October 25, 2026

How to save plants from frost when you can't cover everything

You have a frost alert at 5 p.m., a few hours of light, and more tender plants than covers. Save them in the order a frost would kill them. Read your adjusted low, sort your plants against it, move the pots, cover the beds with the most harvest left, pick what you can eat, and let the rest go. That order puts your sheets and your evening on the plants that need them.

Why the order matters

On a clear, still night the soil and leaves radiate heat to the sky. Leaf surfaces shed heat more quickly than the surrounding air cools. A leaf can ice over while a head-height thermometer still shows a reading above freezing. Ice forming in tender tissue tears the cell membranes, and the plant goes black and limp by morning. Hardy crops load their cells with sugars that lower the freezing point, so the same night barely marks them. Your covers do the most good on plants with no such defense.

1. Read the adjusted low

Siltwell runs its check on every garden daily at 17:00 local time, reading the forecast lows for the next two days. The alert runs on every plan, free and Pro. It fires when the adjusted low reaches your frost threshold, which defaults to 0°C (32°F). On the free plan, the adjustment is the manual offset you enter in Settings → Garden → Edit garden → Microclimate offset. Pro users who connect a station get a learned offset instead. Once about 14 days of overlapping readings exist, Siltwell sets the offset by comparing the station's outdoor air temperature against the forecast.

The offset changes which nights count. Our own station reads 7°F colder than the forecast. Our published first frost date is October 22, and after the learned offset it moves to September 30. The first fall frost we recorded came on October 3. Without a measured offset of your own, start from the range the microclimate checker gives. It sets the forecast model against readings from home stations near you.

2. Sort your plants against that low

Split the garden into three groups. The temperatures each crop tolerates decide where a plant lands.

• Tender. Tomatoes, peppers, cucumbers, zucchini, winter squash, and melons. They evolved in warm climates and carry no sugar defense, so frost on the leaf kills them.
• Damaged above ground. Lettuce, celery, potato tops, and strawberry flowers. Frost burns the exposed tissue while roots, tubers, and crowns below the soil survive.
• Hardy. Spinach, carrots, garlic, and onions. Sugars in their cells and the soil around their roots carry them through a light frost.

Suppose your adjusted low is 31°F tomorrow night. Every tender plant is at risk, the middle group needs a cover only if you want that harvest, and the hardy group needs nothing. Cross the hardy list off before you fetch a single sheet.

3. Move what moves

Pots lose heat from every side, so roots in a container freeze before roots in the ground. Carry them into a garage or shed, or line them up against the house wall. The wall gives back heat it stored during the day. Moving costs you no covers, so do it first.

4. Cover the tender plants with the most harvest left

Rank the tender group by what each plant still owes you. A tomato loaded with full-size green fruit outranks a zucchini that has fed you for weeks. Drape each cover to the ground and weigh the edges. The cover holds the heat rising from the soil, and an open bottom lets that heat escape. Water any dry bed before dark, because moist soil stores more heat and conducts it to the surface overnight. The frost protection methods guide compares materials, and this one covers what you can grab from the house.

5. Pick what you can't cover

Go through every uncovered tender plant with a basket. Take ripe fruit, near-ripe fruit, and anything of usable size. Full-size green tomatoes keep ripening indoors because the fruit makes its own ethylene. Peppers, cucumbers, and zucchini stop growing once picked, so take only what you'd eat. For a larger tomato haul, see freezing tomatoes before the first frost.

6. Let the rest go

Leave the plants that have little fruit left and were already slowing down. Shorter days and cooler nights had ended their season before the frost arrived. Note in your journal which plants you let go and what the morning showed. That record sets next year's order.

What can go wrong

• Wind. A cover holds heat only in still air. On a windy night a cold air mass moves through and carries the trapped warmth away.
• Low spots. On still nights the heavier cold air slides downhill and settles in whichever bed sits lowest. The adjusted low is one number for the whole garden, and it cannot tell you which bed sits in that pocket.
• A second cold night. The alert looks at tomorrow and the day after. Keep the covers by the door if both nights are cold.
• Late damage. Leaves can look fine at dawn and blacken by afternoon as thawed cells leak. Wait a day before you judge, and read whether one night of frost kills a plant.

If you're on the free plan with no offset set, run the microclimate checker tonight. Enter its median as your Microclimate offset in Settings, and the next 5 p.m. check will use it.