Row covers function to enhance growth and yield by modifying the temperatures around plants in the spring and fall, or in combination with low tunnels, during the winter. There are two general types: slitted or perforated plastic, and spun-bonded fabric. Heavier weight row covers can provide several degrees of frost protection, while lightweight "non-heating" or summer weight covers offer less heat enhancement and can be used in summer for insect protection. These materials can be used with or without hoops ("floating row cover") depending on its weight and the fragility of the crop underneath. Newer types of knitted or woven lightweight row cover (for example, 'Proteknet' by Agrinovations) are becoming available; they can be used with or without hoops, are quite durable, and will exclude insects.
Row covers are installed right after planting and are left covering the crop for several weeks, depending on crop type and season. For fruiting crops and cucurbits, covers can be left in place for approximately 3-5 weeks until pollination is needed or the crop outgrows the space under the cover. Other crops that are low-growing and do not require pollination can remain under cover as long as the temperature benefit is useful. Sweet corn may be left covered with spun-bonded row cover until pretassel stage. If the crop is pressing against the cover, either loosen or remove it. Row cover removal timing is more critical in some crops, such as tomato and pepper, as they cannot tolerate extremely high temperatures that might develop under the covers (especially polyethylene). Covers must be removed for crops requiring insect or wind pollination.
Slitted or perforated row covers are clear polyethylene films with slits cut or holes drilled to provide ventilation when the plastic loosens under hot conditions. Under cool conditions, the plastic is taut and the slits remain closed. Very little water condensation occurs under perforated plastic covers. There is generally less frost protection under slitted or perforated row covers than under a solid cover.
Plastic row covers will require support with wire hoops. A piece of No. 9 wire cut about 65" long makes a hoop that is about 3' wide at the base and 14" tall in the center of the row after inserting each leg of the hoop in the soil. Secure the edges of the cover with soil.
If you have a diversified vegetable and/or berry operation, row covers can be a cost effective and convenient tool for producing early, high quality crops. Edges are usually held down with soil, soil-filled bags, boards, smooth saplings or tree limbs or rocks. Row covers provide sufficient growth enhancement by raising air temperatures during the day and moderating cold temperatures at night. They also allow light and water to penetrate to the crop. The result is earlier harvests, and in some cases, higher total yields. While row covers do not provide reliable frost protection, they may be helpful when temperatures drop 2º-3ºF below freezing. Heavyweight covers can provide more frost protection, but they block much of the sunlight, resulting in slower growth. A key benefit of row covers is that, if they are sealed along the edges, they exclude a wide range of insect pests that can damage crops.
Which types to use? There are several different weights, measured in ounces per square yard or grams per square meter. Materials that are 0.5-0.6 oz/yd2 provide growth enhancement and insect control, have high light transmission (85%-90%), and are less expensive than heavier materials, but are more likely to rip from wind and sharp objects (fingernails, boots, deer hooves, stakes, etc.). One can expect 2 seasons with careful handling. A row cover that is 0.9-1.25 oz/yd2 is heavy enough to be more tear resistant and last several seasons, has somewhat lower light transmission (70%), and provides growth enhancement and some frost protection in spring and fall. The heaviest covers are 1.25-2 oz/yd2, have lower light transmission (30%-40%), are used mainly for frost protection or for overwintering, and are durable enough to last for several seasons when handled with care. Non-heating row covers are useful when an insect barrier is needed during the hot part of the season.
Support and Fastening. Many crops can handle floating covers without any support, including lettuce, greens, crucifers, onions, potatoes, strawberries, sweet corn. Those with tender, exposed growing points (tomatoes, peppers, and vine crops) should have some support to prevent damage from wind abrasion. Wire hoops or short stakes with a smooth top to prevent tearing provide good support. Secure the edges of the cover with soil, with soil-filled plastic bags, or with metal or plastic pins or staples. For holding the cover in place, soil is the most secure in high winds, but the edges are difficult to unearth after repeated wetting and drying, while soil bags make it easier to lift or move covers and prolong the life of the material.
Widths. Row covers can be purchased in widths ranging 3-60 feet and in lengths 20-2,550 feet. Wider covers are more labor efficient because they have less edge to bury per covered area - but don't try to lay them in a strong wind!
Weed control. Watch for weed growth under the cover because they provide a good environment for weeds too. Covers can be rolled to the edge of the bed for cultivation, and then replaced.
Storage. Row covers should be stored away from direct sunlight as soon as they are removed from the field. While many have been treated to reduce UV degradation, they will last longer if unnecessary UV exposure is prevented. Fold or roll covers in a systematic way so they can be carefully unfolded for next year's use.
Insect Control. Only use covers on rotated fields because some insects overwinter in the soil and could emerge under the cover. Also, seal the edges of the cover immediately after installation. If you pull it back for cultivation, do so when insects are less active, such as on a cloudy day or in the morning.