Jan 17, 2011
Colored mulches have their advantages

When Joe Nunez, a University of California Cooperative Extension field adviser in Bakersfield, Calif., started working with colored and reflective plastic mulches in pepper fields, his intention was to see how well these mulches cut down on the incidence of aphids. Aphids are the main vectors in transmitting the cucumber mosaic virus CMV).

What Nunez found was not only do many colored and reflective mulches do a good job repelling aphids, but also that some of these mulches – particularly red and green mulches – seem to help increase plant sizes and yields.
When Nunez started his experiments, he realized that CMV, which can reduce yields by up to 50 percent, had become a growing problem in bell peppers and chilies in Kern County.

And while many growers were vigilant in applying systemic insecticides in their fields to kill aphids, this approach wasn’t always effective in stemming the prevalence of CMV. The reason for this is most likely because once an aphid infected with the CMV virus lands on a plant, it transmits the virus almost immediately, Nunez said.

“So, even if you apply insecticides and the aphids are killed 30 to 60 minutes later, the damage has already been done because the virus has already been transmitted,” Nunez said.

That’s where reflective mulches come into play. Insects don’t like reflective mulches because they confuse them, Nunez said.

“The sunlight coming from up above is polarized, and that’s what lets the insects know how to turn left and right. But when light is coming from the bottom as well as from the top, it’s like the light is coming from them in all different directions. This totally confuses them and they just try to stay away from it.”

In his experiments, Nunez looked at how well different colored mulches fared in comparison to various botanical oils and floating cover crops in terms of repelling aphids. Nunez found that the mulches did a much better job at repelling aphids than the other treatments. The various botanical oils actually seemed to fare worse than the control in terms of insect control. In regard to yields, the colored mulches got 35 percent higher yields than the floating cover crops and 65 percent more than the control experiment.

In terms of the mulches themselves, Nunez found that the white and silver reflective mulches were more effective than the other mulches in repelling the aphids. This was most likely because the silver and white mulches reflected more light than the other mulches, Nunez said. The problem with the mulches, though, is that they can be expensive to apply and dispose of.

That’s when Nunez made the decision to paint the dirt in the fields and the canopies of the plants white, with a mixture of kaolin and clay. This is similar to a Surround-type mixture. Both sprays, especially the one on the ground, seemed to do a good job repelling the aphids, he said.

The good news for farmers is that instead of using mulches, they can use this spray on their fields at a much lower cost.

“All the farmer would have to do is transplant his peppers, spray in the Surround material and, when he’s done, just disc his field under like he normally would,” Nunez said.

Although the Surround and the white and silver reflective mulches were most effective in repelling aphids, Nunez found that when he used green and red reflective mulches, the plant sizes grew 15 percent to 20 percent larger in the fields.

Nunez did four replicated field trials with the different colored mulches last year and this year, and so far the results look consistent, he said.

“It’s hard to explain why the plants in the red and green mulches are bigger. There are different theories about it, but no one knows for sure.”

Some people speculate that the red and green mulches heated up the soil faster, which helped the plants grow bigger, Nunez said.

“There aren’t significant temperature differences between the black, red and green mulches. But the plants in the red and green mulches sure look a lot better,” he said.

Ron Goldy, a Michigan State University (MSU) Extension educator, has also worked with different-colored mulches.

“I’ve seen growers using a whole rainbow of colors, but the primary ones growers use in Michigan are black and white,” Goldy said.

Michigan growers use black early in the season because they need the extra heat. Later in the season, in June or July when growers put in late plantings, many of them use white mulch to help cool the soil down.

Goldy said not many Michigan vegetables growers use red and green mulches.

“I’ve seen growers try red, but they never go back to it a second year because the red mulch is more expensive and they don’t find it economical. Some growers use green mulches, but it generally only works for earlier plantings.”

It’s likely, however, that colored mulches might work differently in various microclimates across the country, Goldy said.

Mathieu Ngouajio, an associate professor of vegetable crops at MSU, has also experimented with colored mulches.
“What we noticed was that some colored mulches provided the same level of weed suppression as black mulch. We also noticed that some of the reflective mulches, including the green, were repelling insects as well,” he said.

Ngouajio didn’t see any significant yield increases with the green mulch, however.

He said colored mulches might very well have their place in different growing applications. But in the Great Lakes area, the ideal mulch would be one that starts off black early in the season when temperatures are cool, then turns to white when the weather heats up.

It’s possible to design such a mulch, but the main challenge with mulches is coming up with one that’s biodegradable, Ngouajio said.

“We’ve done some work with biodegradable mulches, but what’s commercially available is too expensive for growers,” he said.

A less expensive way to dispose of the mulch is to find another way to refine it, something plastic engineers are working on now. Another possible approach to making the best use of plastic mulches is to re-use them, Ngouajio said.

“We’ve found that you can ‘double-crop’ mulches by reusing them again in areas where there aren’t problems with deer traffic,” he said. “In this case, you can cut down your use of plastic by 50 percent and the system works perfectly.”

Lisa Lieberman

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