Plastic Mulches

One important component of plasticulture is plastic mulches that have been used commercially for the production of vegetables since the early 1960's, and their usage is still increasing throughout the world. Plastic mulches provide many positive advantages for the user, such as increased yields, earlier maturing crops, crops of higher quality, enhanced insect management, and weed control. They also allow other components, such as drip irrigation, to achieve maximum efficiency. Although a variety of vegetables can be grown successfully using plastic mulches, muskmelons, tomatoes, peppers, cucumbers, squash, eggplant, watermelons, and okra have shown the most significant responses. The production of strawberries and cut flowers, like vegetables is greatly improved by the use of plasticulture. The selection of which mulch type to use will depend on factors such as the crop to be grown, season of the year, whether double or triple cropping is contemplated, and if insect management is desired.

Much of the early work on the use of plastic mulches for vegetable production was to define the impact that differently colored mulches had on soil and air temperatures, moisture retention, and vegetable yields. Based on this work three main colors of black, clear, and white predominate commercial vegetable production today, although white has been replaced largely by a co-extruded white-on-black. Plastic mulches directly impact the microclimate around the plant by modifying the radiation budget (absorbitivity vs. reflectivity) of the surface and decreasing the soil water loss. The color of a mulch largely determines its energy-radiating behavior and its influence on the microclimate around a vegetable plant. Color affects the surface temperature of the mulch and the underlying soil temperature.

Another important factor is the degree of contact between the mulch and soil or by not being taut, often quantified as a thermal contact resistance, will greatly influence the performance of a mulch. If an air space is created between the plastic mulch and the soil by a rough soil surface, soil warming can be less effective than would be expected from a particular mulch. The soil temperature under a plastic mulch depends on the thermal properties (reflectivity, absorbitivity, or transmittancy) of a particular material in relation to incoming solar radiation. Black plastic mulch, the predominate color used in vegetable production is an opaque blackbody absorber and radiator. Black mulch absorbs most ultra-violet (UV), visible, and infrared wavelengths (IR) of incoming solar radiation and re-radiates absorbed energy in the form of thermal radiation or long-wavelength infrared radiation. Much of the solar energy absorbed by black plastic mulch is lost to the atmosphere through radiation and forced convection. The efficiency with which black mulch increases soil temperature can be improved by optimizing conditions for transferring heat from the mulch to the soil. Because thermal conductivity of the soil is high relative to that of air, much of the energy absorbed by black plastic can be transferred to the soil by conduction if contact is good between the plastic mulch and the soil surface. Soil temperatures under black plastic mulch during the daytime are generally 5° F higher at a 2-inch depth and 3° F higher at a 4-inch depth compared to those that of bare soil.

In contrast, clear plastic mulch absorbs little solar radiation but transmits 85% to 95%, with relative transmission depending on the thickness and degree of opacity of the polyethylene. The under surface of clear plastic mulch usually is covered with condensed water droplets. This water is transparent to incoming shortwave radiation but is opaque to outgoing longwave infrared radiation, so much of the heat lost to the atmosphere from a bare soil by infrared radiation is retained by clear plastic mulch. Thus, daytime soil temperatures under clear plastic mulch are generally 8 to 14° F higher at a 2-inch depth and 6 to 9° F higher at a 4-inch depth compared to those of bare soil. Clear plastic mulches generally are used in the cooler regions of the United States, such as the New England states. Using clear plastic mulch will require the use of a herbicide, soil fumigant, or solarization to control weeds.

White, coextruded white-on-black or silver reflecting mulches can result in a slight decrease in soil temperature -2° F at 1-inch depth or -0.7° F at a 4-inch depth compared to bare soil, because they reflect back into the plant canopy most of the incoming solar radiation. These mulches can be used to establish a crop when soil temperatures are high and any reduction in soil temperatures is beneficial. Depending on the degree of opacity of the white mulch, it may require the use of a fumigant or herbicide because of the potential weed growth. Another family of mulches includes the wave-length-selective or photoselective mulches, which selectively transmit radiation in some regions of the electromagnetic spectrum but not in the photosynthetic region. These mulches absorb protosynthetically active radiation (PAR) and transmit solar infrared radiation (IR), providing a compromise intermediate between black and clear mulch in terms of increasing soil temperature. The color of these mulches can be blue-green (IRT-76, AEP Industries Inc., Moonachie, N.J., or Climagro, Leco Industries, Inc., Quebec, Canada) or brown (Polyon-Barkai, Poly West, Encinitas, Calif.) These mulches warm up the soil like clear mulch but without the accompanying weed problem. An above-ground spectral response exists in addition to the response t o elevated soil temperatures, and may be physio-chemical (e.g. phytochrome regulation) or radiative (e.g., increasing or decreasing the heat load on the foliage). For example, in a pepper canopy, twice as much reflected photosynthetically active radiation (PAR) was measured above clear plastic mulch than above black plastic or bare soil. Although both red and black plastics raised soil temperatures similarly, higher early yields and less foliage were observed in plants grown on red plastic. Both red and black mulches reflected about the same amount of PAR, but red plastic increased the ratio of red:far-red wavelenghts (R:FR) in the reflected light. The R:FR ratio and the amount of blue light reflected toward the canopy apparently are critical. In turnips, blue and green mulches induced longer leaves and higher shoot:root ratios than white mulch. The R:FR ratio reflected from white plastic is lower than that of sunlight.

Additional colors that are being investigated currently are red, blue, yellow, gray, and orange, which have distinct optical characteristics and thus reflect different radiation patterns into the canopy of a crop, thereby affecting plant growth and development. This light reflectivity can affect not only crop growth but also insect response to the plants grown on the mulch. Yellow, red, and blue mulches increased green peach aphid populations, and the yellow mulch, which attracted increased numbers of striped and spotted cucumber beetles and Colorado potato beetles. Yellow has long been used in greenhouses to monitor the population of insects. Mulches with a printed silver surface color have been shown to repel certain aphid species and reduce or delay the incidence of aphid-borne viruses in summer squash. Similar to a white mulch, the degree of opacity of a gray mulch may require a herbicide or fumigant to be used to prevent weed growth. Some of these colored mulches (blue and red) had a dramatic impact on the soil temperatures, raising soil temperatures to 167 and 168° F, respectively, at the 2-inch depth when the ambient air temperature was 104° F.

At the Center for Plasticulture extensive research is being conducted on different formulations and well as colors of plastic mulches and their impact on plant growth and yields both in the field and in high tunnels. Results of this research and publications on this topic will be found in this section of the website.

Related Articles