Cover tomato plants when the overnight temperature drops below 50°F, when frost is forecast, or during the first two weeks after transplanting

Tomato plants need protection on cold nights because they are warm-season crops that stop growing and can suffer damage when temperatures fall below 50°F. Frost — a hard freeze at 32°F or lower — kills the leaves and stems of unprotected plants. The exact timing depends on your location's last spring frost date, your plant's age, and the weather forecast for the coming night.

Young transplants need more protection than established plants. A seedling moved into the garden for the first time is stressed and vulnerable; covering it for the first two weeks gives it time to harden off and develop stronger tissue. Mature plants with deep root systems tolerate cold better, but still need covering when frost threatens.

Key Takeaways

  • Cover tomato plants whenever the forecast calls for temperatures below 50°F at night, or when frost is predicted.
  • Young transplants need covering for their first two weeks in the garden, even if temperatures stay above 50°F, because they are still adjusting to outdoor conditions.
  • Frost cloth, old blankets, or row covers work better than plastic alone because they trap heat; plastic touching the leaves can cause damage.
  • Remove covers during the day once temperatures warm above 60°F to prevent heat buildup and allow the plant to photosynthesize.
  • Stop covering plants once nighttime temperatures consistently stay above 50°F and your area's last frost date has passed.

The 50°F threshold and why it matters

Tomato plants begin to slow their growth when nighttime temperatures drop below 50°F. Below 40°F, growth nearly stops. At 32°F or lower, frost forms on the leaves and stems, rupturing the plant's cells and killing the tissue. A single frost night can destroy an entire young plant or strip the leaves off a mature one.

Check your local weather forecast the evening before. If the low is predicted to be 50°F or below, cover the plants before sunset. Do not wait until the temperature actually drops — by then it is too late. Weather services usually issue frost warnings 12 to 24 hours in advance, giving you time to prepare.

Protecting transplants during their first two weeks

A tomato seedling moved from a greenhouse or indoor shelf into the garden has been living in a stable, sheltered environment. The outdoor air, wind, temperature swings, and direct sun are all new stresses. Even if the forecast shows temperatures staying above 50°F, covering the transplant for its first 10 to 14 days outdoors reduces transplant shock and helps it establish roots faster.

Use a lightweight row cover or frost cloth during this period. Leave it on during the day as well if the plant looks wilted or stressed. Once the plant has been in the garden for two weeks and shows new growth, you can stop covering it on warm nights. Continue covering on cold nights until your area's last frost date passes.

Materials that work and materials that don't

Frost cloth and row covers are the best choice. They are lightweight, allow light and water through, and trap a layer of warm air around the plant. A standard frost cloth rated for 20°F protection will keep the plant safe down to about 28°F. Thicker versions rated for 4°F protection work in harder freezes.

Old blankets, sheets, and burlap work in a pinch. Drape them loosely over the plant and anchor the edges with soil, rocks, or stakes so wind does not blow them away. Remove them during the day if the temperature climbs above 60°F.

Plastic sheeting alone is not reliable. Plastic does not insulate well and can touch the leaves, causing frost damage where the plastic makes contact. If you use plastic, create an air gap by draping it over stakes or cages so it does not touch the foliage. Remove it during the day — plastic traps heat and can cook the plant in morning sun.

Timing: when to put covers on and take them off

Put covers on in the late afternoon or early evening, before the temperature starts to drop. This traps the warmth the soil and plant have absorbed during the day. Remove covers in the morning once the temperature climbs above 50°F, or by mid-morning at the latest. Leaving covers on during warm daylight hours reduces the light the plant receives and can cause the leaves to yellow or the plant to become leggy.

On a night when the low is forecast to be 45°F, you can cover the plant at sunset and remove the cover by 9 or 10 a.m. the next day. On a night when frost is predicted, leave the cover on until the sun has warmed the air to at least 50°F, which may not happen until late morning in spring.

Knowing when to stop covering

Stop covering tomato plants once two conditions are both true: nighttime temperatures have stayed above 50°F for at least five consecutive days, and your area's last frost date has passed. Your local cooperative extension office or a quick online search for "last frost date [your city]" will give you the date for your location.

In most of the United States, the last frost date falls between mid-April (in the warmest zones) and late May (in cooler zones). Once that date passes and the forecast shows no more frost, you can leave the covers off permanently. Tomato plants grow faster and produce more fruit when they are not covered, so remove protection as soon as it is safe to do so.

What happens if you cover too late or forget

If frost hits an uncovered plant, the damage is usually visible by morning — the leaves and stems will look blackened, wilted, or mushy. A light frost may kill only the outer leaves while the main stem survives. A hard freeze can kill the entire plant.

If the main stem is still green and firm below the damaged leaves, the plant may recover. Prune off the dead tissue and leave the plant uncovered during warm days so it can photosynthesize and regrow. If the main stem is mushy or black all the way through, the plant is dead and should be removed and replaced.

Frequently Asked Questions

Do I need to cover tomato plants if they are in a greenhouse?

No. A greenhouse, even an unheated one, retains enough warmth to protect plants from frost. The glass or plastic walls trap heat during the day and release it slowly at night. Greenhouse tomatoes need covering only if the forecast calls for temperatures below 25°F or if the greenhouse has broken panels.

Can I use a heat lamp or light bulb under the cover to keep plants warm?

Yes, but only if you are careful. A single incandescent bulb (60 to 100 watts) placed under a frost cloth can raise the temperature by 5 to 10 degrees. Keep the bulb at least 12 inches away from the leaves to avoid burning them. Turn the light off during the day and never leave it unattended, as it is a fire hazard.

What if the temperature drops faster than the forecast predicted?

If you wake up to frost on the ground but forgot to cover the plants, cover them when ready anyway. Even a late cover can slow the damage by trapping the remaining ground heat. Once the sun comes up and the frost melts, remove the cover and assess the damage. Many plants survive a frost they were not covered for, especially if the freeze was light.

Should I water tomato plants before covering them?

Yes. Moist soil holds heat better than dry soil. Water the plants in the afternoon before a cold night so the soil is damp but not waterlogged. The moisture will release heat slowly as the temperature drops, providing a few extra degrees of protection.