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As you’ve been flipping through the pages of all the gardening and seed catalogs that landed on your doorstep this winter, chances are you’ve seen a page or two dedicated to plastic mulches in every color of the rainbow. Plastic mulches, particularly black, white, and clear, are nothing new in the commercial vegetable production arena. However, it is fairly recently that these and other colored mulches have made their way into the realm of home gardening. Plastic mulches offer a variety of benefits: they can extend the growing season by warming garden soil, they can improve weed and insect management, they can help retain soil moisture, and they can increase crop yields and quality. Black, white, brown, red, silver, green, or blue--which color is best? That depends on the crop(s) you’re growing and the effect(s) you desire from the mulch. And because research on colored mulches is not entirely conclusive, most of these plastics are sold with a “for trial use only” caveat.
Mulches by Color
Black—The most widely used, available, and inexpensive of the colored mulches, black plastic mulch has excellent weed suppression ability because of its opacity. It is also useful for warming soil during the growing season, particularly if as much of the plastic as possible is in contact with the soil below. Research at Penn State has shown that soil underneath black plastic can be up to 5 °F warmer at a 2-inch depth and up to 3 °F warmer at a 4-inch depth than uncovered soil at the same depths. This means that plants can be set out earlier than on bare soil, and may result in earlier maturing fruit. For example, collaborative trials between US Department of Agriculture and Auburn University indicate that okra crops mature earlier with higher yields on black plastic than on bare soil. (Blue and red plastic also work well for okra.)
White—White plastic may be of less interest to us cool-weather, short-season gardeners in Massachusetts since it tends to keep soil temperatures cooler rather than warming the soil as does black plastic. The benefits of white plastic in keeping weeds at bay, retaining soil moisture, and keeping the soil cool around the roots of crops such as peas, broccoli, cabbage, and cauliflower may be more easily and inexpensively achieved with a biodegradable mulch, e.g., straw.
Brown—Brown Infrared Transmitting (IRT) plastic mulch is a fairly recent innovation. It warms garden soil better than black plastic early in the growing season and also controls weeds. IRT is a technology that combines the weed-suppressing properties of black plastic with the heat absorbing qualities of clear plastic. (Clear plastic is better at warming soil than black plastic but does not control weeds as well.) But be warned—not all brown plastic on the market is IRT, according to University of Vermont Extension.
Red—Researchers at the USDA and Clemson University noted that certain crops performed better when grown in red mulch as opposed to black mulch: tomatoes, which yielded 20% more fruit; basil, the leaves of which had greater area, succulence, and fresh weight; and strawberries, which smelled better, tasted sweeter, and yielded a larger harvest. Penn State researchers found yield increases for tomatoes and eggplants on red mulch compared to black.