October 27, 2026

Cold damage on plants when nights stay above freezing

Peppers, cucumbers, melons, and basil can be hurt on nights that stay above freezing. Long hours of cool air stiffen the membranes in their cells, the cells start to leak, and growth stalls with no ice anywhere in the leaf. Gardeners call this chilling injury. A frost alert set to the default 0 °C stays quiet through those nights, so raise the threshold while tender crops are out and cover them on cool nights too.

What happens inside a chilled cell

Every plant cell is wrapped in a membrane built from fatty molecules. At warm temperatures those molecules stay loose and fluid. The proteins set into them pump water, sugars, and ions in and out of the cell. Crops that evolved in warm climates have membranes that firm up into a stiffer, gel-like state well above freezing. The pumps slow down. Potassium and other solutes leak out through gaps in the stiffened membrane.

The leaf's power systems fall out of step too. Chloroplasts keep catching light while the chilled enzymes that use that energy run slowly. The surplus energy forms reactive oxygen, which damages the leaf from inside. A bright, clear morning after a cold night makes this worse, because the leaf takes in full sun before it has warmed.

Roots suffer as well. Cold soil thickens water and slows uptake at the root, while the leaves keep losing moisture to the air. A chilled tomato can wilt in a wet bed.

Lettuce, spinach, peas, and garlic carry more unsaturated fats in their membranes. Those membranes stay fluid in the cold, so these crops grow through nights that set a pepper back for a week. Frost damage works by a different route, with ice forming between cells and drawing water out of them. We cover that in what happens inside a freezing plant.

How time and temperature add up

Chilling injury builds with exposure. A short dip before dawn does little. A night that sits cool from dusk to sunrise does more, and several such nights in a row stack their damage. Brief chilling often reverses once warm weather returns, because the membranes loosen again and the cell repairs itself. Long chilling kills cells, and that damage stays.

The temperature where chilling starts varies by crop, by plant age, and by how hardened off the plant is. We won't put one number on it. The figures we've seen differ from crop to crop and from study to study.

Which crops are sensitive

• Tomatoes, peppers, and eggplant. Seedlings stall and turn purple, and fruit set slows after cool nights.
• Cucumbers, melons, squash, and zucchini. Leaves bronze at the edges, and cucumber fruit develops sunken pits.
• Basil. Leaves blacken in patches, often the morning after one cool night.
• Sweet potatoes and beans. Growth stops until the soil warms.

In our own garden one bed sat at both ends of this scale. The ground bed held watermelon and red kuri squash, and on July 29 we sowed lettuce and spinach in it as well. The melon and squash take damage from cool nights. The greens beside them grow through those same nights.

What the damage looks like

Symptoms often appear a day or two after the cold night, usually once a sunny afternoon has stressed the injured tissue.

• Water-soaked patches. Spots on the leaf look dark and wet, then dry to bronze, brown, or papery tan.
• Purple undersides. Cold roots take up phosphorus slowly, and young tomato and pepper leaves turn purple underneath.
• Wilting in moist soil. Chilled roots can't keep up with the leaves, so the plant droops while the bed is wet.
• Stalled growth. The plant sits without new leaves for days after the cold spell ends.
• Pitted fruit. Peppers and cucumbers develop sunken spots, and rot gets in through them.

If you're deciding whether a plant will come back, our guide to cold shock recovery covers what to watch over the following week. If you think ice formed, check whether your yard froze.

When chilling nights reach our garden

A forecast describes the wider area. Cold, dense air drains into low ground on still nights, and bare soil radiates its heat to a clear sky, so single yards often run colder than the forecast grid. Our weather station has learned that our garden runs 7°F colder than the forecast. A night forecast as mild for the area can be a chilling night in our beds.

The same offset moves our calendar. Our published last frost date is April 28, and with the offset applied it lands on May 20. Our last spring frost in 2026 came on April 30. The weeks after a last frost carry the longest chilling nights, because the soil is still cool and has little stored heat to give back after sunset. We set our tomatoes out on May 18, two days ahead of our adjusted date. The peppers, jalapeños, and cucumbers went in on May 22, the watermelon on May 28, and the squash on June 1.

Fall runs the same way in reverse. Our published first frost is October 22, the offset pulls it to September 30, and our first recorded fall frost in 2025 came on October 3. Chilling nights arrive ahead of any of those dates, while squash and peppers are still ripening fruit. To see how far your own lows sit from the forecast, read frost temperature and your real low or run the microclimate checker.

Why the default frost alert stays quiet

Siltwell's frost alert runs for every garden, free and Pro. At 17:00 local time it reads the forecast low for tomorrow and the day after, applies your offset, and fires when either low is at or below your garden's threshold. The default threshold is 0 °C. The alert arrives as a banner on Today, a push, and an email.

A chilling night stays above 0 °C, so the default never fires for it. The garden owner can move the threshold anywhere from −20 to 20 °C. The threshold is one number for the whole garden, and every bed and every crop shares it.

Suppose you raise your threshold to 10 °C the week your peppers go in. Any night forecast at 10 °C or colder, after your offset, now brings the alert with its line “Cover tender plants.” Your lettuce gets the same warning and can ignore it. Once the nights warm and stay warm, set the threshold back to 0 °C so the alert goes back to flagging frost. Raise it again in late summer while fruit is still on the vines.

The offset shapes every one of these alerts. On a free account you enter it by hand in Settings, under Garden, Edit garden, Microclimate offset. Pro accounts with a connected station skip that step. Siltwell compares the station's outdoor air readings against the forecast, and after about 14 days of overlap it sets the offset on its own.

The alert reads one number per night, the forecast low. Chilling injury builds with the hours a plant spends cold, and a night that touches your threshold for an hour before dawn raises the same alert as one that sits there from dusk onward.

What it changes in your garden

• Plant tender crops late. Set out warm-season transplants once nights have warmed, timed from your adjusted frost date.
• Harden them off. Several days of outdoor hours before transplanting let the membranes adjust. Hardened plants handle cool nights better.
• Cover on cool nights. Row cover traps the heat the soil radiates after dark and holds it around the leaves. The methods in our frost protection guide work for chilling nights as well.
• Hold the straw. Thick mulch shades the soil and keeps it cool, so wait until the soil has warmed before mulching peppers and melons.
• Keep harvests out of the fridge. Tomatoes, peppers, cucumbers, and basil suffer the same injury in cold storage. They lose flavor and pit within days.

Before your next tender transplant goes in, raise your garden's frost threshold. Put a reminder on your calendar for the week you'll lower it again.