Growing Magazine - March, 2010

FEATURES

A Rainbow of Colors to Increase Yields

Plastic mulch comes in more than just basic black
By Marcia Passos Duffy

Using plastic mulches to extend the growing season is not a new idea. Black plastic mulches have been routinely used by commercial growers for this purpose since the early 1960s.

Tomatoes growing in red plastic.
Photo by the American Society for Plasticulture.

What’s new is the rainbow of colors that are now available that have taken plastic mulches to an entirely new level. The variety of colors—from blue and green, orange, yellow, red and even metallic silver—reflect color waves that help certain crops grow faster, stronger and produce more fruit.

Color mulches: dressing for success?

Researchers across the country have studied the effects of these varying colors and plastics on plant growth and found that the right colors placed under the right crops can provide good weed and insect control and give a grower two to three times more row crops that mature faster and earlier while being grown on less land and using half as much water.

One study conducted by the U.S. Department of Agriculture’s ARS found that tomatoes flourish when planted with red plastic mulch (with yields increased by 10 to 15 percent); they also discovered that potatoes grow best with pale blue, white plastic or shiny silver mulch (with 15 percent more potatoes harvested), and turnips grow best, and even tastier, when planted with orange plastic mulch.

The reason the different colored mulch works to increase yields is that plants are super-sensitive to the color of the reflected light and react and grow accordingly. The colored mulch “tricks” the plant into sensing that there’s intense competition for resources with other plants, and it will put more energy into growing in order to outgrow other plants.

USDA researchers discovered that the best color for the mulch depends on whether the grower is producing a root or plant crop. In general, the less red color reflected (with the white and blue mulches) means the plant will grow more in the roots. More red spectrum (in reds and oranges) means more shoot growth.

Black mulches warm the soil, but colored mulches do more

Black has traditionally been the color for plastic mulch in the northern states to absorb warmth (used throughout New England, Middle Atlantic states, the Ohio Valley and central states regions). Clear mulch is also used in northern states to warm up the soil, although keeping weed growth down under clear mulch is a problem. In the warmer southern states, white plastic film is used to reflect the sun’s heat and keep down the soil temperature for maximum growth.

All plastic mulch can create a microclimate in the soil to create better conditions for plant growth, says Michael Orzolek, professor of vegetable crops and director at the Pennsylvania State Center for Plasticulture (http://plasticulture.cas.psu.edu), who has been working with plastic mulch colors and reflective material for the past 35 years to look for innovative ways to extend the growing season.

Orzolek’s research into color plastic mulches has confirmed that different colors are most effective on specific plants, regardless of the region.

Red Mulch

“I recommend red for tomato and eggplant,” says Orzolek. Red mulch has many of the weed suppressing and warming properties of black mulch, but tomatoes, in particular, like the extra dose of red light and increase yields. Research has also shown that red mulch reduces the incidence of early blight (compared with plants grown on black mulch). Other research has shown that other crops respond with higher yields when grown on red mulch such as peppers, melons and strawberries. Red mulch has also been found to suppress nematodes.

Silver (Reflecting) Mulch

Orzolek found that silver reflecting mulch increases yields in onions, summer squash and peppers. Peppers in particular respond more to silver mulch when compared to black mulch, and had an average of 20 percent increase in marketable yield as well as fruit size. Silver also repels thrips, aphids and whiteflies. Potatoes and onions responded well to a variety of different colors, including red and metallized silver, but potatoes grown on metallized silver had the highest marketable yields, coolest soil temperature and the least number of Colorado potato beetle adults. In cool years, however, the metallized silver mulch may reflect too much heat and result in a lower yield.

IRT (Infrared Transmitting) Brown and Blue-Green Mulch

This type of mulch provides good weed control, much like black mulch, but is better at warming the soil. Special pigments that are added to film during production block out blue and red wavelengths and inhibit weed growth. These mulches are available in brown or blue-green colors. Orzolek said that the green color is good for cantaloupes (with 35 percent more marketable yield), cucumbers (30 percent more marketable yield), watermelons and summer squash.

Yellow Mulch

Yellow mulch attracts insects (whitefly, cucumber beetle and some aphids) and can serve as a trap that prevents damage to the plant itself.

Investment costs

Color plastic mulch costs more than black plastic mulch. Based on 8,000 linear feet of plastic mulch per acre, the cost of the colored plastic mulch (two 4,000-foot rolls) plus installation would cost $275 to $300 per acre, depending on the type and color; black mulch costs roughly half that price.

The investment is worth it, says Orzolek, with a 10 to 1 return on investment for vegetables. “Also, expect greater market yields, higher fruit quality, better postharvest storage and earlier maturity of the crop,” he says.

Installation tips

Plastic mulch is best installed in the spring as soon as the soil can be worked. Orzolek says that installation is ideal after the grower is done with the primary and secondary tillage to the field. The grower can then install a raised bed, mulch layer and a drip irrigation applicator unit. The mulch can be applied by hand, but it is awkward and time-consuming; a mechanical mulch layer will do the job quicker.

Never use any kind of plastic mulch without an irrigation system in place, since the plastic won’t allow water to seep through. Researchers recommend that growers place drip irrigation tape beneath the plastic mulch 2 to 4 inches beneath the soil surface. Also, make sure beds are level, the drip irrigation tape is in a straight line in the center of the planting bed and water is pumped through the irrigation system right after transplanting.

Since many growers inject soluble fertilizers through their drip irrigation system, Orzolek recommends that growers only apply 40 percent to 60 percent of their N-P-K requirement per acre and than inject the additional N-P-K in a soluble form during the growth and development of the crop during the growing season.

Disposal issues

There are two downsides to color—or any type—of plastic mulch. First, it only lasts one growing season, and, second, it is hard to dispose of when it has outlived its usefulness. After harvest, plastic mulch needs to be removed from the field and disposed of properly. Recycling agricultural plastics is still not widely available; most municipalities require these plastics to be taken to the landfill or a municipal incinerator. Also note that if you are a certified organic grower, there are restrictions in using plastic mulches. For example, PVC plastics cannot be used and all plastic mulch has to be removed from the field before it degrades.

One alternative is to use biodegradable mulch, which can be plowed into the soil after use, although a range of colors are not currently available. Mater-Bi is the most widely known and available material in agricultural biodegradable mulch. It is made by Dubois Agrinovations under the trade name BioTelo, but colors are limited to black and clear (which the company recommends for use with sweet corn only).

The future of colored plastic mulch

Using colorful mulches is an opportunity for growers to use a simple technology to grow more abundant and profitable vegetables.

One way to be sure that this kind of plastic mulch is right for your growing operation is to conduct your own research. This season, experiment with a row or two of crops using the suggested colored plastic mulch for your crop. At harvesttime, compare size and yield with those crops that are planted with basic black mulch.

Colored Plastic Mulch Supplier

Robert Marvel Plastic Mulch
Annville, Pa.
Phone: 717-838-0976
www.robertmarvel.com

Beemis Plastics
Rissler-Marvel Farm
Harrisburg, Pa.
www.bemisplastics.com/agriculture/

Climagro
St-Laurent, Que., Canada
Phone: 514-454-5352
www.climagro.com

American Society for Plasticulture
166 Horvath Drive
Elysburg, Pa. 17824
Phone: 570-898-3710
Fax: 888-577-6801
E-mail: info@plasticulture.org
www.plasticulture.org

The author is a freelance writer from Keene, N.H.