How to Protect Garden Plants from Frost and Cold Nights

One cold night can end a garden that grew all summer. Frost damages plant cells, and tender plants such as tomato, chilli, okra and curry leaf turn black and limp by morning.

Many gardeners learn about frost the hard way, by walking out one morning to find the basil black and the tomato leaves hanging limp. It is a sad moment, but a little planning prevents most of the damage.

The good news is that frost protection is simple and cheap if you plan ahead. Here is what temperatures to watch and how to cover plants so they get through the cold.

At a glance

Frost: temperature at or below 32°F (0°C)
Light frost: 29-32°F (-2 to 0°C) damages tender plants
Hard freeze: below 28°F (-2°C) kills most vegetables
Cover plants before sunset and remove the cover in the morning

Why frost damages plants, and why protecting them is worth it

When the temperature falls to freezing, the water inside plant cells can turn to ice. The ice crystals damage the cell walls, and when the plant warms up in the morning the tissue collapses and turns dark and soft. Plants from warm climates, like tomato, chilli, okra and curry leaf, have no built-in protection against this, while hardy plants like spinach, kale and coriander can adjust and cope much better with cold.

Protection is worth the effort because it can extend your growing season by weeks. A few covered nights in autumn can let you pick tomatoes and chillies a month longer, and a little cover over winter greens keeps them producing. It saves money too, because an established plant like a curry leaf costs far more to replace than a bedsheet costs to throw over it for a night.

Where and when frost happens

Frost is part of winter in many places. In the northern United States, Canada, the UK and most of Europe, the first autumn frost usually arrives sometime between September and November and the last spring frost between March and May, depending on how far north you live. In India frost is mostly a concern in the north, in parts of Punjab, Haryana, Rajasthan, Uttar Pradesh and Himachal Pradesh, during cold waves in December and January, and it is a real risk for crops like tomato, brinjal and potato. In the southern hemisphere, in places like Australia, New Zealand and South Africa, the frost months are roughly May to September.

Frost is most likely on clear, still, cold nights, because there are no clouds to hold the day’s heat and no wind to mix the air. That is why the coldest hour is usually just before sunrise, and why covering plants in the evening works so well. Cloudy or windy nights are much less likely to bring frost, even when it feels cold.

Which plants need protection

Not every plant needs the same care. Know which of your plants are tender and which are hardy.

Tender, protect early: tomato, chilli, okra, bitter gourd, brinjal, basil and curry leaf. They slow down when nights drop below 50-55°F (10-13°C) and are killed by frost.
Hardy, can take light frost: spinach, coriander, methi, peas, kale, cabbage, carrots and radish.
Mint is a perennial. Its leaves die in a freeze but the roots usually survive and grow again in spring.

Watch the forecast

Check the overnight low every evening in the cold season. Frost can happen even when the forecast says 36-38°F (2-3°C), especially on clear, still nights, in low spots and in open areas.
Know your local first frost date in autumn and last frost date in spring.
In north India, cold waves in December and January are the time to be careful.

How to protect plants on a cold night

The good news is that protecting plants does not need special equipment. Most of what you need is already in the house.

Water the soil in the afternoon. Moist soil holds heat better than dry soil. Don’t wet the leaves.
Cover plants before sunset with frost cloth, a light blanket, old bedsheet, burlap or a cardboard box. Cover down to the ground so the heat from the soil is trapped.
Use stakes or a frame so the cover doesn’t rest on leaves. If you use plastic, keep it off the leaves, because plastic touching leaves can freeze them.
Remove covers the next morning once the temperature rises so plants can get sun and air.
Add a 2-3 inch (5-8 cm) layer of mulch such as dry leaves, straw or wood chips around plants to protect the roots.

Protecting potted plants

Pots are more exposed than plants in the ground, because they can get cold from every side, but they have one big advantage: you can move them.

Move pots against a sunny wall, under a porch or roof, or inside the house. Pots lose heat quickly from the sides.
Group pots together, because they keep each other warm.
Lift pots off cold concrete or tile with wood or pot feet.
Wrap pots with bubble wrap or old cloth for extra insulation.
Bring tender plants like curry leaf and chilli inside before nights fall below about 50°F (10°C), and place them in a bright window or under a grow light for 12-14 hours a day.

After a frost

If leaves turn black, don’t cut right away. Wait for the weather to settle, then trim dead parts. The dead leaves give some protection to the plant.
Water gently and avoid fertilizing a stressed plant.
Some plants, like spinach and coriander, may look frozen in the morning but recover as the day warms up.

Frequently asked questions

At what temperature should I protect my plants?

Cover tender plants when the forecast is 36°F (2°C) or below, because actual frost can be colder than the forecast at plant level.

Can I use a plastic sheet as frost cover?

Yes, but keep it from touching leaves and remove it in the morning. A cloth or frost fleece is safer.

Should I water plants before frost?

Yes. Water the soil during the day. Moist soil stores and releases more heat than dry soil.

Read our kitchen garden guide for beginners and how to grow coriander, methi and spinach, which all handle cool weather well.