Jose Andino, Carl Motsenbocker and Ramon Arancibia
Plastic mulch has been used in the production of warm-season crops such as watermelon and other horticultural crops to reduce water evaporation, decrease soil compaction and fertilizer leaching, modify soil temperature, control weeds and increase yield. In general, black plastic mulch is recommended in Louisiana for spring and fall vegetable production while white plastic mulch is recommended for the summer growing season.
Black plastic mulch absorbs most if not all light striking it, and the plastic becomes warm and heats the soil underneath the mulch. Warm-season vegetables, such as watermelon, are often responsive to higher soil temperatures produced by black mulch in the spring. White mulch, on the other hand, reflects most of the light and is thus often much cooler than bare soil or black mulch. Black mulch becomes too hot in Louisiana summers, so white mulch is used for warm-season vegetables like melons. Previous field research with black or white plastic mulch indicated higher watermelon yield compared to bareground culture.
Colored plastic mulches, which are wavelength selective and/or reflective, are relatively new materials that have advantages similar to black or clear mulch. They let certain wavelengths of light through and absorb or reflect other wavelengths. Colored plastic mulches are made with different dyes and other enhancements to change their basic properties. They have additional benefits related to better management of soil temperature by allowing specific light wavelengths to strike the soil. The altered quantity and quality of reflected light in the plant canopy may influence plant growth and productivity. These mulches also block the spectrum of light required for photosynthesis and therefore limit weed growth underneath.
Silver Mulches Reduce Insects
In addition to changing soil temperature and light reflectance into the plant canopy, colored mulches have been shown to influence pest pressure. Researchers have documented that yellow and blue plastic mulches often attract specific insects. In contrast, silver mulches have been shown to reduce insect pressure because of disorientation of insects around the plastic mulch. The highly light reflective silver mulch has been beneficial in commercial tomato production by reducing pest pressure and pesticide use.
The effects of colored plastic mulches on plant growth and yield have been studied in a number of vegetable crops such as bell pepper, cowpea, muskmelon, tomato and Irish potato. Little research, however, has been conducted to evaluate watermelon crop response to colored plastic mulches in the field. Other researchers have demonstrated that vegetative plant growth can be directly affected by the quality of radiation reflected from particular mulches. The effect of light quality on the growth of young watermelon plants in controlled environments was evaluated previously; watermelon plants responded to light environment changes by partitioning more sugars to vegetative parts and producing plants with longer vines. This effect in watermelon is attributed to the light reflectivity of a particular mulch color – red. Even though the effect of colored plastic mulches on watermelon is temporal until the vines cover the mulch, the hypothesis is that early plant response to the light environment can induce modifications in the plant growth that continue after the vines cover the plastic.
The objective of this research was to determine the effect of different reflective and wavelength-selective plastic mulches on insect populations, plant growth and yield of field-grown watermelon. Various colored mulches were compared with black and white mulches and bare ground (no mulch) during the spring growing season for the effect on watermelon production.
Watermelon Tested at Burden
Field experiments were conducted at the Burden Research Center, Baton Rouge, La. Raised beds were established with trickle irrigation tubing, and plastic mulch treatments were installed using a commercial plastic layer machine. Commercial colored mulch materials were used where available, or mulches were painted (blue for two years and yellow for one year) because these particular colors were not available. Containerized transplants of Honeyheart, a yellow-flesh variety commonly called “seedless,” and Sangria, a red-flesh seeded variety, were hand-transplanted into the field at four weeks into the two center rows. Crimson Sweet was used on the two outside guard rows and together with Sangria, both common watermelon varieties, served as source of pollen for Honeyheart.
Insect, disease and weed management practices were conducted in accordance with LSU AgCenter recommendations, and standard fertilization procedures – preplant and fertilizing through the irrigation system – were followed. Insect populations were sampled by counting all insects present, and main vine length was measured twice after transplanting. Plots were harvested three times, with harvests one week apart. At harvest, fruits from each plot were counted, measured and weighed individually and then classified as marketable or cull fruit for separate analysis.
Colored mulches affected cucumber beetle populations; the red and yellow plastic mulch plots had among the highest cucumber beetle populations recorded in both varieties while the silver plastic mulches had among the lowest. Our research results are similar to other research that has reported that silver mulch is effective in reducing insect populations, presumably because of enhanced light reflection and disorientation of insects.
Most mulched plots had longer vines than the bare-ground treatment with few differences in vine length among the colored-mulch treatments by four weeks after transplanting. The increased plant growth found with mulch use is presumably because of higher temperatures recorded under the plastic mulch and the enhanced growth response of watermelon, a warm-season crop.
There were no differences among mulch treatments in first and total Honeyheart harvests. Two green wavelength-selective mulches and silver-on-black (nonselective, reflective) mulch had the highest first Sangria harvest and were among the highest total Sangria harvest. Plants in plastic mulch treatments had higher yields as a result of higher fruit numbers per area or fruit per plant. Fruit weight, length and diameter and sweetness for both cultivars were not affected by colored plastic mulch treatments.
Further research is required to investigate the influence of colored mulches on watermelon growth and yield and to determine the specific physiological effects on watermelon plants.
Jose Andino, former graduate student, Carl Motsenbocker, Professor, and Ramon Arancibia, former Research Associate, Department of Horticulture, LSU AgCenter, Baton Rouge, La.
(This article appeared in the summer 2005 issue of Louisiana Agriculture.)