How to Protect Plants from a Late Frost

A late spring frost can wipe out tomatoes, peppers and squash in a single night. The good news: most frosts are light, and a simple cover put on in the evening usually saves the plants. This guide covers how to see a frost coming, how to protect plants, and which ones need it.

Know your risk window

Frost risk is highest in the weeks around your average last frost date, and some risk lasts until the safer planting date, after which only 1 year in 10 has another frost. Look up both for your ZIP code with Frost Dates by ZIP Code. Any tender plants in the ground before the safer date need a cover on hand.

Signs a frost is coming

• A low of 36°F or below in the forecast. Thermometers are usually 5 feet above ground, and the ground can be several degrees colder, so frost can form at plant level when the forecast low is in the mid 30s.
• Clear skies and calm wind. On clear, still nights the ground loses heat fast. Clouds and wind act like a blanket and reduce the risk.
• Dry air. A low dew point in the evening means temperatures can keep dropping overnight.
• National Weather Service alerts. A Frost Advisory means frost is likely; a Freeze Warning means temperatures are expected to reach 32°F or lower.

Low spots and the bottoms of slopes are coldest, because cold air sinks and collects there. Beds against a south-facing wall stay warmest.

How to protect plants

Cover them in the evening

Covers trap the heat that rises from the soil overnight. Put them on in the late afternoon or evening, before the temperature falls.

• Frost cloth or floating row cover is the best option: light, breathable and reusable. Heavier fabrics protect against colder temperatures.
• Old sheets, blankets or burlap work well in a pinch.
• Buckets, pots and boxes turned upside down protect small plants.

Let covers reach all the way to the ground and weigh down the edges with stones or bricks; the warmth comes from the soil, so a cover that stops at the leaves does little. Use stakes or hoops to keep heavy covers off the plants.

Avoid plastic touching the leaves

Plastic sheeting conducts cold, so leaves that touch it can freeze. If plastic is all you have, hold it above the plants on hoops or stakes, or put a sheet underneath it.

Water the day before

Moist soil holds more heat than dry soil and releases it through the night. Water the bed during the day before a frost, but don’t soak the leaves in the evening.

Use cloches for early plantings

Individual covers help transplants through a cool spring:

• Wall-of-water cloches hold water that warms in the sun and releases heat at night.
• Cut-off milk jugs make cheap mini greenhouses for single seedlings. Take the cap off during the day so plants don’t overheat.

Remove covers in the morning

Take covers off once temperatures rise above freezing the next morning. Left on in the sun, they can overheat plants underneath.

Which plants need protection

Protect from any frost	Usually fine in a light frost
Tomatoes	Peas
Peppers	Spinach
Eggplant	Lettuce (may be slightly damaged)
Basil	Kale and cabbage
Cucumbers	Broccoli
Squash and zucchini	Onions and garlic
Beans	Carrots and beets
Sweet corn	Radishes

Seedlings of any crop that were just moved outside are more delicate than established plants. Cover them too if a hard freeze, in the upper 20s or below, is forecast.

After a frost

• Wait before judging. Damaged leaves turn dark and limp within a day or two. Plants often regrow from undamaged buds.
• Don’t prune right away. The damaged leaves protect the rest of the plant if another cold night follows. Trim once the danger has passed.
• Replant if needed. If tomato or pepper plants are killed, there is usually still time to replant; check the Seed Starting Calculator for timing.

The simplest protection: wait

The surest way to avoid frost damage is to keep tender crops indoors until the safer planting date and nights stay above 50°F. Plants set out a week later into warm soil often catch up with, and outgrow, those planted early into cold soil.