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Colored Plastic Mulches for Vegetable Gardening

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General Principles

In northern climes, heat-loving vegetables are obviously something of a challenge to grow, or at least to grow well. Any and every trick available to the gardener is wanted. Plastic sheet mulches are one such tool, and an important one.

A plastic mulch can do some or all of several things. First, it can greatly reduce evaporation from the soil, so that water needs are easier to manage (and most heat-loving plants are also thirsty plants). Second—and the commonest reason for its use—it can hold the temperature of the soil significantly above what it would otherwise be. Third, it can virtually eliminate weeds, and thus weeding. Fourth, if its color is matched to the vegetables growing in it, it can materially enhance productivity.

Note carefully all the uses of can in those statements. Except for reducing evaporation, none of the possible benefits are automatic, but depend very much on the exact type of mulch selected, and on its proper use by the gardener.

Soil temperature

Plastic mulches have long been used by gardeners, home and commercial, in the belief that they will materially raise soil temperatures. They can do so, but application is crucial: the underside of the mulch must be in direct, firm contact with the soil over the entire mulched area. Commercial growers can manage that by use of large, expensive, highly specialized machines; home growers have to rely on a degree of care that too few use. Just spreading a sheet of plastic polyethelene over a strip of soil will most likely not provide any temperature benefit, because the plastic will be in, at best, irregular spot contact with the actual soil surface. To make plastic mulches effective as soil-temperature raisers, we need to make sure that the soil to be covered is as smoothed-out as we can make it, preferably with a slight curvature to it (highest in the middle, sloping slightly down to the edges) and that we then apply the mulch as tightly and firmly as possible, and somehow seal it well at the strip edges. (Typically, one buries the edges slightly into the soil, though laying pieces of rebar on the edges can also work.)

(It should be obvious that sheet mulch is only going to be useful for vegetables that are not closely planted, else the plastic would be riddled with holes. Such sheet mulch is for larger plants with greater spacing—most or all Solanaceae, melons, things like that.)

Plain black plastic was long the material of choice for mulching, and still predominates in cool climates (in hot areas, white plastic is often used to reduce otherwise-excessive soil temperatures). An alternative is clear plastic: it provides substantially greater soil heating than black, but at a high price—rampant weed growth under it.

The physics of the thing is this: black-plastic mulch, when struck by sunlight, absorbs most of the energy in that light at all the wavelengths of significance—ultra-violet, visible, and infrared—then re-emits that energy as long-wavelength (“thermal”) radiation. Because both faces of the sheet—the groundward and the skyward—radiate, a lot of the absorbed energy can end up lost, pumped back into the air. But, if the mulch is in thorough contact with the soil under it, a much larger fraction of the re-radiated energy will be carried into the soil, because soil has far higher thermal conductivity than air. Soil temperatures under well-fitted black plastic mulch during the daytime are generally 5° F. higher at a 2-inch depth, and 3° F. higher at a 4-inch depth, than those of bare soil.

Clear plastic, on the other hand, obviously absorbs very little solar energy itself, but passes the great majority through to the soil under it. Owing to the increased soil warmth and the inpenetrability of the mulch to moisture, the underside of a clear-plastic mulch is normally covered with condensed water. That film of water essentially acts as a filter, being transparent to the incoming shortwave radiation from the sunlight but opaque to the outgoing longwave infrared re-radiation from the soil. In plainer English, that means that the combination of mulch-plus-moisture-film lets energy go down from sun to the soil, but blocks energy from going back up from soil to the air—just like a greenhouse wall. Daytime soil temperatures under clear plastic mulch are generally 8° to 14° F. higher at a 2-inch depth, and 6° to 9°. F. higher at a 4-inch depth, than those of bare soil.

Wouldn’t it be nice if one could somehow “average out” the two mulch types, black and clear? Nowadays, that very thing is possible, with the advent of so-called “Infrared Transmitting” (IRT) plastic mulches. Those wonderful toys absorb (which is to say block) photosynthetically active radiation (PAR), the stuff weeds thrive on, but transmit solar-infrared radiation (IR), which warms the soil: the weeds we don’t want under the mulch are starved of light energy, while the heat-bearing IR goes on to warm the soil. Weeds are stopped, yet we get a heating effect notably greater than that from plain black mulch (“ Crops grown on IRT mulch will develop 7 to 10 days earlier than crops grown on black plastic (Mallikarjuna Rao, 2015).”).

(Note: “IRT” is still commonly used as a generic name for this type of mulch, but it is now formally a trade name, belonging to a particular manufacturer of it. There are apparently “grades” of it—depending on its exact degree of transmissivity—such as IRT 76 or IRT 100.)

Color Effects

In recent years, it has been shown that selecting a plastic mulch of the right color—with “right” depending on the particular plants being grown—can materially improve growth and yield. There is nothing mystic or new-age about that: it’s simple physics and biology. Plants grow by converting sunlight to chemical energy by photosynthesis, a process depending on the marvellous properties of chlorophyll. But the physics and chemistry of that process make it quite sensitive to the exact wavelength of the light—plants can’t use all light with equal facility, which is why we have to pay through the nose for fancy “grow-light” fluorescent tubes for seedling lights (and why leaves are green, not pink or black or blue). The colored mulches reflect particular wavelengths of sunlight back up onto the leaves of the plants above them, and certain plants prefer certain wavelengths (colors) of light.

Once that basic principle was realized, researchers started trying different colored mulches and discovered that different vegetables thrive on different light wavelengths, and hence on different-colored mulches. The results are, even to this day, still coming in, but it is clear that colors do have nontrivial effects, and picking the right color of mulch for a given vegetable can indeed augment its growth.

One other factor that needs consideration is “biodegradability”—the extent to which a sheet of plastic mulch will or will not degrade over time. For the commercial grower, this is one sort of problem: will the stuff remain after harvest as a clean-up problem. For the home gardener, it’s more or less the reverse: will I be able to re-use the material for more than one season? But, for both, there remains the reality that at some point the immediate usability is over, yet the plastic remains to be disposed of. If your mindset is that having the garbage truck take it away, or taking it yourself to a landfill, ends the issue, so be it; but some will have concerns about the long-term effects on the environment. Biodegradable colored mulches are under development, but they still haven’t managed to make any with no plastics at all in them, and are not certifiable as organic (and often don’t even fully degrade). So it’s up to your conscience.

Practical Usage

What to Use Where

Farther on you will find a set of links that can give you quite an education in uses of plastic mulches, but for the home gardener it can all be simplified quite a bit.

Recall that the prudent home vegetable gardener will use a bed-rotation scheme, both to help avoid diseases that can build up in the soil when the same crop is grown again repeatedly, and to help keep the soils balanced as to nutrients. Perhaps the ideal is a four-bed scheme, grouping Solanaceae (tomatoes, peppers, eggplants, and their kin); Brassicas (broccoli, cabbage, cauliflower, and their kin); corn; and root and other miscellaneous veggies.

Corn can be mulched with plastic, but it’s problematic because corn is shallow-rooted and does not transplant at all well—but you can scarcely start seeds with mulch already down. With corn, you are much better off to suppress weeds and warm the soil with an organic mulch, anything from straw to a low-growing cover crop.

The root and miscellaneous crops are mostly (though not entirely) things that get (especially in a deep-dug bed) planted quite closely, and so also are (again, mostly—lettuces are an obvious exception) not candidates for plastic mulch.

Brassicas are cool-weather crops, best planted in mid or late summer for harvest well into fall, or even into winter, because we want them to get that “kiss of frost” that sweetens them up. Thus, there is no point is using soil-warming mulches for a Brassica bed. There could be weed-suppression purposes, but for that, as with corn, an organic mulch is better.

If we seem to have discounted everything, let's not forget those Solanaceae, some of the most important vegetables we grow. For them, a red plastic mulch reportedly will increase production by 15% to 20%, plus producing crop earlier.

We can also remember that potatoes are Solanaceae. But even if you have the space and urge to grow potatoes, there is the same issue as for corn: you can’t bring rthe crop up through the mulch, and you can’t transplant into it. Again, organic mulch is your answer.

There is, however, one other crop class where plastic mulch can be a huge aid: melons. If you have a separate melon bed, then what you want seems to be green infra-red transmitting plastic mulch (mind that transmissivity: not all green mulch has it).

Also, if you have a distinct bed or part of a bed (in, say, the Miscellaneous bed) where you are growing lettuces, black plastic mulch should help there in the colder months (not in the summer!).

By the way, do not try the cheapskate’s alternative to colored mulches, which is black plastic painted by the user. The “color” requirements include a lot outside the range of human vision, and even experienced colored-mulch experts trying the trick have found that they could not come even close to meeting the standards of integrally colored plastic (their results were later evaluated with measuring equipment).

Use Notes

  1. Use drip irrigation: plants grown with mulch are plants with good-sized inter-plant separations, making drip irrigation a natural complement to the mulching. Think about whether you want the drip system under or above the mulch, and install accordingly.

  2. Lay the mulch as early as you can, to get the most soil-warming benefit by planting or transplanting time; you can even lay it in the autumn, as soon as the beds in question are free and have been treated (that is, re-spaded if necessary, and composted or fertilized as appropriate) as the new crop may require—though consider whether the material can last through a winter then spring then summer without deteriorating (experience helps here).

  3. Make sure the soil is quite moist before laying the mulch; water well as necessary to achieve that state.

  4. Just before laying the mulch, soften and smooth the soil as best possible, possibly giving it a very slight mounding—you want the plastic to bond as closely as possible to the soil surface (plus you absolutely, positively do not want rain puddling on the mulch).

  5. Stretch the plastic as taut as reasonably possible (but do remember that you’re going to have to cut holes in it for the plants, so don’t go crazy); and be sure to seal the edges well.

  6. When planting seeds or seedlings, make the hole cuts as small as possible (X-cuts usually work best).

  7. Be sure to complement plastic mulches with row-cover material of some sort.

Also keep in mind that plastic mulches are not a true commodity—there are substantial differences in quality between brands. The most obvious defect is a bleaching or changing of color during the season, which will greatly reduce the benefits of wavelength-selective mulches. Red-plastic mulches made by Sunoco and Polyon have been reported to be of good quality, but the home gardener may not have the option of selecting a brand name.

It is no longer straightforward to keep a list here of actual manufacturers, but when you see the stuff offered at retail, try to find out who the maker is and remember that Google Is Your Friend.

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