Commercial Vegetable Cooperative Extension Service College of Agriculture and Production With Home Economics Plastic Mulches Guide H-245 George W. Dickerson, Extension Horticulture Specialist This publication is scheduled to be updated and reissued 12/07. In New Mexico, growing vegetables commercially, yields. Black plastic mulch can accelerate crop whether for vegetable sheds or farmers’ markets, re- production as much as one to two weeks. Clear quires intensive management and marketing skills. plastic mulch has been shown to increase earli- Success involves properly managing scarce water ness as much as three weeks in northern cli- supplies, controlling weeds and other pests and sup- mates. Weed growth, however, can be a major plying a high-quality product when the price is high. problem under clear plastic unless appropriate High prices generally are associated with limited sup- herbicides or fumigants are used. Selecting the ply and high demand. crop type (vine crops like pumpkins, squash, melons) also is important in warmer areas of Mulches the state for good ground cover or shading in the summer to prevent excessive heat build up un- Mulching is an agricultural cropping technique that der the plastic. involves placing organic or synthetic materials on the • Reduced evaporation. Plastic mulches help re- soil around plants to provide a more favorable envi- duce evaporation of moisture from the soil. Irri- ronment for growth and production. Organic mulches gation frequency and amounts generally can be traditionally are used in backyard gardens and smaller reduced, although additional water may be truck gardening operations, since materials may be needed to support earlier and greater crop pro- limited and application techniques are labor intensive. duction. A more uniform soil moisture also will Organic mulches provide many of the benefits of result in less plant stress. most synthetic mulches, except soil warming and pe- • Weed control. Since black plastic mulch pre- rennial weed control. Unlike synthetic mulches, how- vents light from reaching the soil, growth of an- ever, organic mulches like hay, straw, grass clippings, nual and most perennial weeds can be pre- and compost tend to return nutrients to the soil vented. Thin black plastic mulch will not, through decomposition. however, control nutsedge. Inorganic plastic mulches have been used by com- • Improved quality. Plastic mulches help prevent mercial growers since the early 1960s, with black fruit crops from touching the soil. This reduces and clear plastics being the most popular. Tomatoes, the incidence of fruit rots and keeps fruit peppers, eggplants, vine crops and okra generally cleaner. respond well to these plastics. Plastic mulches nor- • Reduced soil compaction and root pruning. mally are used in conjunction with drip irrigation to Better weed control results in less cultivation maintain optimum soil moisture and for improved and less root pruning. Undisturbed beds also re- stand establishment. main more friable with less compaction. Weeds between beds can be controlled with directed Advantages of Plastic Mulch (Lamont, 1991) herbicides or by mechanical means. • Reduction in fertilizer losses. Flood and fur- • Earliness and greater yields. Earlier plant row irrigation techniques tend to leach nitrogen growth and earlier crop production are two of and other water soluble nutrients below the root the primary benefits of using black and clear zone. Since plastic mulch techniques generally plastic mulches. Earlier crop production gener- include drip irrigation, nutrient loss is kept to a ally results in higher market prices and higher minimum. Nutrients can be injected into the drip To find more resources for your business, home, or family, visit the College of Agriculture and Home Economics on the World Wide Web at aces.nmsu.edu system and accurately delivered to the root zone and expansion of the mulch. Also, it generally is more as needed. resistant to wind fatigue and cracking. Plastics come • Insect control. In some cases, reflective silver in rolls 2,000 to 4,800 feet long (depending on the and white plastic mulches help repel aphids and thickness) and are 3 to 5 feet wide. other insects that damage plants and are vectors Woven, black polypropylene mulches have been of viral diseases. used in the greenhouse and nursery industries for some time as weed barriers. These ultraviolet, light- Disadvantages of Plastic Mulch stabilized mulches are guaranteed to last up to five years. They allow water and air penetration while • Removal and disposal. Removing the plastic controlling weeds. These tear-resistant mulches (16 mulch after the cropping season is the biggest mil thick) can be reused year after year. In experi- disadvantage. Although removal equipment is ments with small vegetable producers in northern available, plastic tends to become brittle in New New Mexico, these mulches have been used to warm Mexico’s intense sunlight, making it difficult to the soil, control weeds, harvest rainfall and reduce remove in one piece. The “tucks,” or sides of evaporation of moisture from the soil. the mulch buried in the dirt, remain intact, since they are not exposed to the sunlight and separate Mulch Colors from the brittle mulch on the bed top. Little pieces of plastic can scatter across a field. Many A mulch’s color affects the temperatures below landfills also will not accept plastic, and it is and above the mulch through the absorption, trans- difficult to recycle. Photodegradable and biode- mission and reflection of solar energy. This affects gradable mulches have been evaluated, but re- the microenvironment surrounding the plants. The sults have been mixed. Another alternative is degree of contact (thermal contact resistance) be- woven, black polypropylene mulch with an ul- tween the mulch and the soil also affects soil warm- traviolet light inhibitor that can be reused for ing. The better contact the mulch has with the soil, many seasons. the more effective the warming properties of the • Cost. The cost of applying plastic mulch can mulch (Lamont, 1999). be quite high both in terms of materials and • Black. Black plastic mulch is the most popular equipment. The minimum equipment required color used in commercial vegetable production, includes bed-shaping equipment and a mulch especially for weed control. As a blackbody ab- applicator. Other equipment may include a sorber, this plastic absorbs most incident solar drip-line applicator (usually associated with radiation, including visible, infrared and ultra- the mulch application) and a transplanter or violet light. Much of the thermal energy, how- seed planter. The advantages of using a mulch ever, is lost to the atmosphere through convec- for earliness, increased yields, reduced water tion and reradiation. Transferring of thermal application, better weed control and higher energy to the soil can be optimized by maximiz- prices must offset the increased cost of using ing mulch contact with the soil. Soil tempera- plastic mulches. tures under black plastic during the daytime can • Management. With drip irrigation, managing be as much as 5ºF higher at a 2-inch depth and plastic mulch is more intense. Wilting plants 3ºF higher at a 4-inch depth than bare soil at the could mean a plugged drip line, while overly same depths (Lamont, 1999). wet areas could mean rodent damage to the • Clear. Soil temperatures during the daytime un- lines. Drip line problems are hard to evaluate der clear plastic can reach 8-14ºF higher at the when covered with mulch. 2-inch depth and 6-14ºF higher at the 4-inch depth than bare soil at the same depths due to a Types of Plastic Mulch greater (85 to 95%) solar radiation transmit- tance. Clear plastic absorbs very little solar ra- Most commercial plastic mulches are made of ei- diation. Water droplets that condense on the ther linear, low-density polyethylene or high-density underside of clear plastic allow solar light polyethylene. High-density polyethylene is lighter and (short-wave radiation) in, but block outgoing, stronger than the same thickness of low-density poly- long-wave infrared radiation (heat). This heat ethylene. Most plastic mulches vary in thickness from normally is lost to the atmosphere from bare 0.75 to 1.5 mil and may be smooth or embossed soil. Incoming solar radiation, however, makes (McCraw and Motes, 1991). The diamond-shaped weeds a major problem under clear plastic un- pattern on embossed plastics helps reduce contraction less controlled with a herbicide or fumigant Guide H-245 • Page 2 (Lamont, 1999). Solarizing or disinfecting of exhibit improved soil-warming characteristics, the soil has been used in some areas of the although generally not as well as clear plastic. country to reduce soilborne diseases and some Colors range from blue-green to brown weeds. To achieve sufficiently high tempera- (Lamont, 1999). tures for solarization, the soil must remain cov- ered for several weeks during the hot part of the Soil Preparation and Preplant Fertilization summer. Good soil moisture will improve ther- mal conduction of heat into the soil profile The soil should be deep plowed or disked at least (Katan, 1980). one month before bed preparation. Incorporate crop • White. Light is reflected back into the atmo- residues well. Remove all trash, rocks or clods from sphere or the plant canopy from a white plastic the field that may hinder the application of the plastic. mulch, resulting in slightly cooler (-2ºF at Preplant fertilizers can be broadcast and incorporated 1-inch depth) soil temperatures. White plastic into the beds as they are formed. Good soil moisture mulches can be used to establish crops in the (60 to 80% of field capacity) is necessary to make summer, when a reduced soil temperature might firm, smooth beds (Granberry, Kelley, Chance, be beneficial. Coextruded white on black plastic McLaurin, Harrison, Sanders, 1994). It is important mulch helps cool the soil (white) while control- that the bed be firm, so the soil doesn’t settle.