What is Season Extension?
Season extension simply means providing frost protection in the early spring and the late fall. Through the use of row covers, we can plant vegetable crops sooner and keep them alive longer thus extending the growing season. In addition, most early and late-season vegetables benefit from the increased soil warmth and protection from wind and pests that row covers offer.
What Are Row Covers?
Row covers come in two flavors: floating row covers are lightweight fabrics made of spun polypropylene while hoop supported row covers are sheets of clear or white polyethylene. The floating row covers allow sunlight, rain and air through their fabric. The hoop supported row covers allow only sunlight through.
Row covers have five chief benefits:
- Row covers capture warmth which enhances plant growth and earlier yields in cooler regions.
- Row covers reduce wind damage.
- Row covers are an effective - and least toxic - form of insect control.
- Row covers can protect your more delicate crops from light frosts.
- Row covers can protect your crops from insect borne diseases.
About Floating Row Covers
Row covers have been available for nearly 20 years now. Often called floating row covers because they 'float' on top of the crops they're designed to protect, row covers are so lightweight that they do not need additional support. Some crops with a tender, exposed growing point, such as tomatoes and peppers, are an exception.
Row covers can be supported with wire hoops or short stakes to prevent wind abrasion - damage to the plants done by the row cover as it flaps in the wind.
Floating row covers are available in various weights ranging from 0.3 ounces to 2 ounces per square yard. The heavier the cover, the more degrees of frost protection it affords. Sizes range from widths of 1 yard to 60' and lengths of 20' to 2,550'. Wider row covers are more labor efficient as there is less edge to bury per covered area. Durability of the row cover is related to weight, type of material, and the additives used.
Lightweight Floating Row Covers
The lightest floating row covers are used primarily as insect barriers. These floating row covers can protect crops such as cabbage and broccoli from loopers and cabbage worms by excluding the egg-laying moths. Eggplant, radishes, and other favorites of the flea beetle are easily protected by floating row covers. Crops rotation in fields or beds planted under row covers is still necessary since overwintering insects from a previous crop can emerge under the cover. The disadvantage to the lighter floating row covers is that they are easily damaged by deer, dogs, or other animals, and are seldom reusable. The lightest row covers have a negligible effect on temperature and light transmission.
Medium Weight Floating Row Covers
Medium-weight covers are the most commonly available. Medium weight row covers are commonly used to enhance early maturity, increase early yields and total yields, improve quality, and extend the season.
Heavier Weight Floating Row Covers
Heavier row covers, those exceeding 1 ounce per square yard, are used primarily for frost and freeze protection. These row covers are also selected when extra strength and durability are required for extended-season use. The microclimate created by heavier row covers is similar to that created by medium weight row covers, but they can often be reused for 3-4 seasons or more.
Installing Floating Row Covers
Floating row covers can be installed by hand or by a variety of mechanical devices - often attached to a tractor. Floating row covers are often installed immediately after transplanting or direct seeding. The row covers are laid over the area and the edges weighted down or buried. Small-diameter concrete reinforcing bar (rebar), cut to manageable lengths, is excellent for weighting the edges. The row cover should have enough slack to allow for crop growth.
How Long Should You Leave the Row Cover On?
With self-pollinated crops, or leafy vegetables such as lettuce or cabbage, the row covers can be left on for almost the entire duration of the crop cycle. With insect-pollinated crops, such as melons, squash, or cucumbers, the row covers must be removed at the time of flowering to allow for insect pollination. The row covers may, however, be replaced after the crop has been pollinated. Treat the removal of the row covers as a hardening-off procedure. Over the course of a few days, keep the cover off for a longer and longer period. Final removal is best done on a cloudy day, preferably just before a rain. Plants will suffer more of a transition shock if exposed to sunny, windy conditions.
Be careful when using row covers with tomatoes or peppers. Heat-temperatures above 86° F for more than a few hours may cause blossom drop.
About Hoop Supported Row Covers
Row covers made of clear or white polyethylene are too heavy to float above the crop, and the material is not permeable to air or water. These row covers are slitted or punched to allow for ventilation, and attached to or laid on hoops for support. Dimensions vary, but a typical hoop is 14-18" high at the apex, and wide enough to cover one bed. These hoop supported row covers are commonly used in combination with black plastic mulch for weed control. Hoop supported row covers are often called low tunnels. These row covers offer the same benefits as floating row covers, but are more labor-intensive to work with.
Punched row covers are manufactured with small holes punched about 4" apart to provide ventilation of hot air. The punched covers trap more heat than the slitted tunnels. These row covers are best for northern areas and must be managed carefully to avoid overheating crops on bright days. These row covers are useful for peppers, tomatoes, eggplant, most cucurbits, and other warm-season crops with upright growth habit.
Hoops for supporting these row covers are commonly made from 10 gauge galvanized wire. The wires are cut to 65-75" long. Each end is inserted about 6" to 1' deep into the soil to form a hoop over a row or bed. Hoops are spaced 5' to 8' apart. The row covers are drapped over the hoops and anchored along each edge with soil. Tunnels can be set by hand or with machines that resemble plastic mulch layers.
References
Bachmann, J and Earles, R, "Season Extension Techniques for Market Gardeners", Last modified 2000-06, ATTRA, http://attra.ncat.org/attra-pub/seasonext.html, Accessed 2003-08-31.