Plastic mulch is generally 1.1 to 1.5 mils thick, 4 to 6 feet wide, in rolls of 1,000 to 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. 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. Clear plastic has the highest soil warming capability (8°F to 14°F 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 3ºF to 5ºF over bare ground.
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. 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 (muskmelon, watermelon, cucumber, squash, tomato, pepper, and sweet corn).
Apply plastic mulch after fields have been leveled, smoothed, and fertilizer has been applied, and when there is good soil moisture. 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. 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 require Roundup or Gramoxone applications in the spring to knock down.
After harvest, plastic mulches should be removed from the field and disposed of properly according to local ordinances on incineration, landfills, and open burning. Alternatives to removing plastic from fields are biodegradable mulch films and recycling programs to alleviate landfill accumulations. Both options are currently being investigated.
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 that used these products reported uneven and incomplete breakdown, particularly after tillage buried the plastic fragments at the end of the season. However, biodegradable mulches made from plant starches that are broken down slowly by soil microbes into carbon dioxide and water now exist.
The most widely available and studied of the newer biodegradable mulches is marketed and distributed under the trade name BioTelo (Dubois Agrinovations). Another commonly used product is marketed as Eco-One (Heartnut Groves). BioTelo is made from a material made in Italy called Mater-Bi (Novamont). Mater-Bi is a biopolymer made primarily from corn starch plus proprietary biodegradable complexing agents derived from renewable, synthetic, or mixed sources. While BioTelo mulch is approved for use on European and Canadian organic farms, the manufacturer had not petitioned for this material to be approved for use on USDA-certified organic farms. Because organic regulations require that synthetic mulches be removed from the soil at the end of the growing season, biodegradable plastics are currently NOT permitted for use in organic production systems. At this time, no biodegradable plastic mulch is allowed for use on organic farms. None of the commercially available biodegradable plastic mulches have been proven to meet the requirements of the National Organic Standards because there is currently no product that is entirely bio-based and/or produced without use of genetically modified organisms. Organic regulations do allow for the use of synthetic mulches, but they must be removed from the soil at the end of the growing season.
Research at Cornell University has shown that the biodegradable mulch performed comparably to black plastic in raising soil temperatures. Melon yields in comparative trials were also equivalent to those grown on black plastic mulch. Despite some breakdown of plastic during the growing season, the weed control is comparable to that obtained with plastic.
Biodegradable mulches can range from 2 to 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 to 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. Buy what you need each year – do not try to store the 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. 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.
WeedGuard Plus - Non Fert (Sunshine Paper Co.) is a black paper mulch with soil warming properties similar to black plastic and bio-based mulch films. While it is OMRI listed and is effective under good conditions, applying it with a mulch layer can be problematic. It is also a good deal more expensive than even the degradable biobased films.