A row cover protects plants and seedlings in fall or spring while allowing light and rain through the material. Anchor the lightweight cloth with lawn staples, bricks, or rocks to keep it from blowing away.
© Carol Michel
The chill in the air does not have to signal the end of the vegetable harvest. Even in cooler climates it’s not too late to set out transplants of cool-season vegetables, such as broccoli and Brussels sprouts, or to sow seeds for lettuces, spinach, chard, and other leafy greens. Provide a cover for young plants of these cool-season vegetables, and you're good to go for several more weeks. These same techniques can also be employed to protect plants in spring. Here are some techniques:
Row cover material is made out of lightweight, spun fabric that allows light and water to penetrate. Reemay is a widely available brand with different grades; others may be called plant protection fabrics or floating row covers. Inexpensive and reusable, construct a frame of PVC, rebar, or other rigid material to support the fabric above the plants. Reemay protects plants from frost damage to about 28 degrees. Similar materials may keep the air temperature about 4 degrees warmer under the cover than outside. Be sure to buy the grade of fabric that is appropriate for your climate. To keep the material from blowing away, anchor it with rocks, bricks, lawn staples, or zip ties.
Elevate a row cover with a frame to create a low tunnel. This protects plants from frosts in fall and spring. Add a layer of plastic over the row cover to protect plants even more.
Sometimes called a hoop house, a low tunnel usually has just enough height to accommodate the tallest plants growing under it. Heavy duty plastic is strung over hoops of rebar or PVC pipe. Heavy duty, 6-mil thick plastic protects plants when temperatures drop into the teens. Add the thick plastic over the row cover, and when the temperature outside reaches 0 degrees, the temperature inside the hoop will be about 15 degrees. In early fall, be sure to open the ends of the tunnel to keep it from getting too hot. Once winter hits, you can leave the tunnel closed.
Whether protecting plants with a row cover or in a tunnel, you can keep the inside even warmer with jugs of water. Fill clean, empty milk jugs with water and set them between plants. During the day, the sun heats up the water. The heat slowly dissipates at night and slows the drop in air temperature.
Quick covers for an emergency
Although row covers and tunnels are considered semi-temporary, they are more permanent than a quick cover for vegetables when frost threatens. Many gardeners toss blankets, sheets, newspapers or even paper bags over plants at night when frost is forecast. Remove the material during daylight hours.
Plastic is a poor material to use as a quick cover, unless it can be tented so that it does not touch the plants. The reason is that moisture from condensation forms under the plastic and if this freezes, the plants that it touches will be damaged.
Cooperative Extension Offices, http://www.csrees.usda.gov/Extension/
The Year-Round Vegetable Gardener by Niki Jabbour (Storey Publishing)
The Winter Harvest Handbook by Eliot Coleman and Barbara Damrosch (Chelsea Green Publishing)