did get frosted, don't prune the damage immediately; wait a few days to see what's actually dead, since blackened outer leaves often shield living tissue underneath.

The Shoulder-Season Payoff

Growers who cover well typically extend their season 3-5 weeks past their neighbors — often the difference between green tomatoes composted in October and ripe ones picked into November. Frost protection isn't fussy perfectionism; it's the cheapest yield increase available.