Heavyweight rows covers and garden blankets or quilts are used to protect plants from cold temperatures, chilling winds, and frosts–most often in late autumn, winter, and early spring. Lightweight floating rows covers are used to shield plants from insects and diseases in spring and summer. Shade cloth is used to shade soil and plants in hot summer weather.
Row covers and floating row covers are commonly made form spun bonded polyester or polypropylene fabric–often called horticultural cloth. Heavyweight covers are used to extend the growing season by keeping plants up to 10°F warmer during the day and 2° to 3°F warmer at night. Lightweight covers are used to exclude insects but are thin enough to allow light, water, and air to reach plants.
Row covers can be draped over hoops or frames set above rows of plants (to form protective tunnels) or they can be laid loosely or floated over plants. Medium to heavy weight row covers weighing from 1¼ to 2 ounces per square yard should be partially lifted on hot days and removed as the season advances to prevent plants from overheating.
Lightweight row covers that are permeable to air and water and weigh about a half ounce per square yard usually do not require venting but they should be lifted when pollinating insects visit crops. Row covers–depending upon weight–can transmit up to 85 percent of the available sunlight.
Horticultural cloth for covering plants is commonly sold in squares or rolls and is easily cut to size to cover one plant or an entire row. Covers can be held in place by garden staples, soil, boards, or bricks. A floating row cover set in place when seeds are sown or seedlings set out should have enough material for 4 to 6 weeks of plant growth, allowing the cover to float up with plant growth.
Row covers used to protect plants from frost or freezing weather should be lifted up or supported by a frame so that the fabric does not touch plant leaves. Fabric touching plants can wick killing cold temperatures to plant tissue.
Row covers can extend the growing season in cold-winter regions by 3 to 4 weeks in both spring and fall giving seeds and seedlings a faster start and protecting maturing crops until harvest. In warm-winter regions, row covers can protect plants all through the winter and greatly prolong the growing season and harvest.
Use row covers to:
• Warm and speed the growth of seeds and seedlings in spring and protect crops from late frosts.
• Slow the evaporation of soil moisture.
• Protect plants from wind, heavy rain, and hail.
• Keep pests off plants (and stop the spread of disease by insects); be careful not to trap pests under row covers. (Covers will not protect plants from insects that emerge from the soil such as flea beetles, root maggots, tomato hornworms, Japanese beetles, and Colorado potato beetles.)
• Protect late summer crops from fall frost and extend the harvest.
• Protect crops from hot temperatures and burning sun rays.
Row cover weights:
Row covering weights: Row covers are measured or graded by the weight in ounces per square yard of fabric. Heavyweight covers weigh 2 ounces per square yard; medium weight covers weigh 1¼ ounce per square yard; lightweight covers weight a half ounce per square yard.
• Heavyweight row covers. Heavyweight covers (2 ounces per square yard), also called garden blankets or quilts, are commonly made of thick, close knit polypropylene fibers. These covers are best supported above plants by plastic or fiberglass hoops or wooden frames.
Heavyweight covers can protect plants to 24°F and are commonly used in late winter or early spring to protect plants from frost or short freezes allowing the growing season to begin as much as 3 to 4 weeks earlier than average.
In cold-winter regions, heavyweight covers also can be used in late autumn and early winter to keep freezing weather from harming crops until harvest and delay soil freezing (protecting root crops). In mild-winter region, heavyweight covers can used to protect root and leaf crops for harvest all winter or wintering over crops for early spring harvest.
Heavyweight row covers block from 40 to 50 percent of sunlight and are best not used once temperatures have settled above freezing and plants begin active growth. The temperature under a heavyweight cover in warm weather can rise to as much as 20 degrees warmer than the outside temperature
• Medium-weight row covers. Medium-weight or all-purpose row covers (1¼ ounces per square yard) are commonly made from spun-bonded polyester or polypropylene knitted fabric and are placed on supports over young crops or floated over the tops of stronger, mature plants.
Medium-weight row covers protect plants from frost damage down to 28°F (2 to 6 degrees of protection). These covers will protect plants from strong winds, heavy rain, and even hail but allow light, air, and water to reach plants.
Medium-weight row covers allow from 70 to 85 percent of sunlight to reach plants and can be used for light shading in summer. Medium-weight covers also can be used to exclude insects and other pests early in the growing season.
• Light-weight row covers. Light-weight or summer-weight row covers (one-half ounce per square yard) are used to protect plants from insects, birds, and insect-borne diseases. Light-weight row covers transmit up to 95 percent of sunlight to plants. These covers are not used for frost protection, will not overheat plants in hot weather, and readily allow rain and irrigation to reach plants.
• Shade cloth. Akin to row covers is shade cloth. Shade cloth is a knitted fabric often made from polyethylene or polypropylene. Shade cloth is commonly attached to a frame and set above heat sensitive crops in summer. It can extend the growing season of crops such as lettuce and spinach which bolt or flower and go to seed if allowed to sit in the hot sun.
Shade cloth is available in a range of densities to provide 30, 40, 50, 60, 70, 80, or 90 percent shade. A density of 60 percent or less can be used for heat sensitive crops such as salad greens.
Shade cloth should sit at least 24 inches off the ground to minimize the buildup of heat and allow for the circulation of air. Support shade cloth with wire or plastic hoops or on wooden stakes or frames set up over crops.
Seasonal use of garden fabric and row covers:
• Spring. Prepare the soil and sow seed or set out transplants. Place or float garden fabric on the ground directly over sown seeds or seedlings or drape the fabric over hoops or a support structure. Do not stretch fabric tight over seedbeds; allow the cover to float upwards with plant growth. Fabric over hoops can be stretched tight. Anchor the edges of covers in place with garden staples or soil.
Be sure to lift or vent the fabric during the day once daytime temperatures begin to warm to greater than 70°F (remember temperatures underneath the cover can be 10°F or more warmer). Do not allow plants to overheat which will result in wilting and leaf or blossom drop.
Allow pollinating insects such as bees to reach crops that require insect assistance for pollination. (Insect-pollinated crops include beans, peas, pumpkins, squash, and strawberries.) Lift or remove fabric covers during the day when plants begin to flower. For self-pollinating plants such as tomatoes and peppers, covers can remain in place as long as temperatures do not rise to greater than 85°F; when temperatures rise too warm tomato pollen will become sterile and peppers will not set fruit.
Horticultural cloth will block insects from reaching plants (including aphids, potato beetles, Japanese beetles, grasshoppers, leaf miners, cabbage worms, root maggots, and some vine borers) as long as the edges of the cloth are secured. (But garden fabric will not exclude insects that emerge from the soil from insect eggs in the soil before the soil was covered.)
• Summer. Use lightweight garden fabric or shade cloth to protect plants from hot temperatures and intense summer sunlight. Garden fabric and shade cloth will keep soil and plants temperatures cooler. Horticultural cloth will slow soil moisture evaporation.
Choose shade cloth according to the intensity of the sun in your region. Shade cloth is available in weaves that will provide 30, 40, 50, 60, 70, 80, or 90 percent shade. A weave density of 60 percent or less can be used for heat sensitive crops such as salad greens.
Set shade cloth at least 24 inches off the ground to minimize the buildup of heat and allow for the circulation of air. Support shade cloth with wire or plastic hoops or on wooden stakes or frames set up over crops.
• Fall. Use medium-weight garden fabric in autumn to protect cold-sensitive crops such as tomatoes and peppers from early frosts or chilling winds. Row covers put in place in the fall slow the dissipation of soil heat built up over the summer and reserve daytime solar heat into cooling autumn nights. As daytime autumn temperatures continue to drop, medium-weight fabric can be replaced with heavyweight fabric; these fabrics should be supported by hoops or structures.
Row covers used in autumn can extend the harvest of warm-weather crops by two to three weeks in almost all regions, longer in mild-winter regions. Row covers can extend the growing season for fall cool-weather crops by a month or more.
• Winter. Medium- and heavyweight row covers in mild-winter regions can be used to extend the growing through the winter. Leaf and root cool-weather crops can be protected by row covers for harvest or storage through the winter. In cold-winter regions replace row covers with plastic in winter to create a coldframe or cold-weather plastic tunnel.
Common commercial brands of horticultural cloth: Horticultural cloth for row covers is sometimes sold under brand names including Remay, Agribon, Harvest Guard, Typar, and Agryl.