The advantages of using plastic mulches for the production of high-value vegetable crops have been recognized since the late 1950s (Emmert 1957, Schales and Sheldrake 1965, Waggoner et al. 1960).
Some of the advantages that plastic mulch provides are as follows.
The greatest benefit from plastic mulch is that the soil temperature in the planting bed is raised, promoting faster crop development and earlier harvest. Black plastic mulch can give a harvest earlier by some 7-14 days, while clear plastic may advance the harvest date by 21 days.
Soil water loss is reduced under plastic mulch. As a result, a more uniform soil moisture is maintained and irrigation frequency can be reduced. The growth of plants on mulch can be twice that of plants in unmulched soil. Because larger plants will require more water, mulching is not a substitute for irrigation.
Fewer Weed Problems
Black and white-on-black mulches will reduce light penetration to the soil. Weeds cannot generally survive under such a mulch.
Reduces Fertilizer Leaching
Excess water runs off the impervious mulch. Fertilizer beneath the mulch is not lost by leaching, so that fertilizers are optimally used and not wasted.
Reduces Soil Compaction
The soil under plastic mulch remains loose, friable and well-aerated. Roots have access to adequate oxygen, and microbial activity is enhanced.
Root Pruning Eliminated
Cultivation is eliminated, except in the area between the mulched strips. Weed growth in these areas can be controlled by cultivation or by the use of a chemical herbicide.
Cleaner Vegetable Produce
The edible product from a mulched crop is cleaner and less subject to rot, since no soil is splashed onto the plants or fruit. To keep plants clean, they should be grown in a raised bed that is firm and slightly convex, with the highest point down the center of the row, while the plastic should be stretched tight to encourage the run-off of water. There should be no puddles on the mulched beds.
Mulch film is nearly impervious to carbon dioxide, which is necessary for photosynthesis. Research has shown that high levels of carbon dioxide may build up under the plastic. Because the film does not allow the gas to penetrate, it has to escape through the holes punched for the plants and a "chimney effect" is created, resulting in abundant CO 2 for the actively growing leaves.
Mulches increase the effectiveness of chemicals applied as soil fumigants. Because of the impervious nature of the plastic mulch, it acts as a barrier to gas escape and keeps gaseous fumigants in the soil.
Reduces Drowning of Crops
Water is shed from the row area by the raised tapered bed so that excess water runs off the field, thus reducing drowning and other excess soil water stress.
Assists in Insect Management Strategies
Use of reflective mulches to delay onset of aphid vectored viruses in summer squash has been well documented in the research literature (Black 1980, Lancaster 1987, Conway et al. 1989 and Lamont et al. 1990).
There are also several disadvantages to using plastic mulch:
Removal and Disposal
A major problem with plastic mulch is removing it from the field after completion of the cropping season for proper disposal. Plastic mulches, especially black plastic, do not break down and should never be disked or incorporated into the soil. Clear plastic does break down over time, but small pieces may remain in the field for several years. Research is now addressing this problem, and several new photodegradable and biodegradable mulches have been developed. This removal/disposal issue is being vigorously debated by the plastics industry, university scientists and agricultural producers in such forums as the American Society of Plasticulture, which has formed a Plastics Disposal Committee to investigate recycling and energy reclamation of agricultural plastics.
Greater Initial Costs
The use of plastic mulch will increase the cost of production for a given crop. These costs should be offset by increased income due to earlier harvests, better quality fruit and higher yields. Mulch Types in Commercial Use in the United States
There are three primary non-degradable mulch types used commercially in the production of vegetable crops: black, clear, and white (or white-on-black). Black plastic is the most popular, because it retards weed growth and warms the soil in the spring. Clear mulch is used in the northern United States because it provides an even warmer soil environment (mini-greenhouse effect) than black plastic mulch. However, a drawback to clear mulch is that it will require the use of a herbicide or fumigation to prevent weed growth beneath the mulch. White or white-on-black mulch will provide a cooler soil temperature than either clear or black. This mulch is good for establishing crops under hot summer conditions (e.g. fall tomato).
In the early 1960's, it was recognized that photo- or biodegradable plastic could offer a solution to the disposal problem associated with plastic mulches. Work on biodegradable starch-based film (Otey and Westoff 1980) and photodegradable polyolefin polymer and polyethylene polymer films (Carnell 1980, Ennis 1987) is under way. However, resulting mulches have been quite variable in their rate of degradation (Chu and Matthews 1984 and Wien 1981).
Recently, newer photodegradable products have shown more satisfactory degradation characteristics when tested in different regions of the USA (Clough and Reed 1989, Sanders, et al. 1989; Kostewiz and Stall 1989; Johnson 1989 and Lamont and Marr 1990). Three major commercial products are Plastigone, an ultraviolet-activated, time-controlled biodegradable plastic; Biolan, an agricultural mulch film designed to photodegrade according to a predetermined schedule into harmless particles, which then biodegrade into carbon dioxide; and Agplast, a photodegradable material made by Lecofilms.
Another major family of plastic mulches are the reflective mulches. The reflective properties of a aluminum-faced plastic have been shown to interfere with the movement of aphids by reflecting both short and long wave-lengths of light from their surface, disorienting the aphids' flight. Several species of aphids are vectors for watermelon mosaic virus (I and II), zucchini yellows mosaic virus, and serious diseases of squash, watermelon and pumpkin. These viruses cause symptoms on the plant leaves and fruits that can range from a mosaic of green/yellow to deformation of squash fruit into knobby, misshapen forms. Random probing of hosts and nonhosts by winged aphids makes it difficult if not impossible to achieve acceptable control of these viruses by insecticide spray programs in a given crop (Broadbent 1957). Painting the shoulders of black mulch with aluminum paint increases its reflectivity, and is also effective in delaying the onset of virus symptoms in squash fruit (Lamont et al. 1990). By using reflective mulch, a grower may be able to harvest marketable produce for a longer period of time.
The last and also the newest, type of mulches are the photoselective films. These mulches transmit a high proportion of solar infrared radiation, while blocking most photosynthetically active radiation (Loy et al. 1989). The reason why this new family of mulches is under development is because such plastics improve soil warming properties while retaining adequate weed suppression under the mulch. These mulches are a compromise between clear and black mulch films, providing soil temperatures midway between those under black and clear mulches, but largely suppressing the weed growth which is a problem with clear mulches (Loy and Wells 1990).
In commercial applications, mulch is typically laid down by machine. There are basically three operations involved in applying the mulch: 1) bedding the soil, 2) pressing the bed, 3) laying the plastic mulch, drip tube and fumigation (if needed). These can be accomplished as separate operations or in combination.
Bedding and Pressing the Soil
Several bedding machines are available to growers, including both single and multiple row models. With "superbedders", the soil is raised and bedded in one operation. With other types of machinery, the soil is first raised in one operation with hilling discs or double disc hillers on a tool bar. The bed is then compressed to a uniform height and density, using a bed press pan. The bedded rows should be spaced with their centers 1.5 to 1.8 m apart, depending on the equipment. A bed 10-15 cm high, 76 cm wide, with a slope from the center to the edge of 3.17 cm is commonly used for vegetable production. Such a slope will allow excess rainfall to run off the mulch.
Application of the Mulch,Drip Tube and Fumigation (If Required)
The soil must contain adequate moisture for seed germination when the plastic mulch is laid. Temperatures should be at least 10Â°C, and the soil should be well worked, and free from undecomposed plant plant debris if a fumigant is used. Fumigants are used primarily to control nematodes and soilborne diseases. Researchers are currently exploring the potential of delivering a fumigant (e.g. Vapam) via the drip/trickle irrigation tubing, not only to resanitize the area under the mulch cover between crops, but to actually kill the remains of the previous crop.
Plastic mulch comes in rolls 1.2 or 1.5 m wide and 608-730 m long. The thickness is usually 125-150 microns (1.25-1.50 mm). Mulch is available with an embossed (diamond shape design) pattern on the film which helps to hold the mulch tight against the soil.
Laying the Mulch
The grower should take time to adjust the machine so that the press wheels hold the plastic firmly against the bed, and the covering discs place soil halfway up the side of the bed but not on top of if.
Only approved herbicides can be used between rows of plastic. A cultivator or hoe can be used to control the weeds, but the grower must be sure not to rip or tear the mulch.
Planting the Crop
Vegetable crops can be established in plastic mulch by transplanting or direct seeding. Transplanting can be accomplished by hand or with a mechanical transplanter. For very early crops of green pepper and tomato, large container cells with a diameter of 5-7.6 cm should be used. For other vegetable crops, a cell size between 2.5 and 5 cm is generally suitable. When plants are being set in by hand, several tools can be used to make holes in mulches. A long-handled bulb setter, or a can or steel cylinder welded onto the end of a handle, can be used. The hole should be 7.6-10 cm wide, and deep enough to hold the plants. A hand tobacco setter works well once holes have been made in the plastic mulch. With both hand setting and machine setting, the use of a "starter solution" â€” a soluble fertilizer high in phosphorous (P) â€” will often get the plants off to a good start. A suitable proportion would be 10-52-17 or 10-20-10. (see Table 1 for suggestions on plant spacing when plastic mulch is used).
Drip irrigation is recommended for use with plastic mulches. Plastic mulch should not be used without irrigation of some kind.
Double Cropping with Plastic Mulch
Once the first crop has been harvested, a second crop should be grown on the mulch. Intensive cropping of this type gives two crops for a single annual expenditure on mulch and drip tubing ( Table 2). The second crop can be fertilized through the drip irrigation line ("fertigation"), using soluble fertilizers and a fertilizer injector. Fields should never be planted twice in the same crop in one year. Diseases and insect cycles can best be broken by a rotation of three or four years.
Windbreaks, consisting of strips of winter wheat, rye or barley, should be established to protect vegetable seedlings from prevailing winds. Each strip of grain should be the width of a small grain drill. Enough room should be left between grain strips to accomodate five or six mulched beds 1.5 or 1.8 meters apart. Grains strips planted in the fall will promote early growth of, and protect, young vegetable transplants. Spring fertilization of the strips by topdressing will help assure a dense stand.
Significant Yield Increases
Plastic mulch systems can produce significant yield increases, if managed properly. Some examples are given in Table 3.
With proper planning, attention to detail and careful management of all aspects of the cropping sequence, earlier and higher yields are possible using plastic mulches.
- Black, L.L. 1980. Aluminum mulch: less virus disease, higher vegetable yields. Louisiana Agriculture 23: 16-18.
- Broadbent, L. 1957. Insecticidal control of the spread of plant viruses. Annual Review of Entomology 2: 239-354.
- Carnell, D. 1980. Photodegradable mulch eliminates costly removal steps. Proceedings, National Agricultural Plastics Congress 15: 94-96.
- Chu, Chang-chi and D.L. Matthews. 1984. Photodegradable plastic mulch in central New York. HortScience 19: 497-498.
- Clough, G.H. and G.L. Reed. 1989. Durability and efficiency of photodegradable mulches in drip-irrigated vegetable production systems. Proceedings, National Agricultural Plastics Congress 21: 42-45.
- Conway, K.E., B.D. McCraw, J.E. Motes and J.L. Sherwood. 1989. Evaluation of mulches and row covers to delay virus diseases and their effects on yield of yellow squash. Applied Agricultural Research 4, 3: 201-207.
- Emmert, E. 1957. Black polyethylene for mulching vegetables. Proceedings, American Society of Horticultural Science 69: 464-469.
- Ennis, R.S. 1987. Plastigoneâ„¢ a new, time-controlled, photodegradable, plastic mulch film. Proceedings, National Agricultural Plastics Congress 20: 83-90.
- Johnson, H. 1989. Plastigone photodegradable film performance in California. Proceedings, National Agricultural Plastics Congress 21: 1-6.
- Kostewicz, S.R. and W.M. Stall. 1989. Degradable mulches with watermelons under north Florida conditions. Proceedings, National Agricultural Plastics Congress 21: 17-21.
- Lamont, W.J., K.A. Sorensen and C.W. Averre. 1990. Aluminum strips on black plastic mulch reduces mosaic on yellow squash. HortScience 25, 10: 1305.
- Lamont, W.J. and C.W. Marr. 1990. Muskmelons, honeydews and watermelons on conventional and photodegradable plastic mulches with drip irrigation in Kansas. Proceedings, National Agricultural Plastics Congress 22: 33-39.
- Lancaster, D.M., H.K. Whitam, and L.L. Black. 1987. Reflective mulch delays virus spread in summer squash. Agriculture 30: 16-17.
- Loy, B., J. Lindstrom, S. Gordon, D. Rudd, and O. Wells. 1989. Theory and development of wavelength selective mulches. Proceedings, National Agricultural Plastics Congress 21: 193-197.
- Loy, B. and O. Wells. 1990. Effect of IRT mulches on soil temperature, early vegetative development in muskmelon and weed growth. Proceedings, National Agricultural Plastics Congress 22: 19-27.
- Otey, F.H. and R.P. Westoff. 1980. Biodegradable starch-based plastic films for agricultural applications. Proceedings, National Agricultural Plastics Congress 15: 90-93.
- Sanders, D.C., C.A. Prince and P.P. David. 1989. Photodegradable plastics in North Carolina. Proceedings, National Agricultural Plastics Congress 21: 11-16.
- Schales, F.D. and R. Sheldrake. 1965. Mulch effects on soil conditions and muskmelon response. Proceedings, American Society for Horticultural Science 88: 425-430.
- Waggoner, P.E., P.M. Miller, and H.C. DeRoo. 1960. Plastic mulching: Principles and benefits. Connecticut Agricultural Experiment Station Bulletin, New Haven 634-44 pp.
- Wien, H.C. 1981. Are photodegradable mulches practical for New York State growers? Proceedings, National Agricultural Plastics Congress 16: 103-110.
- Does red colored mulch give any significant increase in tomato production?
- A. I have not myself tried color-selective mulches, but I think that in future we are likely to see an increase in the use of colored mulches: yellow for insect control, blue etc. For example, yellow is known to attract insects, and we could use a yellow strip running along the plastic mulch with an insecticidal oil. However, it is difficult to obtain small quantities of colored mulch for testing. (W.N. Chang)
Index of Images
Table 1 Suggested Plant Spacing for Vegetables Grown on Plastic Mulch
Table 2 Suggestions for Double-Cropping of Vegetables Using Plastic Mulch
Table 3 Average Yields (Kansas, Usa) of Crops Grown under Plastic Mulch with Drip Irrigation