Plastic mulches have been used commercially on vegetables since the early 1960s. Three basic mulch types have been used in commercial production: black, clear, and white-on-black plastic. 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. The degree on contact between the mulch and soil, often quantified as a thermal contact resistance, can affect greatly 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 UV, visible, and infrared wavelengths 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 5o F higher at a 2-inch depth and 3o 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 14o F higher at a 2-inch depth and 6 to 9o 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. Clear mulch is used primarily in our region of the country because it provides an even warmer soil environment –a mini-greenhouse effect– that allows the early production of sweet corn and other crops. 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 o F at 1-inch depth or -0.7o 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 a white mulch, it may require the use of a fumigant or herbicide because of potential weed growth.

The newer family of highly reflective silver mulches has been used primarily to repel aphids and thus delay the onset of virus symptoms in a fall squash crop. The soil temperatures under these mulches will be several degrees (5-8o F) cooler when compared to a black plastic mulch. In research on the effect of mulch color on yields of Irish potatoes, silver mulch had the highest yields when compared to red, black or bare ground. This may have been due to the silver mulch providing a cooler soil environment, which favors potato growth and development.

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 wavelength. These mulches absorb photosynthetically active radiation (PAR) and transmit solar infrared radiation, providing a compromise between black and clear mulches. These infrared-transmitting (IRT) mulches afford the weed control properties of black mulch but are 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.

Newer colors that are currently being investigated are red, yellow, blue, 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.

Red mulches were the first really new color to be investigated, other than the ones mentioned above and have started to be used commercially. There have been a lot of trials on tomatoes, some that have shown a benefit of either improved yields or enhanced ripening and quality of the fruit. In other trials, there has not been a response. There is also some indication from different trials that red mulch may also be reducing the severity of early blight on tomatoes. This is indeed an interesting finding that may have some real benefit. Red plastic mulch has also been shown to increase yields in zucchini and in honeydews and muskmelons. In a study in New Hampshire, researchers found that differences in reflectivity among a red, black and red on black mulch were minimal at 16 inches above the mulch surface and on the shaded side of the row. They speculate that for red mulch reflectivity to have a more sustained and more consistent effect on biomass accumulation and yield in tomato, the rows may need to be oriented in a North-South direction.

A summary of two years research at Penn State on crop response to red, brown IRT, green IRT, black, silver, white, blue (light and dark) and yellow color mulch is presented below:

1. Tomatoes.  No significant difference in the yield of marketable tomato fruit (cv. Sunbeam) from clear, yellow, black, silver, red or brown IRT mulch.
2. Peppers. No significant difference in yield of marketable peppers fruit (cv. Enterprise) from clear, yellow, black, silver, red and brown IRT mulch. Fruit grown on yellow produced the smallest fruit. Silver, red and clear mulch appeared to hasten maturity of the peppers harvested compared to the black or yellow treatments. In 1998, plants grown on silver mulch significantly produced more marketable pepper (cv. Marengo) compared to those grown on white mulch. There were no significant differences in marketable pepper among the other colors (green and brown IRT, black, red, yellow and blue). Pepper plants grown on either silver or green IRT mulch produced larger fruit compared to brown IRT, black, red, white, yellow or blue mulches. Highest soil temperature, taken on August 3, 1998, was recorded under green IRT mulch 103 o F and coolest under white (89o F).
3. Muskmelons. Plants (cv. Cordele) grown on green IRT, blue, red, silver mulch produced significantly more fruit (total yield) than plants grown on white mulch. In addition, plants grown on green IRT or blue mulch produced significantly more fruit compared to plants grown on black mulch. Larger fruit was harvested from plants grown on brown IRT mulch and the smallest from plants grown on black mulch.
4. Zucchini and honeydew. Blue-colored mulch improved yields of zucchini, honeydew.

This light reflectivity can affect not only crop growth but also insect response to the plants grown on the mulch. Examples are yellow, red, and blue mulches, which increased green peach aphid populations, especially the yellow color, which attracted increased numbers of striped and spotted cucumber beetles. There may exist the potential to use this information in developing an insect management program where a row of yellow mulch is laid in the field after a certain number of rows of whatever mulch is being used for the crop. It would be considered a trap row. In a trial in Pennsylvania, the highest yield of peppers was from yellow mulch. Since it has been proven that insects are attracted to this color, a grower has to really be on top of their scouting for insects. Yellow has long been used in greenhouses and now in high tunnels to monitor insects.

Similar to a white color mulch mentioned previously, the degree of opacity of these newer colored mulches may require a herbicide or fumigant to be used to prevent weed growth. Some of these colored mulches, for example blue and red, can have a dramatic impact on the soil temperatures, raising soil temperatures to 167o and 168o F, respectively, at the 2-inch depth when the ambient air temperature was 104 o F (my research in Kansas).

Summary. There are still many aspects of colors that we really do not understand. We know that we can build a mulch to specific spectral parameters or wavelengths and that will determine the color. The color of the mulch will influence the soil temperature, the surface temperature of the mulch and the light reflected by into the plant canopy. We know the blue color in the 440-495nm wavelength band will cause a plant response- phototropism, photosynthesis; while red color in the 625-800nm wavelength band will influence photosynthesis, seed germination, seedling/vegetative growth, and anthocyanin synthesis. Another impact on the effectiveness of a color is if the mulch is applied to a raised bed or is laid flat on the ground. This can cause a difference in the impact a mulch can have on the soil and plant microenvironment. The last consideration is the difference in color retention, film appearance, and film longevity of mulches currently on the market. This is the critical question of what really is a red, blue or yellow mulch and how best do we define it. To anyone who has ever looked at a color additive chart, the problem is readily apparent. A lot more research still needs to be done on the effect different colors have on the microclimate, vegetable crop growth, yields and earliness.

By: William James Lamont Jr., Extension Vegetable Specialist, Department of Horticulture, 206 Tyson Building, Pennsylvania State University, University Park, PA 16802, E-mail: wlamont@psu.edu

Originally published: Proceedings. 1999. New England Vegetable and Berry Growers Conference and Trade Show, Sturbridge, MA. p.299-302.

Reviewed by: T. Jude Boucher, UConn IPM, 2012

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