Cold Frame vs Row Cover vs Low Tunnel
When to use row cover, a low tunnel, or a cold frame for frost nights. Degree gains from Maryland and New Hampshire extension, plus venting rules.
Published by ThriveThyme
season extension row cover cold frame low tunnel frost
Cold Frame vs Row Cover vs Low Tunnel
Pick the cover that matches the crop and how many nights you will use it. All three trap soil heat. None of them heat the garden on their own.
Use the Season Extension Desk for a frost countdown and a crop list sorted by what to pick, cover, or leave in.
Quick Pick
| Method | Typical gain | Best when |
|---|---|---|
| Light row cover | About 2F | One light frost, insects, or a thin safety layer |
| Medium row cover | About 4 to 6F | Fall greens and brassicas through early freezes |
| Heavy row cover | Up to about 8F | Overwintering spinach and kale on short days |
| Low tunnel | Same as the fabric (more with clear plastic) | Whole beds you will cover many nights |
| Cold frame | Often about 5 to 10F on clear nights | Salad greens you tend by hand and can vent daily |
Sources: University of Maryland Extension (row cover weights) and University of New Hampshire Extension. Cold frame gain varies with sun, lid seal, and insulation.
Row Cover Flat on the Bed
Drape fabric to the soil on all sides and pin it. Fast for a single night. Where fabric touches leaves, those spots get less protection, so tall crops do better on hoops.
Light fabric buys a couple of degrees. Medium is the usual fall choice. Heavy fabric cuts a lot of light, so save it for cold nights and short winter days.
Low Tunnel
Wire or PVC hoops about 15 to 30 inches tall, 2 to 4 feet apart, with fabric or clear plastic over them (University of Maryland Extension). The cover stays off the leaves, so plants take less damage and you can walk covers on and off faster.
Clear plastic holds more heat on still nights and overheats fast. Open the ends on sunny days above about 50F. Fabric tunnels need the same venting when days warm.
Details: How to use row cover and low tunnels to extend the season.
Cold Frame
A low glazed box blocks wind and traps sun. It is slower to set up than fabric and needs daily attention on sunny days. Prop the lid when the sun is out. Close it before a hard freeze night.
Cold frames shine for salad greens and cool transplants you check often. They do not replace a greenhouse, and a thin lid still loses to a long, clear hard freeze.
More: Cold frames and winter sowing and How to vent a cold frame on sunny days.
What Not to Expect
- Tomatoes, peppers, and basil: a cover may ride out one light frost (about 32F). It will not hold them through a hard freeze below about 28F. Pick first. See Frost Harvest Desk.
- Kale, spinach, and collards: covers stretch harvest deeper into winter. Growth nearly stops once days are under about 10 hours of light, so size the plants before then.
- Any cover on dry soil: water in the afternoon if beds are dry. Moist soil holds more heat than bone-dry soil.
Next Steps
- Build a frost-night kit so covers are ready before dusk.
- See which crops to cover on a frost night.
- Check your first frost date and the forecast low each evening.