Row Covers for Pest Control
Row covers are among the most reliable no-spray pest controls a home gardener has. A lightweight fabric or fine insect netting, installed at planting with the edges sealed, keeps out flea beetles, cabbage caterpillars, root maggots, cucumber beetles, and squash vine borers. They fail for predictable reasons: gaps at the edges, pests already in the soil underneath, and crops that need insects to pollinate them.
What row covers stop
Row cover works by physically keeping insects away from the crop. Extension sources list these pests as well suited to exclusion:
| Crop group | Pests row cover can exclude |
|---|---|
| Brassicas (cabbage, broccoli, kale, cauliflower) | Imported cabbageworm, cabbage looper, diamondback moth, cabbage maggot, flea beetles |
| Cucurbits (squash, cucumbers, melons) | Cucumber beetles, squash bugs, squash vine borer, aphids |
| Eggplant and potatoes | Flea beetles, Colorado potato beetle, aphids |
| Onions, seeded crops | Onion maggot, seedcorn maggot |
Sources: University of Wisconsin Extension and UNH Extension. Wisconsin also notes that covers can reduce thrips and the spread of aster yellows, a disease carried by leafhoppers.
Cucurbits are where covers have some of the clearest numbers. In Cornell trials of mesotunnels (netting over hoops about 36 to 48 inches tall), organic muskmelon had roughly three times fewer cucumber beetles and about six times more marketable fruit weight than uncovered plots. Cucumber beetles also spread bacterial wilt, so keeping them out protects against disease, not just chewing damage.
Choosing a weight
Row cover fabric comes in different weights. For pest control, lighter is usually better because it lets in more light and traps less heat. From the New England Vegetable Management Guide and University of Maryland Extension:
| Grade | Weight | Light transmission | Frost protection | Best use |
|---|---|---|---|---|
| Lightweight | 0.45 oz/sq yd | About 90% | About 2°F (UMD) | Insect exclusion, can stay on some crops to harvest |
| Medium | 0.5–1.0 oz/sq yd | 70–85% | 4–6°F | Spring and fall season extension |
| Heavy | 1.5–2.2 oz/sq yd | 30–50% | Up to 8°F | Frost protection, overwintering |
Insect netting is an alternative for crops that do not need pollination. UNH notes it excludes insects "without increasing temperatures or providing frost protection," which makes it a better choice in summer. UMD says netting should last at least five growing seasons, compared with one to three years for fabric row cover.
Mesh size matters for small pests. For swede midge, whose adults are about 1.5 mm long, Cornell Cooperative Extension recommends a mesh of 0.35 mm.
Timing: on at planting, off at bloom
Row cover has to go on before the pest arrives. Wisconsin states it "must be put on at planting." Covering plants after flea beetles have found them just traps the beetles inside with your seedlings.
Removal depends on the crop:
- Leafy greens, brassicas, root crops. These do not need pollinators for the part you eat. UMD notes lightweight cover can stay on crops like beets and salad greens from seeding to harvest, as long as heat is not an issue.
- Squash, cucumbers, melons. These need insect pollination. The New England guide says covers on fruiting crops typically stay about three to five weeks, until pollination is needed. For squash vine borer, UMD recommends covering "until flowering." Cornell's mesotunnel guidance is to open the ends temporarily at peak bloom to let pollinators in, which is not necessary for parthenocarpic varieties that set fruit without pollination.
- Eggplant. In a Cornell trial against flea beetles in Buffalo, the cover came off eggplant at the end of June, when plants began to flower.
Wisconsin is candid that the bloom requirement "obviously limits its use for pest exclusion" on insect-pollinated crops. You get protection during the vulnerable early weeks, then the crop is exposed.
Common mistakes
Gaps at the edges
Insects find openings. Wisconsin says effective exclusion requires completely burying the edges. Cornell recommends that edges be "sealed and secure to the soil." Check the edges after wind and after you open the cover to harvest or weed.
Pests already in the soil
Flea beetles, root maggots, and some other pests overwinter in or near the soil. If you cover a bed where the same crop family grew last year, the pests can emerge under the cover. Wisconsin advises against enclosing plants over infested areas and recommends rotation. Cornell warns that "if any pests are present under the row cover, their population can increase rapidly."
Heat
Covers raise temperature and humidity. The New England guide suggests more ventilation once outside air reaches about 80°F. Netting runs cooler than fabric.
Fabric resting on plants
Cornell recommends keeping fabric off the leaves to avoid scorching and so insects cannot feed through it. Hoops help. UMD describes low tunnels 15 to 30 inches high with hoops spaced 2 to 4 feet apart. A simple version from the same guide: cut #9 gauge wire into 7-foot lengths and push the ends into the soil to make arches about 3 feet wide and 2 feet tall at the center. UNH suggests doubling the hoops at the ends of a row, where the cover pulls hardest.
Forgetting about weeds
Weeds grow well under row cover too, and pulling them means opening the cover. Cornell suggests planning ahead, for example by mulching before covering.
Combining covers with other methods
Row covers pair well with rotation, which reduces the soil-emerging pests that defeat them, and with resistant varieties. Perimeter trap cropping is another option for squash once covers come off at bloom; see companion planting evidence for how it works. Row covers used for pest control can also double as frost protection early in the season, as covered in season extension and cold frames.
Related
Row covers are the exclusion layer of No-Spray Pest and Disease Management. For the pests covers cannot address, such as those on crops that must be open to pollinators, habitat for natural enemies helps; see attracting beneficial insects.