Plastic-covered trenches permit early seeding, early plant growth and some frost protection. Polyethylene is generally strentched across a trench about 7 inches deep. A mechanical mulch layer can be used to apply the plastics, mounding soil along the edges of the bed to keep the plastic taut.The application of a pre-emergence herbicide is important to control weeds. Periodic holes can be made in the plastic as temperatures rise in the trench. The plastic should be removed when plants first come in contact with the bottom of the row cover.
Floating row covers:
Floating row covers are often used to protect both cool season and warm season crops. Made of spunbonded polyester or polypropylene, covers may range from 5 to 50ft and up to 800ft long. In the manufacturing process, the polyester or polypropylene is extruded or spun in a web onto a collection belt. The small-diameter filaments are then heated and pressure treated to form a thin, whitish sheet of porous fabric. Covers may range in weight from 0.3 to 2.0 oz/sq.yd. Row covers from 0.5 to 1.25 oz/sq.yd will give 2 degrees to 6 degrees F frost protection in the spring. More protection can be realized during the fall due to warmer soils. Row covers weighing 1.25 oz/sq.yd will generally transmit enough light(70 percent) for optimum plant growth with good frost protection. Lighter-weight materials, that provide minimal frost protection, are generally used to protect crops from insects and birds.
Floating row covers are generally applied loosely over the top of the crop and secured along the edges with fabric pins and/or soil. Insect control is more effective when the sides are completely sealed with soil. Covers should be applied when there is minimum wind. Allow the cover to drape over the crop, leaving room for crop expansion. Floating row covers should not be applied over one-stemmed crops like peppers, tomatoes, eggplant or even squash, since stems can be broken on windy days.
Spunbonded polyester and polypropylene have the advantage over polyethylene in that they don't usually have to be vented. Air and water will pass relatively freely through spunbonded covers.
Hoop-supported row covers:
Unlike floating row covers, hoop-supported row covers or low tunnels are supported by wire hoops, eliminating abrasive damage to the crop associated with floating row covers. Hoops are generally made from 9- ro 10-gauge galvanized wire, 48 to 60in long, spaced 4 to 5ft apart in the row. Wires are generally shaped to form an arch. Wider tunnels will generally result in greater warming but are more vulnerable to wind damage. Crops are often irrigated with drip irrigation to help conserve moisture and reduce weed populations.
California system:The California system of hoop-supported row covers involves two sheets of 36in wide, 1.5ml clear polyethylene supported by wire hoops spaced 5 to 7ft apart. Edges of the plastic are secured with soil. Wooden stakes approximately 30in long and one inch in diameter are spaced at 10- to 25-ft intervals in the bed. A 16- gauge wire is stapled to the top of the stakes and edges of the plastic are clipped together at the wire with clothespins. Covers are closed and opened manually. Hoops are generally attached to the wooden stakes and wire for greater stability under windy conditions. A second set of wire hoops may be placed over the top of the tunnel allowing the plastic to slide down between the hoops when venting.
Perforated plastic tunnels:Solid plastic tunnels must be vented in some way to prevent heat damage to plants. Venting can be very time consuming and a major problem in windy weather. Perforated plastic allows for ventilation without removing the plastic and reduces damage from wind. Perforations are generally 3/8-inch in diameter but may vary with manufacturer. Slit row covers have the same effect. Very little water condensation occurs under perforated plastic covers.There is generally less frost protection under perforated and slitted row covers than under a solid cover.
Spunbonded fabric tunnels:Spunbonded polyester and spunbonded polypropylene fabric tunnels are two of the more popular row cover tunnels since they don't have to be vented and don't tear as easily as polyethylene. Edges are secured with fabric pins and/or with soil.
Above words are cited from George W. Dickerson's essay in October 2004
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