Row Covers

For most crops, floating row covers require no support because they are lightweight. They literally "float" or lie directly over the crop, whether direct seeded or transplanted. Materials include perforated plastic, spunbonded polyester, and spunbonded polypropylene; they do not impede seedling emergence or subsequent growth of the crop. (For an exception, see below.) To secure the cover against wind, all the edges are buried or weighted down with sandbags, rocks, or other materials.

There are two types of these covers: perforated polyethylene which is about 1 mil thick and spunbonded polyester or polypropylene which is available in several weights (rather than in thickness).

Perforated polyethylene has a uniform pattern of 3/8-inch holes (74 holes/sq ft) for ventilation. However, the holes allow for heat loss at night and are an entry point for insects. Overall, many growers have found these covers to be very beneficial for growth enhancement.

Spunbonded covers are comprised of a thin mesh of white synthetic fibers which entrap heat and serve as a barrier to wind, insects, and varmints. Water from rain or overhead irrigation freely passes through. The weight of these covers range from 0.3 to about 2.0 oz/sq yd (10 to 68 grams/sq meter). The lightest covers are used primarily for insect exclusion while the heaviest of the covers are used for frost protection. The most common weights are 0.5 to 1.25 oz\sq yd (17 to 42 grams/sq meter). With covers under 0.5 oz there is minimal heat retention at night; and over 1.75 oz, there is a significant reduction in light transmission. The heavy covers are used for nighttime frost protection only since they do not transmit sufficient light for optimum crop growth.

Spunbonded row covers in the 0.5 to 1.25 oz range provide 2o to 4o F frost protection in the spring. In the fall, there is more protection because there is a larger reservoir of heat in the soil in the fall than in the spring. These covers perform very well in protecting late-season tomatoes and pepper from early frosts.

Floating covers require much less installation labor than hoop supported covers. The wider and longer the covers, the less labor required per unit area since only the edges are secured. These covers vary in width from 6 to 50 feet and up to about 800 feet long. One way to evaluate labor needs is to compare the time that it takes to apply one piece of cover 50 feet wide x 200 feet long (500 perimeter feet) versus the time it takes to apply 5 pieces 10 feet wide x 200 feet long (2100 perimeter feet). Regardless of width, the cover is secured by weighting down the edges (sides and ends) with soil, sand bags, stones, or long pins. In extremely windy areas, additional weighting in the middle of the cover is advisable.

Insect control is effective with spunbonded covers when all of the edges are completely sealed. For example, maggots in radish (and in other crops in the cabbage family) are controlled when a cover is applied at seeding with a complete seal around the edges. When the adult maggot flies search out the young seedlings for laying eggs, the flies will be unable to get under or through the cover. A continuous furrow of soil along the edges is probably the best method.

It is feasible, however, that row covers could actually increase insect damage on some crops. The environment under the cover is pretty "cozy" for insects, so it is important that all transplants that are to be covered are free of insects at transplanting. To prevent a heavy population of Colorado potato beetles under a cover, do not plant potatoes in the same place in successive years and use a cover the second year. Overwintering adult beetles could emerge from the soil under the cover and lay an abundance of eggs which would soon hatch into a dense population of larvae.

There are a couple of disadvantages of floating covers. One is the weed pressure under the covers. Between strips of plastic mulch or with crops such as sweet corn where plastic mulch is not used, weeds grow rapidly and competitively. When using herbicides, the highest labeled rate is generally needed to adequately control weeds. There is also some inefficiency in covering the space between strips of plastic, but here again the trade-off is reduced installation labor of wide covers versus narrow covers.

Most all vegetable crops as well as strawberries, raspberries, and cut flowers have been grown with row covers. Although the primary crops for row cover use are high value crops such as melons, tomatoes, pepper, summer squash, sweet corn, and strawberries, many growers find row covers to be valuable for a number of crops for varying reasons -- earliness, higher yields, overwintering protection, and insect and varmint control. Crop selection and cover selection are important to the economical success of the extra inputs. Crop distinctions such as temperature sensitivity, pollination methods, and growth habit dictate the type of row cover that is best to use.

Even though most crops can be grown without damage under floating covers, tomatoes and pepper are an exception. If the spunbonded material is not supported with wire hoops for these crops, flapping of the cover in the wind will damage the growing points of young plants. Also, with summer squash under windy conditions, many of the leaves might be broken by the cover. For these three crops, a series of strategically placed wire hoops will prevent crop damage.

At the Center for Plasticulture, evaluation of floating row cover materials is being conducted on crops in the field as well as crops being grown in high tunnels.