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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. Mulches are one such tool, and an important one.
A plastic mulch can do some or all of several things. First, it greatly reduces 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 use by the gardener.
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 deal 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.)
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 ends 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, which has a 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 soil warmth and the inpenetrability of the mulch to moisture, the underside of a clear-plastic mulch is normally covered with condensed water. That water is transparent to the incoming shortwave radiation from the sunlight, but opaque to the outgoing longwave infrared re-radiation from the soil. In English, that means that the combination of mulch-plus-moisture-film lets energy go down from sun to soil, but blocks energy from going up from soil to 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? Nowadays, that very thing is possible, with the advent of so-called "Infra-Red Transmitting" (IRT) plastic mulches. These wonderful toys absorb photosynthetically active radiation (PAR) but transmit solar-infrared radiation (IR); since it's the PAR that energizes plants--the weeds we don't want under the mulch--they're 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 (intermediate between black and clear mulch in soil-temperature effects).
(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 transmissivity--such as IRT 76 or IRT 100.)
In recent years, it has been shown that selecting a plastic mulch of the right color--with "right" depending on the 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. The colored mulches reflect particular wavelengths of sunlight back up onto the leaves of the plants above them.
Once the basic principle was realized, expermienters tried different colored mulches and discovered that different vegetables thrive on different light wavelengths, and hence on different-colored mulches.
(When reading anything about colored plastic mulches, it is very important to look at the date of the article: this is a very fast-developing field, and reports from the middle or even late 1990s may, or may not, be well out of date.)
Research continues, most notably at the Pennsylvania State University Plasticulture Center; most of the information to be found today is simply their work parroted back by some other source.
In a summary they set forth in 2000, they noted that though many vegetables can be grown successfully using plastic mulches, cucumbers, eggplant, muskmelons, okra peppers, squash, tomatoes, and watermelons showed the most significant responses. By vegetable, their notes, here quoted nearly verbatim, are:
Cucumber - this crop appears to respond better to dark-blue mulch (as compared to to black), with an average 30% increase in marketable fruit yield over a 3-year period (there was a difference in yield response between an open-pollinated and hybrid variety). Lowest yields of marketable cucumber at this location were from plants grown on yellow mulch, but in more southerly climates, below North Carolina, cucumber response to yellow mulch may be entirely different.
Eggplant - this crop appears to respond better to red mulch (as compared to black), with an average 12% increase in marketable fruit yield over a 2-year period. The greatest response of eggplant to red mulch was observed when plants were growing under stress conditions of temperature and water). There may be varietal differences of response in eggplant to the use of plastic mulch.
Muskmelon - this crop appears to respond better to green IRT or dark-blue mulch (as compared to black), with an average 35% increase in marketable fruit yield over a 3-year period. The lowest yields of marketable cantaloupe at this location were from plants grown on either white or black mulch. In more southerly climates, below North Carolina, cantaloupe response to white or black mulch would be entirely different.
Onion - this crop appears to respond to several different mulch colors, including red, metallized silver, and black, as compared to no plastic mulch at all, with an average 24% increase in marketable bulb yield over 8 varieties. There was a significant difference in yield response between specific onion varieties and mulch color. (This trial evaluated red onions, but other onion types should respond the same as the red-onion varieties grown in this trial.)
Okra - [no comment in the summary]
Pepper - this crop appears to respond better to silver mulch (as compared to black), with an average 20% increase in marketable fruit yield and fruit size over a 3-year period. The lowest yields of marketable peppers at this location were harvested from plants grown on either white or light-blue mulch. In more southerly climates, below North Carolina, pepper response to white mulch would be entirely different. Pepper plants grown on green IRT had marketable-fruit yields similar to plants grown on black mulch.
Potato - this crop appears to respond better to any of several different mulch colors, including red, metallized silver, and black, compared to no plastic mulch, with an average 24% increase in marketable tuber yield. There was a significant difference in yield response by mulch color by specific potato variety. Generally, though, potatoes grown on a metallized-silver mulch may have the highest marketable tuber yields, coolest soil temperatures, and fewest Colorado Potato beetle adults. (But field-laying metallized-silver mulch to a suitably tight fit over raised potato beds can be difficult--and in cool years a metallized-silver mulched potato crop may, compared to black or red plastic mulched, have the lowest plant population owing to poor plant emergence. Use of black or possibly red plastic mulch will produce the highest yield of quality potatoes.
Summer Squash - this crop appears to respond more to dark-blue mulch compared to black with an average 20% increase in marketable fruit yield over a 2 year period. Lowest yield of marketable zucchini squash was harvested from plants grown on yellow mulch at this location. In more southern climates, below North Carolina, cucumber response to yellow mulch may be entirely different.
Tomato - this crop appears to respond more to red mulch compared to black with an average 12% increase in marketable fruit yield over a 3 year period. There appears to be a reduction in the incidence of early blight in plants grown on red mulch compared to plants grown on black mulch. When environmental conditions for plant growth are ideal, tomato response to red mulch is minimal.
Watermelon - [no comment in the summary]
Why onions were reported on even though not listed as showing "significant responses" is unclear, as is the omission of okra from the summary; watermelon, one supposes, can be assumed to respond much like muskmelons ("cantelopes"). Moreover, the potato comments, even though cleaned up a bit above, are still more than puzzling as to optimum mulch color. The metallized-silver mulch has the effect of lowering soil temperatures, which is said to increase potato tuber production, hence the concern about emergence. Our guess is that the home gardener wanting to experiment with metallized-silver plastic mulch for potatoes would be best to wait for emergence before laying down the mulch, something impossible for commercial operators but possible, by careful hand labor, for the home gardener.
In their literature, the researchers also reported that yellow plastic seems to be quite attractive to unwanted insects, and it is sometimes used in commercial practice as a "catch" for that purpose (that is, a few rows are set out here and there on yellow, and pest control vigorously applied to those particular rows).
If we turn their summary "upside-down"--that is, list by mulch color instead of vegetable--what we get is this:
Red - eggplants, tomatoes.
Metallized Silver - peppers, potatoes.
Dark Blue - cucumbers, summer squash, muskmelons ("cantelopes"), probably watermelons.
Green IRT - muskmelons ("cantelopes"), probably watermelons.
(Another source of plastic-mulch information is the American Society for Plasticulture.)
As a possible alternative to plastic, colored-paper mulches are being developed. Their benefits are chiefly to commercial growers, who would be relieved of the task of taking up plastic after the growing season, as the paper sorts are intended to biodegrade. Such materials are still under development.
First, get this clear: folklore and many reports notwithstanding, black plastic mulches in the home garden will, in general, not provide much soil-warming effect; it takes heroic efforts to get plastic mulch tautly bonded to the underlying soil well enough to make the kinds of differences potentially available. Their real use is conserving soil moisture and defeating weeds. For material soil warming (that does not rely on letter-perfect installation), it's clear or IRT mulches: clear ones if weed competition is not a big factor, else IRTs.
One scheme some have used is a season of black plastic, followed by a season of clear--the black having eliminated most of the weeds, and thus their seed, in its time. Also, some plants, given warm soil, will grow so quickly that their leaf canopy will shadow out most weed competition (that is especially in a deep-dug bed, where plants can be spaced close enough that their leaf growth provides essentially a near-solid canopy, or "living mulch"). Whether a season of IRT could be alternated with a season of clear is not certain, but it should work, and would not require a season of only minimal soil warming.
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).
There is some question--the literature is unclear and often contradictory--on whether colored plastic mulches provide any soil-warming benefits beyond what simple black does. Other than the IRT ones, whose color is more or less incidental to their function, there doesn't seem to be any reason why they should. Some sources report tomatoes (the favorite subject crop) growing better on red, while others find growth just as good on IRT mulches. Yet other sources claim substantial soil warming from colored mulches.
One thing that seems to emerge from the discussions is that the value of colored mulches at least, if not all mulches, decreases as conditions more nearly approximate ideal for the plant in question; in other words, they help make up for deficiencies, and probably wouldn't have much effect in a perfect garden.
In our climate, the warm-weather crops can be very roughly divided into two classes: the ordinary and the difficult. The "ordinary" would be those crops--such as tomatoes, peppers, squash, or potatoes--for which reasonable care in selecting sowing and transplanting dates, plus some "insurance" in the form of row cover or heat traps (Walls o'Water or water-filled clear-plastic bottles among the seedlings) should be more than enough in any but the strangest of seasons to assure a good crop. The difficult would be those crops which we are really stretching to grow here--eggplants, melons, cucumbers, and the like.
With the "ordinary" warm-weather crops (mostly the Solanaceae), we don't urgently need the soil-warming effect--though any extra helps--and so we'll go for the color-reflection benefits; for the tricky ones, we'll opt for the IRT type and its soil-warming effects. After a season or two, we'll see what there is to see and decide whether to change course.
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.
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).
Make sure the soil is quite moist before laying the mulch; water well as necessary to achieve that state.
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).
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.
When planting seeds or seedlings, make the hole cuts as small as possible (X-cuts usually work best.
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.
Knowledge in this field expands annually. Here are a few further sources, but only a few:
Why Plastic Mulch? (commercial site)
Biodegradable Mulch Product Testing 2006 (PDF file)
Plastic Mulch (a commercial site, included to allow a quick look at representative prices, not as an endorsement of a particular product)
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