How to Use Row Cover and Low Tunnels to Extend the Season
How many degrees row cover and low tunnels add, which weight to buy, and how many weeks of fall harvest they realistically buy. From Maryland and New Hamp...
Published by ThriveThyme
row cover season extension frost protection fall
How to Use Row Cover and Low Tunnels to Extend the Season
Row cover is a white spun fabric that traps heat from the soil. It does not make heat. It buys a few degrees on a cold night, which is often enough to carry greens past the first frosts.
How Many Degrees Each Weight Adds
| Fabric weight | Light through | Frost protection | Best use |
|---|---|---|---|
| Light, about 0.45 oz per sq yd | 90 to 95% | About 2F | Insect barrier, light frost |
| Medium, 0.5 to 1.0 oz | 70 to 85% | About 4 to 6F | Fall greens, light frost nights |
| Heavy, 1.5 to 2.2 oz | 30 to 50% | Up to about 8F | Overwintering, hard cold |
Source: University of Maryland Extension, row covers. University of New Hampshire Extension gives about 2F for light fabric and 6 to 10F for heavy fabric.
Two layers add more than one. Fabric that touches leaves protects less where it touches, so a frame helps.
Low Tunnels
A low tunnel is row cover or plastic over wire or PVC hoops, about 15 to 30 inches tall with hoops 2 to 4 feet apart (University of Maryland Extension). Fabric on hoops gives the same degrees as the fabric alone, with room for plants to grow and less leaf damage.
Clear plastic over hoops holds more heat on still nights, but it can overheat plants fast. Open the ends on any sunny day above about 50F.
How Many Weeks It Really Buys
Be realistic. Row cover protects a few degrees, so it gains time only while nights stay near the edge.
- Tomatoes, peppers, basil: a cover gets them through one or two light frosts, often 1 to 3 more weeks of picking. It will not save them from a hard freeze.
- Lettuce, chard, Asian greens: often 2 to 4 more weeks of harvest in fall.
- Kale, spinach, collards: a medium or heavy cover, or a low tunnel, can carry them well into winter in zones 6 and warmer, and through much of winter in zone 7 and up. Growth nearly stops once days are under about 10 hours of light, so plant fall greens to reach full size before then.
How to Use It
- Lay covers before dusk on a forecast frost night. The soil heat you trap is what protects the plants.
- Anchor every edge with sandbags, boards, or soil. Wind lifts loose fabric and lets cold in.
- Water dry soil earlier in the day. Moist soil stores more heat.
- Pull the cover back on warm afternoons if plants need light or air, and close it again before evening.
Check each crop's kill temperature on the Frost Harvest Desk, then see which vegetables survive a hard freeze and protecting plants from early frost.