Plastic Mulch and Row Covers

Plastic mulch and row covers are cultural tools that can improve earliness and yields of many vegetable crops. The primary function of these materials is environment modification, and their effectiveness is strongly influenced by the weather during a particular growing season. Row covers can also have pest management benefits. They can be used as a part of an overall production management program.

Plastic Mulch Films

Plastic mulch is generally 0.75-1.25 mils thick, 4-6 feet wide, in rolls 1,000-4,000 feet long. It is available in a multitude of colors ranging from clear (transparent) to opaque (black or brown). Recently, colored mulches have been investigated for their influences on insect control and plant yields. For example, reflective or silver mulches have been shown to reduce the incidence of onion thrips and aphids. Check with your local extension office for the most recent research findings proven to work in your area.

Plastic mulch functions to warm the soil, conserve moisture, and prevent nutrient leaching. It also protects ground-level fruit from soil pathogens. However, plastic mulch restricts rainwater from reaching to the roots. Therefore, drip irrigation should generally be used with plastic mulch. Clear plastic has the highest soil warming capability (8-14ºF over bare soil), but weed growth underneath can be extreme. An herbicide is necessary to keep weeds under control with clear mulch. Black mulch will prevent weed growth by prohibiting light transmittance to the soil and will warm the soil 3-5ºF over bare ground. On the other hand, white-on-black (white on the top) mulch is used to cool the soil.

Wavelength selective or near-infrared transmitting mulch (formerly referred to as IRT mulch, but now "IRT" is part of a trade name) is a "hybrid" of black and clear mulch characteristics. They are more expensive than conventional plastics. Specific pigments incorporated into the film during manufacture selectively block out blue and red wavelengths of light (which cause weeds to grow). This inhibits weed growth similar to black mulch. At the same time, infrared light is transmitted through the mulch warming the soil (similar to clear mulch). The wavelength selective mulches are generally brown or green in color. However, don't purchase them on color alone. The pigments embedded in the plastic impart these specific properties. Commercial recommendations are to lay wavelength selective mulches 7 days prior to transplanting. Within reason, the additional cost for this mulch film is compensated for by increased yields due to early soil warming. On small farms or in small fields, black, brown, or wavelength selective mulches are often the preferred way to eliminate the use of herbicides. This is a viable option for weed control on many organic farms. Crops that respond best to mulching are those that require higher soil temperatures (e.g. muskmelon, watermelon, cucumber, squash, tomato, pepper, okra, and sweet corn).

Apply plastic mulch after fields have been leveled and smoothed and fertilizer has been applied, and when there is good soil moisture (at or near field capacity, which is the amount of moisture left after a rain or irrigation event after surplus water has moved out of the root zone by gravity). In the case of black mulch, good uniform soil contact is essential as the soil is warmed by heat conduction. Commercially, the simplest way to apply mulch film is with a mechanical mulch layer. Plastic mulch can be laid flat against the ground or on raised beds. Raised beds offer additional soil drainage and early warming. Hand application is an option, but applying more than a half-acre can be difficult and time consuming.

Generally, plastic mulch is laid in the spring as soon as the land can be prepared. However, some spring seasons are wet and can delay normal land preparation and planting activities. An alternative is to lay plastic in the fall. Fall mulch application will require similar land preparation as in the spring, but use of a cover crop between the rows is recommended to prevent soil erosion. Oats will winter kill, but winter rye will need to be terminated by using an herbicide (such as Roundup or Gramoxone), or by mowing and cultivation.

After harvest, plastic mulches should be removed from the field and disposed of properly according to local ordinances on incineration and landfills. Alternatives to minimize disposal challenges of used PE are biodegradable mulch films and recycling programs to alleviate landfill accumulations. Recycling is very difficult to implement because mulches are dirty after field use, recycling facilities are limited, and it can be challenging to transport used plastic to recycling facilities. Soil and plant debris adhere to the mulch, adding up to 70% by weight and the presence of soil can abrade the recycling equipment. Research is ongoing to assess the potential for recycling the plastic into higher value products through pyrolysis and other chemical recycling methods that can accept some level of soil and debris in the used plastics.

Biodegradable Plastic Mulch

Degradable plastic mulch has been in development for decades. Some of the first commercialized products were photodegradable, and would break down when exposed to light. Many growers who used these products reported uneven and incomplete breakdown, particularly after tillage buried the plastic fragments at the end of the season. However, degradable mulches prepared from biodegradable polymers now exist. They are designed to be tilled into the soil after their service life, after which they will undergo aerobic biodegradation by soil microorganisms, producing CO2, water, and microbial biomass.

The most widely available and studied biodegradable polymer is Mater-Bi, made in Italy by Novamont. Some mulches that use this polymer are Bio360 and BioTelo (Dubois Agrinovations) and BioAgri (BioBag Americas). Mater-Bi is made primarily from starches, cellulose, vegetable oils plus proprietary biodegradable complexing agents derived from renewable, synthetic, or mixed sources. While Bio360 mulch is approved for use on European organic farms, at this time no biodegradable plastic mulch is approved for use on USDA-certified organic farms. This is because currently available biodegradable plastic mulches have a maximum 25% biobased content while one of the requirements of National Organic Program is that the mulch must be completely biobased. Further, most commercially available biodegradable plastic mulches are produced through fermentation using genetically modified yeast and bacteria for increased productivity, and that is not allowed in US organic agriculture. US organic regulations do allow the use of synthetic (polyethene) mulches, but they must be removed from the soil at the end of the growing season.

Research has shown that the biodegradable plastic mulches performed comparably to polyethylene mulch in controlling weeds, raising soil temperatures and increasing crop yields despite some breakdown of biodegradable mulch during the growing season. Biodegradable mulch does not have a significant impact on soil quality. Research at Washington State University modeled five years of mulch degradation data from a field study and predicted the timeframe of 21 to 58 months for 90% degradation of biodegradable plastic mulch after tillage. As biodegradable mulch starts to degrade during the growing season, mulch adhesion to fruit surface can be an issue for heavy-fruited crops like pumpkin and watermelon, where fruits rest on the mulch for extended period. Up-to-date information can be accessed at the Washington State University Small Fruit Horticulture Research & Extension Program's Plastic Mulches page,

Biodegradable mulches can range from 2-3 times the cost of standard black plastic, but end-of-season labor and disposal costs are avoided. The mulch is thinner (it comes in 0.5-0.8 mil thicknesses) than typical black polyethylene (1.25 mil), and when starting to lay the plastic, extra care is required to prevent tears. When laying mulch, do not stretch as tightly as you normally would with black plastic. Applying in early morning when temperatures are cooler can help. The mulch starts to break down more quickly when stretched. Apply right before planting because the mulch will start to break down as soon as it makes soil contact. Buy what you need each year – do not try to store biodegradable mulch. The mulch can start to break down in storage, particularly if storage conditions are moist and/or warm. Store the mulch upright, on ends of rolls. The mulch can start to degrade or stick together under pressure of its own weight. Biodegradable plastic mulches undergo degradation even under ideal storage conditions and may perform best if deployed within 2 years of their receipt date.

WeedGuardPlus (Sunshine Paper Co.) is a brown paper mulch with soil-cooling properties. It is OMRI listed and is effective under low rainfall and low wind conditions. WeedGuardPlus is also effective in controlling nutsedge unlike polyethylene and biodegradable plastic mulches. However, it is more expensive than biodegradable plastic mulch. 

Slitted and Floating Row Covers

Row covers function to enhance growth and yield by modifying the temperatures around plants in the spring and fall, or in combination with low tunnels, during the winter. They are also used for frost, hail, and wind protection, and to exclude certain pests. There are two general types: slitted or perforated plastic, and spun-bonded fabric. Heavier weight row covers can provide several degrees of frost protection, while lightweight "non-heating" or summer weight covers offer less heat enhancement and can be used in summer for insect protection. These materials can be used with or without hoops ("floating row cover") depending on its weight and the fragility of the crop underneath. Newer types of knitted or woven lightweight row cover (for example, 'ProtekNet' by Agrinovations) are available; they can be used with or without hoops, are quite durable, and will exclude insects. 

Row covers are installed right after planting and are left covering the crop for several weeks, depending on crop type and season. For fruiting crops and cucurbits, covers can be left in place for approximately 3-5 weeks until pollination is needed or the crop outgrows the space under the cover. Other crops that are low-growing and do not require pollination can remain under cover as long as the temperature benefit is useful. Sweet corn may be left covered with spun-bonded row cover until pretassel stage. If the crop is pressing against the cover, either loosen or remove it. Row cover removal timing is more critical in some crops, such as tomato and pepper, as they cannot tolerate extremely high temperatures that might develop under the covers (especially polyethylene). Covers must be removed for crops requiring insect or wind pollination.

Slitted or perforated row covers are clear polyethylene films with slits cut or holes drilled to provide ventilation when the plastic loosens under hot conditions. Under cool conditions, the plastic is taut and the slits remain closed. Very little water condensation occurs under perforated plastic covers. There is generally less frost protection under slitted or perforated row covers than under a solid cover. 

Plastic row covers will require support with wire hoops. A piece of No. 9 wire cut about 65" long makes a hoop that is about 3' wide at the base and 14" tall in the center of the row after inserting each leg of the hoop in the soil. Secure the edges of the cover with soil.

If you have a diversified vegetable and/or berry operation, row covers can be a cost effective and convenient tool for producing early, high quality crops. Edges are usually held down with soil, soil-filled bags, boards, smooth saplings or tree limbs or rocks. Row covers provide sufficient growth enhancement by raising air temperatures during the day and moderating cold temperatures at night. They also allow light and water to penetrate to the crop. The result is earlier harvests, and in some cases, higher total yields. While lightweight row covers do not provide reliable frost protection, they may be helpful when temperatures drop 2-3ºF below freezing. Heavyweight covers can provide more frost protection, but they block much of the sunlight, resulting in slower growth. A key benefit of row covers is that, if they are sealed along the edges, they exclude a wide range of insect pests that can damage crops.

Which types to use? There are several different weights, measured in ounces per square yard, ounces per square feet, or grams per square meter. Materials that are 0.5-0.6 oz/yd2 provide growth enhancement and insect control, have high light transmission (85-90%), and are less expensive than heavier materials, but are more likely to rip from wind and sharp objects (fingernails, boots, deer hooves, stakes, etc.). One can expect 2 seasons with careful handling. A row cover that is 0.9-1.25 oz/yd2 is heavy enough to be more tear resistant and last several seasons, has somewhat lower light transmission (70%), and provides growth enhancement and some frost protection in spring and fall. The heaviest covers are 1.25-2 oz/yd2, have lower light transmission (30-40%), are used mainly for frost protection or for overwintering, and are durable enough to last for several seasons when handled with care. Non-heating row covers are useful when an insect barrier is needed during the hot part of the season.

Support and fastening. Many crops can handle floating covers without any support, including lettuce, greens, crucifers, onions, potatoes, strawberries, sweet corn. Those with tender, exposed growing points (tomatoes, peppers, and vine crops) should have some support to prevent damage from wind abrasion. Wire hoops or short stakes with a smooth top to prevent tearing placed at 3- to 6-foot intervals provide good support. Secure the edges of the cover with soil, with soil-filled plastic bags, or with metal or plastic pins or staples. For holding the cover in place, soil is the most secure in high winds, but the edges are difficult to unearth after repeated wetting and drying, while soil bags make it easier to lift or move covers and prolong the life of the material.

Widths. Row covers can be purchased in widths ranging 3-60 feet and in lengths 20-2,550 feet. Wider covers are more labor efficient because they have less edge to bury per covered area - but don't try to lay them in a strong wind!

Weed control. Watch for weed growth under the cover because they provide a good environment for weeds too. Covers can be rolled to the edge of the bed for cultivation or herbicide application, and then replaced.

Storage. Row covers should be stored away from direct sunlight as soon as they are removed from the field. While many have been treated to reduce UV degradation, they will last longer if unnecessary UV exposure is prevented. Fold or roll covers in a systematic way so they can be carefully unfolded for next year's use. 

Insect control. Some insects overwinter in the soil where the crop was grown, and emerge in the next spring. In such cases, only use row covers on rotated fields. Also, seal the edges of the cover immediately after installation. If the cover is removed for cultivation, it should be done when insects are less active, such as on a cloudy day or in the morning.

Insects That Can be controlled by Row Covers

Cabbage root maggot fly. This pest is a concern in spring or fall crucifer crops. Pupae overwinter in the soil wherever they fed on fall brassicas. First generation adults fly from April to May and lay eggs at the base of the crop stems. Maggots feed on roots and kill early cole crop seedlings. Immediately after planting, place spunbonded row covers in the field and seal the edges to keep cabbage maggots out. It is important to rotate crops as pupae can overwinter in the soil and flies may emerge under the row covers and damage the crop. 

Flea beetles. There are many different species of flea beetles, each with a specific host crop. Because they typically spend the winter as adults around field edges, they can be effectively excluded by row covers if covers are in place soon after planting. Crucifer and striped flea beetles are tiny, black or striped beetles which cause shot-hole feeding patterns on any of the cabbage family crops. Covers can be used with spring or fall transplants, or all summer on direct-seeded crops, but are too hot for transplants in midsummer. Potato flea beetle causes similar damage to eggplant, tomato, and potato. Corn flea beetles cause feeding damage but are primarily a concern because they vector Steward's wilt. Excluding beetles with row covers prevents infection of young corn plants.

Spinach leafminer and Beet leafminer. These are pests of spring spinach, beets, and chard. The adult black fly emerges from overwintering sites in the soil and lays small eggs in the underside of leaves. Maggots tunnel inside the leaf, making unsightly pathways that render greens unmarketable. Row covers prevent flies from laying eggs on the leaves.

Striped cucumber beetle. This is a pest of cucumber, melons, summer squash, winter squash, and pumpkins. Row covers prevent feeding damage and transmission of bacterial wilt vectored by the beetle. Remove when flowers appear to allow for pollination by bees.

European corn borer. Adults emerge in late May or early June and lay eggs on corn. If row cover is left on into mid- to late June, after flight is peaked, it provides excellent protection to corn. If removed just as flight starts (e.g., first week in June) the larger, healthy corn that was covered may be just as infested as corn that was never covered. Row covers can be left on until tassel if enough slack is left for 3 to 4 feet of stalk growth.

Colorado potato beetle. This insect moves into potatoes and eggplant in late May and early June. Row covers should be removed before tuber initiation, which usually coincides with flowering, to prevent excessive heat.

Potato leafhoppers (PLH). Adults migrate from southern states where they overwinter. Adults usually arrive, reproduce and damage beans, potatoes and sometimes eggplants in June and July, but may last until September. Feeding causes a symptom known as hopper burn, where tips and edges of leaves begin to yellow, curl and die back. Adults and nymphs hide on the underside of leaves. Row covers can be used on beans from emergence until bud stage or the start of bloom. If removed for bloom, damage can be avoided and yields maintained. Lightweight row covers can also be used on potatoes to exclude PLH, flea beetles and Colorado potato beetles.