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Protecting Plants From Frost: FAQs

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You needn’t worry much about protecting plants from frost when it comes to hardy vegetables like broccoli, carrots, spinach, cabbage, cauliflower, and radishes. They’re used to nippy temperatures. Protecting these plants from frost is almost a waste of time.

But heat lovers are another matter. As winter turns to spring, tomato plants go in the ground. And as summer turns to fall, tomatoes (especially indeterminates) continue to produce. 

However, Mother Nature ignores the dates on the calendar. She sprinkles frost and cold temperatures as she likes. A gardener needs only to lose tomatoes and peppers and other heat-loving plants once to a late season freeze or an early fall polar wave to experience disappointment.  From then on, it’s all hands on deck for protecting plants from frost.

Even if you wait well beyond the last frost date in the spring to set out plants – or if you keep your eye on the forecast in the fall – you can face two main scenarios. Maybe you’ll experience a short-term cold snap. Or your region could be in for a long-term cold wave. Either way, keep your eye on the forecast. And check out these frequently asked questions to know how to manage Mother Nature’s temperature swings.

Protecting plants from frost: frequently asked questions

Q. What temperature do you need to protect plants from frost?

Water freezes at 32 degrees F. But tomatoes can survive a light freeze (28-30 degrees F) if all is dry. The real problem? Moisture. 

When low temperatures are accompanied by any bit of precipitation, wetness, or humidity, then frost develops. Tomatoes cannot withstand frost. Low temperatures + any form of moisture = frost. Bottom line: if your forecast is 40 degrees F and below, then take precautions.

Q. How long can you keep plants covered to protect from frost?

Is your forecast for a short-term frost or freeze, as in overnight? Then remove coverings in the morning or once temperatures rise over 50 degrees F to give them extra light and warmth. But you want to avoid keeping plants covered for days on end. They need light. For a cold spell that lasts more than a day or two, consider a longer-term solution like Wall-o-Water or even a cold frame. 

Q. Will cardboard boxes protect plants from frost?

Yes. Save those Amazon delivery boxes and brown paper grocery bags. They’re easy to use, keep their shape, and best of all, they’re free. Use them as frost protection both in the spring and the fall and then recycle them. 

Q. Can you put plastic over plants to protect from frost?

Yes. You can use oversized trash bags or plastic grocery bags – depending on the size of your plants.

Make sure there is plenty of air space between the plastic and the plant. You may even use stakes, sticks, or cages to hold up the plastic bag to ensure it doesn’t touch the plant. Reason: cold plastic damages plants at the point of contact.

Secure the edges of the bags or sheets of plastic with bricks or clips so the plastic doesn’t slip and give frost an opening. Remove plastic bags during the daytime so that soil can reheat and light can warm plants.

Q. Will a tarp protect plants from frost?

A tarp can protect plants from frost provided  …

• It’s not too heavy. An overweight tarp can stress your tomato plant stem and branches – even break them.
• It doesn’t touch the plants. Use stakes, cages, or other supports to make sure air can circulate around the tomato plant. A plastic tarp, like a plastic bag, can damage tomato plants when it gets too cold.

Q. What are the best methods for protecting plants from frost with plastic?

Hands down: plastic milk jugs, plastic juice jugs, and plastic vinegar jugs are the best plastic to use. Best of all, they’re free. Of course, this method works only if plants are smaller than the size of the jug.

Cut out the bottom of the jug on three sides. Use the fourth side to anchor the jug. You can insert the fourth side into the soil (which may damage roots) or secure it with a brick or rock (best method.) Leave the cap on the top of the jug to create a miniature greenhouse for the plant. Heat and moisture will gather inside the jug. Remove the jug in the morning when temps rise about 50 degrees F so plants can get plenty of sunshine.

Q. What are the best methods for protecting plants from frost with water?

Water your tomato plants before a frost or freeze. Be sure to water at the soil line. Avoid getting water on the stems or leaves, because it may freeze at worst or at best get very cold and damage your tomato plant.

Beyond that, consider several types of water insulator products that are on the market for protecting plants from frost.  They go by brand names like Wall-o-Water, Season Starter Plant Insulators, and Kozy Coats. Wrap these sheets around your plants and fill the troughs with water, which absorbs heat during the day. Heat from the water protects plants at night. 

Q. What are the best methods for protecting container plants from frost?

Set individual plant covers over container plants to protect them from frost

Or throw a tarp, old sheet, large towel, or blanket over container plants to protect them. Just make sure they’re not too heavy or they’ll weigh down your tomato plants and even break some branches.

Q. What’s the best long-term solution for protecting plants from frost?

Consider using tunnel row covers in the spring to protect tomato plants from late frost. Tunnels protect plants from wind as well as frost and cold temperatures. Tunnels are made of wire or fiberglass hoops covered with nonwoven fabric or plastic. You can purchase pre-made tunnel row covers or make your own. 

Or you might consider one of the more lasting solutions for protecting plants from frost. If your growing season is short or if you want to get a jumpstart on all the gardeners in your neighborhood, then consider growing your tomato seedlings in a cold frame or a greenhouse. This way you needn’t worry about the occasional frosty evening. Your tomato plants can grow big and strong until all danger of cold nights has passed. A cold frame or greenhouse can perform double duty in the fall and even into the winter. You can transfer plants there when temperatures begin to dip.

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