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To begin your cover, build hoop supports of PVC pipes over which your cover material will rest. Attach 8-inch sections of 1-inch solid PVC pipe to the inside long walls of the bed, securing them with tube straps. Space the pipes no more than 4 feet apart and position them directly across from one another. Then slip 1/2-inch flexible PVC pipe (or stronger tubing such as electrical conduit) into these solid supports. For a 4-foot-wide bed, the lengths of flexible pipe should be about 6 feet long.
Now you can cover the hoops with a fabric that suits your needs, securing it with purpose-specific clips, available from garden suppliers. For frost and insect protection, cover the hoops with floating row cover material or poly film. Row covers are typically spunbonded polyester or polypropylene, which are extremely lightweight and can protect plants from frosts down to about 26 F. Heavy-duty poly film, such as polyethylene, polycarbonate, or polyvinyl chloride, will turn your raised beds into mini greenhouses, retaining heat and protecting from frosts down to 22 F.
Poly film is sold by thickness up to 24 mils. The higher the mils, the greater the protection, but less light and air can penetrate it. The heavier films are available from farm supply centers or online greenhouse suppliers, but the 4- to 8-mil poly film found at the hardware store or garden center is sufficient for most people and most uses. Be sure to get poly with UV light inhibitors, which lasts longer than untreated plastic. You can repair any tears or punctures with clear poly film repair tape.
While it keeps your plants safe from hail, wind, and other threats, poly film can trap heat when temperatures start to rise. So remove it during the day or when all danger of frost is past, or buy vented film. Furthermore, any covering will also keep out pollinating insects that some crops need, so you may need to remove the film when the bees are buzzing around.
In regions with hot summers, exposure to too much sun can scorch seedlings and cause leafy greens to bolt. Prevent this by covering your crops with shade cloth. It can reduce light (and hence heat) up to 90 percent, though most home gardeners should look for 50 percent blockage provided by a UV-stabilized woven or knitted fabric in green, white, or black.
All these coverings will keep out flying and hopping insects. But if birds are your primary pest problem, cover the hoops with bird netting (especially valuable for seedlings and fruit).
Four-footed pests like deer, rabbits, dogs, cats, raccoons, and squirrels will make short work of any fabric or plastic coverings. To discourage these visitors, you'll need chicken wire or hardware cloth that surrounds the bed rather than covers it. Pound 2-foot sections of rebar or wooden stakes about 18 inches into the ground at each corner of the bed. Slip 2- to 4-foot pieces of 3/4-in solid PVC over the stake, then secure the wire mesh to the posts. That way you can easily slip the posts and wire off the stakes when you need to tend to your beds.