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Plastic Mulch Primer

by Vern Grubinger
Vegetable and Berry Specialist
University of Vermont Extension

The use of plastics -- to cover greenhouses and high tunnels, as row covers, and for soil mulches – has resulted in a horticultural revolution of sorts. Largely unheard of just two generations ago, it’s almost impossible now to find a farm that grows horticultural products without plastic. Taken together, the use of agricultural plastics for modifying crop microclimates to enhance quality and yield is known as ‘plasticulture.’

Your markets, your climate and your pocketbook all play a role in determining how much and what type of plasticulture to use. If you want to push harvests up by a week or two in order to meet market demand and fetch good prices, then plastic mulch by itself may be sufficient. To gain additional earliness, plastic mulch as well as row cover may be required. Obviously there has to be an economic benefit to using both methods in combination because the cost is higher. More extensive, and expensive, measures to extend the growing season, such as high tunnels and greenhouses, are justified only with crops that have very high economic values per unit of land.

The first use of polyethylene as a greenhouse cover in the United States was in 1948, when Professor Emery Myers Emmert at the University of Kentucky employed this material in place of glass, the traditional but more costly greenhouse covering. Dr. Emmert is known as the father of agricultural plastics in the U.S. and he developed the use of agricultural plastic through his research on greenhouses, row covers and mulches. More recently, in the Northeast, Dr. Otho Wells of the University of New Hampshire played a key role in developing plasticulture techniques suitable to our climate and markets and then he helped educate growers and colleagues about their use.

Let’s consider just one component of plasticulture: mulch. Plastic mulch has been used in commercial vegetable production since the 1960’s. The potential benefits of using plastic mulch include: soil warming, reduced evaporation, increased yield and earliness, reduced nutrient leaching and improved nutrient uptake. Of course, there are downsides, too. These include: cost of the material and greater labor and equipment expense to apply and remove the mulch, as well as disposal fees that can be significant.

There is a wide variety of plastic mulch on the market today that can reflect, absorb or transmit sunlight. The color of mulch has a big influence on how it affects the microclimate around a crop plant.

Black plastic mulch is typically used for spring-seeded crops because it increases soil temperatures about 5ºF at a depth of 2 inches compared to bare ground. Black mulches absorb heat from sunlight then pass it on to the soil below. That’s why black plastic must be laid flat and tight to the ground if it is to provide the maximum warming effect. A big advantage of black mulch is that it blocks visible light, so it is effective at reducing weed growth (except in the planting holes!).

Clear mulch is the most transparent to infrared radiation and warms the soil to the greatest extent of all plastic mulches, increasing daytime soil temperatures from about 7º to 13ºF at a depth of 2 inches. For warm season crops, such as vine crops, earlier yields and higher yields can be achieved with clear mulch. However, clear mulch is also transparent to visible light, which contributes to weed growth under the mulch.  Unless herbicides are applied under the mulch, or the crop canopy covers the plastic quickly, those weeds can become a big problem.

Infra-Red Transmitting (IRT) mulches warm the soil intermediate to clear and black mulch. They are pigmented to reduce the amount of visible light transmitted in order to reduce weed growth under the mulch. IRT mulches are usually green or brown, and they contain specific pigments that let them transmit a maximum of near infra-red radiation and a minimum of visible light. Not all green or brown mulches are IRT.

The benefit of IRT mulch is highest early in the season. Late-planted crops don’t usually show a yield improvement with IRT compared to black plastic. Cloudy cool weather can also lead to very little difference in yield between IRT and black mulch. Since IRT costs more than black plastic, it is best suited to early plantings of tomatoes, peppers, and vine crops. If you are using plastic row covers, or tunnels, with tomatoes and peppers, the air temperature over the IRT will not be as warm as over black mulch, reducing the risk of blossom drop caused by high air temperatures in the tunnels.

White or co-extruded white-on-black mulch can reduce surface soil temperatures slightly, by about a half a degree at a 2-inch depth, relative to bare soil because it reflects most incoming radiation. This mulch is useful when lower soil temperatures are desired for summer production.

A wide variety of other colored plastic mulches, including red, yellow and silver have been developed. Each of these colors has distinct effects on how light is reflected as well as the radiation balance in and below a crop canopy. That can influence crop performance.

Studies of red mulch have not shown consistent results. Red mulch sometimes increases yield of tomatoes, but not always. Different tomato varieties also appear to respond differently to the red mulch. The effect of red mulch, like that of IRT, depends on sunlight levels, so the variation results in studies may be due to different light conditions, as well as different temperature conditions. Red mulch does tend to increase soil temperatures much like an IRT, so in early plantings it is likely to promote earlier yields. When used with main season plantings, the effects of red mulch may be more variable.

Yellow is generally very attractive to insects—that’s why it is the color of sticky cards used to monitor pests in the greenhouse.  I’m not sure why a grower would want to use yellow plastic mulch except maybe to create a ‘trap crop’ that lures pests out of other parts of the field.

Highly reflective or shiny aluminum plastic mulches have been shown to repel certain aphids. They are therefore useful to reduce or delay the onset of mosaic viruses in squash, melons, and other crops, that are spread by aphids. These are the mulches of choice when insect-spread virus management is your principal goal. Recently, a mulch has been introduced that consists of a strip of black plastic down the center that is flanked on either side by metalized reflective plastic. This plastic combines the advantages of black plastic over the seed row, to help heat the soil, with the reflective characteristics of metalized plastic for insect and disease management.

For more information on plasticulture and plastic mulches, see:
Penn State Center for Plasticulture: plasticulture.cas.psu.edu.
American Society for Plasticulture: www.plasticulture.org.

Published: November 2004
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