Best Row Covers for Keeping Pests Off Vegetable Beds

Which Row Cover Keeps Insects Off Vegetables?

For insect protection, buy the lightest grade of spun-bonded row cover, about 0.45 ounce per square yard, or a fine-mesh insect netting. University of Maryland Extension describes the lightweight grade as the one marketed as an insect barrier. It passes 90% to 95% of sunlight and adds only 2°F of frost protection, so it traps the least heat of any grade. Put it on at planting, seal every edge to the ground, and take it off insect-pollinated crops when they bloom.

Heavier fabric is sold for a different job. As weight goes up, the cover holds more warmth on a cold night and lets less light through to the plants. A cover that suits a frosty April night is a poor choice over lettuce in July, and the thin insect barrier does little against a hard frost. Decide which problem you have this season before you compare products. If frost is the main worry, our guide to cold frames and row cover types compares fabric with rigid frames and low tunnels.

Row Cover Grades Compared

Row cover is a white, non-woven fabric made from spun-bonded polyester or polypropylene, and it is sold by weight. University of Maryland Extension sorts it into three grades.

Grade	Weight (oz per square yard)	Light transmittance	Frost protection	Main use
Lightweight	0.45	90% to 95%	2°F	Insect barrier through the growing season
Medium-weight	0.5 to 1.0	70% to 85%	4 to 6°F	Spring and fall season extension
Heavyweight	1.5 to 2.2	30% to 50%	Up to 8°F	Frost and freeze protection

Grade names are not standardized. Utah State University Extension, for example, calls 0.50 to 0.55 ounce fabric medium-weight and puts anything from 0.90 to 2.0 ounces in the heavyweight class. Read the ounces and the light percentage on the label and ignore the adjective on the front of the package.

The light figure matters for anything left on for weeks. A heavyweight cover that passes 30% to 50% of light will hold back growth if it stays over a crop through the day, which is why the heavy grades are used for cold snaps and then pulled off.

Insect Netting vs. Spun-Bonded Fabric

Insect netting is a woven or knitted mesh instead of a felt-like sheet. It also lasts longer. University of Maryland Extension says netting should last at least five growing seasons, against 2 to 3 years of reuse for row cover fabric.

With netting, mesh size decides what gets through. Utah State University Extension lists the pests excluded at four mesh sizes. The finest mesh in its table keeps out thrips, aphids, whiteflies and flea beetles along with everything larger. The coarsest stops only moths, stink bugs and squash bugs. A coarse net bought for cabbage butterflies will do nothing against aphids, so match the mesh to the smallest insect you need to exclude.

Fine mesh works in both directions. The same Utah table shows the finer meshes also excluding lady beetles and lacewings, which eat aphids. If aphids get in under a fine net, their predators cannot follow, so check under the cover from time to time.

Which Pests a Row Cover Stops

A cover works against insects that fly or walk in from somewhere else. University of Wisconsin Extension lists these uses for floating row cover:

Pest	Crops
Imported cabbageworm, cabbage looper, diamondback moth	Cabbage, broccoli, cauliflower
Flea beetles	Cabbage, potato, eggplant, salad greens
Onion maggot	Onion
Seedcorn maggot	Beans, corn and other crops
Early season cucumber beetles	Cucumbers, squash and other cucurbits
Aster leafhopper, which carries aster yellows	Carrot, celery, lettuce

Utah State University Extension adds squash bugs, cutworms, Colorado potato beetle and aphids to the list. For the caterpillars on brassicas, the cover is one tool among several, and our guide to getting rid of cabbage worms covers scouting and sprays. Squash growers dealing with borers will find the timing details in squash vine borer prevention.

A cover also keeps out rabbits, deer, birds and groundhogs, according to University of Maryland Extension, which is a useful bonus over a bed of young lettuce.

Timing: On at Planting, Off at Bloom

The cover has to be in place before the insects arrive. Wisconsin’s guidance is that it must be put on at planting, and Maryland’s is to cover spring transplants and newly emerged seedlings as soon as possible. A cover that goes on a week after cabbage butterflies have visited encloses their eggs with the crop.

How long it stays depends on whether the crop needs bees. Insect-pollinated crops such as cucumbers and squash must be uncovered during bloom. Crops grown for leaves and roots have no such limit, and the lightweight 0.45 ounce grade can stay on beets, snap beans and salad greens from seeding to harvest. Cabbage, broccoli and kale are harvested before they flower, so they can stay covered for their whole season too.

Transplants raised indoors still need a gradual introduction to outdoor light and wind before they go under a cover. The schedule in our guide to hardening off seedlings applies whether or not the bed will be covered.

The Rotation Catch: Pests Already in the Soil

A row cover cannot exclude an insect that is already under it. Some pests spend the winter in the soil near last year’s host plants and come up the following spring. University of Maryland Extension names the adult flies of root maggots, flea beetles and Colorado potato beetles as examples. Cover a bed where potatoes grew last year with potatoes again, and the beetles emerge inside the cover with the crop to themselves.

The fix is rotation. Utah State University Extension advises rotating crops by family so that a cover never goes over ground where eggs may have been laid the year before. Wisconsin gives the same advice for crops prone to seedcorn maggot. In a small garden with 3 or 4 beds, that means moving the brassicas, the nightshades and the cucurbits to a different bed each year.

Hoops, Edges and Fit

The lightest fabric can float directly on a crop, but hoops keep it off growing tips and make it easier to lift for weeding. University of Maryland Extension describes low tunnels 15 to 30 inches high with hoops spaced 2 to 4 feet apart. Buy fabric wide enough to go over the hoops and still leave a generous margin on each side to hold down.

The edges are where most covers fail. For pest exclusion the edges must be sealed along their whole length, and Wisconsin’s advice is to bury them completely. Maryland lists sod pins, wooden boards, bricks, sandbags, rocks or soil as anchors. Utah suggests shoveling soil or placing large rocks every 3 feet or so along each side, which also keeps wind from lifting the cover. Leave some slack in the fabric so the plants can push it up as they grow.

Heat and Weeds Under the Cover

Every cover warms the air under it. In summer, University of Maryland Extension reports temperatures under a row cover running 5 to 15°F higher than outside, enough to cause heat stress. Wisconsin notes that heavyweight fabric can raise daytime temperatures 10°F or more. For summer insect control, stay with the lightest grade and watch cool-season crops for wilting on hot afternoons.

Weeds enjoy the same shelter as the crop. A cover acts as a small greenhouse for everything under it, and because the fabric hides the bed, weeds can get large before anyone notices. Wisconsin’s advice is to take the cover off periodically for weeding. Lift one side, pull what has come up, look for insects that slipped in, and seal the edge again.

How Long Covers Last

Expect 2 to 3 years of reuse from row cover fabric and at least five growing seasons from insect netting, going by the University of Maryland figures. The thinnest fabric is the weak point. Utah State University Extension warns that lightweight covers rip easily and may last only a few months, especially when stretched over hoops in wind.

That makes netting worth a look for beds that are covered every summer, such as a permanent brassica bed. For a single spring crop, lightweight fabric is easy to cut to size with scissors.