What's with the plastic mulch in vegetable gardens? I'm about to find out. Farmers and gardeners recognize the benefits of using mulch to reduce weeds and watering as well as to boost yield. However, historically I have been adverse to the oxymoronic concept of covering the ground in plastic sheeting in an organic vegetable garden. After all, where does that plastic go after use? So one year after another, I have tried various biodegradable approaches to mulching my Colorado Front Range vegetable garden, all with unsatisfactory results. And at the beginning of each season, having researched yet again different mulching approaches, I think to myself, "This year, I will achieve mulching nirvana."
The first couple summers, I didn't mulch my vegetable garden at all. Those years, as the growing season progressed, the weeds and sun won. OK, lesson learned. Try mulch. One year, I purchased rolls of biodegradable black paper mulch. While the paper mulch probably helped conserve water, by mid-summer weeds were poking through and where the renegade flora didn't breech the mulch, the birds presented a different problem, pecking and shredding the mulch in search of worms. The following year, I covered the paper mulch with grass clippings, which fermented creating a permeating stench. And I found myself engaging in the tedious process of picking grass clippings when I harvested leafy vegetables like lettuce and spinach. Last year, I tried hay, but made a crucial error when we purchased bales grass hay (albeit organic) and had a bumper crop of grass in the garden, which started growing again this spring. Made me wish I had a goat. (OK, by now any vegetable farmers or horticulturalists reading this are probably laughing out loud at my ineptitude.)
This year, I'm armed with red plastic mulch, which I purchased in 3x3' squares. Red plastic mulch is available from many on-line retailers specializing in garden supplies, plants and seeds, including Burpee and Gardener's Supply Company. Agricultural researchers have been studying the impact of various plastic mulch colors for years. While black is an effective weed barrier and clear can be used early season to heat up soil, red plastic mulch has been shown to increase yields with tomatoes, peppers, eggplants, melons and strawberries.
I must say my husband is not at all impressed with the aesthetics of the red mulch. Tongue in cheek he said, "It looks like plastic bags have blown into the garden. I hope the homeowners association bans the stuff." I prefer to think that as the growing season progresses, we'll see a lovely balance of Christmas red and green.
The impact of various mulch colors on plants is due, at least in part, to changes in soil temperature and the reflection of different wavelengths of light. Red and far-red light (waves between 600 and 800 nanometers) produce the greatest changes in plant growth. However, deciding which color mulch to use, is not only dependent on the intended crop but also geography. For example, in a 2004 study, Texas A&M researchers showed tomato plant yields were higher with silver reflective plastic mulch than with red or black mulch, probably because under the hot Texas sun, the silver reflective plastic limits the soil to a productive rather than inferno-like temperature. Colorado Extension advises that colored mulches are not necessary in sunny Colorado, suggesting black will do, whereas cloudier climates may see stronger results with red mulch versus black. None-the-less, I'm giving the red a whirl.
So just how does the the reflective light trigger increased plant yield? The plants interpret the light waves as signals that other plants must be nearby. Given plants compete for nutrients, light and water, they respond to the signal by producing photochrome, which, in turn, triggers growth in the shoots of the plants rather than the roots, leading to earlier and higher yield. Researchers have also studied the impact of red plastic mulch on flavor, aroma and nutrient content. For example, in a study published in 2001, researchers found that strawberries ripened over red mulch (as compared to black) were "20% larger, had higher sugar to organic acid ratios and emitted higher concentrations of favorable aroma compounds." In other words, the red mulch had a favorable impact on phytonutrient content, flavor and aroma.
A few caveats in considering plastic mulch. If you're aiming for a perennial strawberry bed versus annual planting, you'll need to find another mulching solution. Given the plastic mulch degrades by the end of one or two growing seasons, you'll be challenged to pull it up and replace it without damaging the strawberry plants. Also, keep in mind that if you're in a hot, sunny climate the plastic mulch that heats the soil so effectively early season can overheat roots, stems and leaves later in the season. To compensate, plant over mulch early season, so your crops grow enough to shade the mulch some. And cut circles where you place plants or seeds, rather than cutting an "x" as the hot plastic can burn the plant if it touches leaves or stems.
You may be wondering why, after all these years, I have now justified the use of plastic mulch, considering it will ultimately be landfill fodder. Well, that depends on a few "ifs." If I have increased yield, if I conserve water, and if I reduce weed-related tilling (with its related release of greenhouse gases), I will have improved other environmental aspects of my food production endeavors. Plus, I hope I will be able to use the mulch for two seasons, not just one. But if the red plastic doesn't bring me to mulching nirvana this summer, you'll be the first to know!
