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 (8º-14º 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 3º-5º F 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 https://smallfruits.wsu.edu/plastic-mulches/.
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.