Protecting Harvest from Frost in Organic Gardens

It is Fall and night-time temperatures are getting into the 40s and the sunshine of summer is replaced by rain and overcast. Pole beans are still producing and carrots are not yet ready to harvest.

There is always hope of some warm weather in September, but frost can happen almost as easily, especially if you live in the states and provinces along the US-Canadian border.

When it comes to weather, being safe is a better strategy than being sorry you missed those tasty vegetables. It is time to switch the shade cloth needed to keep lettuce and spinach from bolting in the hot August weather to the row covers that help keep nighttime temperatures more to the plant’s liking.

Row Covers Create a Micro-Climate

Covering plants with a row cover can raise the ambient temperature around protected plants by 4 to 7 degrees Fahrenheit or 1 to 4 degrees Celsius. This may not seem like much, but to your lettuce, there is a big difference between 32 degrees Fahrenheit and 36 degrees Fahrenheit.

Piling straw, leaves, or other mulch around plants can help keep soil temperatures more even, but cover on either plant with mulch can also lead to rot from excess moisture or provide a haven for fungus or disease. Row covers help you avoid introducing problems while you are trying to solve a problem.

Within the protected climate created by a cover, the soil temperature remains higher than in unprotected areas, pests cannot get past the row cover to the plant, and moisture within the covered area condenses on the cover and falls back into the earth. Row covers continue to let sunlight into the protected area, although many reduce the percent of sunlight falling on the plants.

One word of caution, if daytime temperatures are in the 60s, remove the frost protection to avoid overheating the plants under cover.

Different Row Covers Meet Different Needs

Gardening row covers come in several materials and thicknesses. The most popular row cover on the market is a non-woven, lightweight polyester fabric called Reemay (rm). Designed primarily for protecting crops from insects, the 0.6 ounce per square yard polyester fabric provides frost protection down to 30¬∞F and transmission of 75% of available sunlight. You can find Reemay (rm) sellers by searching the Web for “Reemay row cover.”

Agribon is another light weight, spun-bond polyester row cover. It weighs .5 ounces per square yard and claims to transmit 83% of available sunlight. Often used by large scale vegetable farmers, it comes in a variety of widths and lengths.

Row covers are designed primarily to float over plants; you place them directly over the plants and secure them on the sides with ground staples, earth heaped around the edges, or by laying a pipe or board along the edges.

Heavier Row Covers Give Greater Frost Protection

The popularity of Reemay ™ encouraged other manufacturers to create a range of non-woven row covers.

Gardeners Supply sells Garden Quilt, a 1.2 oz. per square yard, non-woven, polyester covering that protects down to 24 degrees Fahrenheit and also protects against wind. On the downside, it transmits only 60% of available sunlight.

When used for frost protection in Fall, however, sunlight is not as critical for plant health as it is in spring when the plant is in growth mode. Gardeners Supply also has a row cover that falls between floating row cover and the Garden Quilt in weight and light transmission.

Garden Quilt can be laid directly on sturdy plants; it is better to lay it over hoops, either wire or PVC, to makes it last longer than when laid directly over plants.

By securing it tautly with ground staples, clothespins, or PVC hoop holders, the fabric gets less dirty and is less likely to be caught by a rake or hoe and torn or snagged.

Clear Row Covers Provide More Light, Less Protection

Clear plastic row covers provide some frost protection in Fall, but are primarily designed for letting in more sunlight to help growing plants in spring and summer. You can use anything from cheap, plastic drop cloths found in the paint department of hardware stores to specially manufactured row covers that may include ventilation strips to prevent heat build up or silver threads to scare away birds.

All plastic row cover should be installed on a framework that keeps it off plants. You can use bendable hoops of metal, plastic, or PVC that you poke into the ground on each side of a row. Lay the cover over the hoops and secure to the hoops with clothes pins or special hoop clips.

Around the edges, secure the cover with ground staples, earth, or, 1 x 2 boards or lightweight pipes. Using boards or pipes makes it easier to lift the cover on one or both sides to harvest or check on your plants.

To avoid the hassle of swapping clear row cover for non-woven row cover each year, leave the clear cover you use during summer in place and put heavier, non-woven row cover over the clear cover.

If you grow in containers or have large, specimen plants, read Special Frost Protection for Special Plants to learn about ways to protect individual plants from frost or freezes..