Yes, row covers protect from frost by raising air temps under the fabric by 2 to 8°F above the open air. The exact boost depends on the cover weight you pick from the shelf. Light covers add just a small bit of warmth. Heavy covers can save your crops from a hard freeze night. A good frost protection fabric is one of the cheapest insurance plans you can buy for your garden.
I saved a tray of twelve tomato transplants with a medium cover during a surprise late spring freeze last May. The forecast said 42°F that night but the temp dropped to 29°F at dawn. My covered plants came through with no leaf damage at all. The two starts I had left out got hit hard and I had to pull them and start over from seed.
Covers work on frost nights through two key tricks at once. First, they trap heat that radiates up from the soil after the sun goes down. The soil holds warmth from the day and slowly gives it off all night long. Second, the fabric blocks convective cooling by stopping cold wind from sweeping warmth off the plant leaves below.
Per Maryland and Wisconsin extensions, the warmth boost runs in three set ranges. Lightweight fabric adds 2°F above the open air. Medium fabric gives you 4 to 6°F of frost guard. Heavyweight row cover can boost canopy temps up to 8°F on a calm night. Wind cuts these gains so a still night gives the best results.
Pick the right weight for the kind of cold snap you face that night. Match cover weight to the gap between forecast temp and the cold tolerance of your crop. A 34°F night on lettuce needs no cover since lettuce takes light frost just fine. A 28°F night on peppers needs a heavyweight cover and maybe two layers for full safety.
Layer two covers for extreme cold below 20°F in your area. I put a light cover right on the plants and stretch a heavy cover over hoops above. The trapped air between the two layers boosts the warmth gain to 10°F or more on calm nights. This combo saved my brassica bed last winter when temps dropped to 15°F for two nights in a row.
Use hoops to hold the cover off the leaves where frost can pass through contact. Any spot where the cover touches a leaf will freeze just like it would in open air. I learned this the hard way on a kale bed where the fabric sagged in the middle and froze a strip down the center of each plant. Hoops are not optional when temps drop hard at night.
Set your hoops 4 to 5 feet apart and pull the cover snug over the top. Freeze protection garden setups need tight edges too so warm air does not leak out the sides. Anchor with soil or bricks every three feet along the bed. With this setup, you can push your growing season four to six weeks longer at both ends of the year.
Read the full article: Row Cover Garden Guide: Weights and Timing
