Alternatives to Plastic Mulch for Organic Watermelon Production

The material below was first presented as a poster: Silvernail, Bomford, and Harvey

Anthony Silvernail, Michael Bomford, and Bryant Harvey. 2006. Alternatives to plastic mulch for organic watermelon production. Kentucky Academy of Science Meeting, Agicultural Sciences Section, November 10, 2006. Morehead, KY.

Abstract

Black plastic mulch is commonly used in conventional and organic production of watermelon (Citrullus lanatus [Thunb.] Matsum. et Nakai) to conserve soil moisture, prevent weed emergence, promote early ripening, and prevent fruit rots. Because it is a costly off-farm input with high embodied energy that generates waste instead of building soil organic matter, we set out to evaluate alternatives suitable for organic watermelon production. In 2006 we grew seeded and seedless watermelon varieties (Sangria and Charleston Jr., respectively) organically, following a winter cover crop of winter rye (Secale cereale L.) and hairy vetch (Vicia villosa Roth). We compared the standard bed preparation system – cover crop incorporation followed by mulching with black plastic to three alternative systems: 1) cover crop roll-down; 2) cover crop roll-down followed by mulching with hay; and 3) cover crop incorporation followed by mulching with newsprint and hay. Three weeks after transplanting we measured vine length and counted and identified weeds. Eight weeks after transplanting we harvested and weighed all fruit and counted and identified weeds. The black plastic system out-yielded all but the newspaper and hay mulch system. Early growth and final yields were lowest, and weed counts highest, in the roll-down system without additional mulch. The newspaper and hay mulch system was the most promising alternative to plastic mulch among the systems tested.

Introduction

Organic weed management tactics should be designed with an awareness of system sustainability, providing sufficient yields for economic sustainability, while maintaining or improving soil quality.¹ Black plastic mulch has long been used in conventional vegetable production as an effective means of weed control, which also warms the soil in spring, allowing earlier production of warm season crops like melons. Organic standards allow the use of plastic mulch, even though its high embodied energy (308 GJ/t),² off-farm origin, and annual disposal needs run counter to the standards’ emphasis on resource cycling. Many organic growers would prefer to use mulches that can be produced on-farm and incorporated into soil to build soil organic matter. Such alternatives include newspaper, hay, and roll-chopped cover crops. We set out to compare organic watermelon yield and weed pressure in soil mulched with black plastic or these alternatives.

Objectives

Determine the early growth response of the two watermelon varieties to the mulch treatments.
Evaluate the efficacy of the different mulches on weed control.
Determine the yield response of the watermelon varieties to the mulch treatments.

Materials and Methods

The experiment was conducted at the Kentucky State University Research Farm during the summer of 2006 using a randomized complete block design with four mulch treatments and three replicates. Plots were 25 m long and spaced 2.5 m apart. Mulches were:

6 mil black plastic (Plastic);
newsprint covered with hay (Newspaper, Fig. 1);
a roll-chopped cover crop covered with hay (Hay); and
a roll-chopped cover crop without hay (Rolldown) (Fig. 2).

Hairy vetch and winter rye were planted in all plots in September of 2005 at 33 and 66 kg/ha, respectively. Plastic and Newspaper treatments were prepared with two passes of a Celli Spading Machine to incorporate the cover crop before mulch application. Hay and Rolldown treatments were prepared with a single pass of a Buffalo Rolling Stalk Chopper to kill and flatten the cover crop without incorporating it. Feather meal fertilizer (Nature Safe, 10-2-8) was surface applied at 78 kg N/ha before transplanting. On 15 June, eight week-old seedless (cv. Charleston Jr.) and seeded (cv. Sangria) watermelon varieties were transplanted into each plot at 3 and 1.5 m spacings, respectively.

The longest vine from the second plant of each variety was measured three weeks after transplanting to evaluate early growth (Fig. 3). All melons were harvested, counted and weighed eight weeks after transplanting. All weeds in two 0.5 m² sample areas in each plot were identified and counted after harvest.

Figure 1. Setting up the Newspaper treatment, with a layer of hay mulch over a single layer of newsprint.

Figure 2. Unplanted plots after:

cover crop incorporation with a Celli Spading Machine (left, Plastic and Newspaper treatments);
mulching with hay (right, Newspaper and Hay treatments); and
cover crop roll-down with a Buffalo Rolling Stalk Chopper (far right, Hay and Rolldown treatments).

Figure 3. Measuring watermelon vine length three weeks after transplanting.

Results

Early season vine measurements showed more robust growth on Plastic than on the other mulches (Fig. 5). Melon growth in the Newspaper and Hay treatments was similar, with growth slowest in the Rolldown treatment.

Weed counts were highest in the Rolldown treatment for both watermelon varieties (Figs. 4 & 7). Total weed counts in the Plastic treatment were negligible, and significantly lower than with other mulches. Similar total weed counts were found in the Newspaper and Hay treatments. Dominant broadleaf weeds were smooth pigweed and lambsquarters. Dominant grassy weeds were crabgrass, goosegrass, stinkgrass, and giant foxtail. Broadleaf and grassy weeds showed similar responses to the mulch treatments.

The yield response to mulch was similar for the seeded and seedless watermelon varieties. Weed density and yield were inversely related for both varieties (Fig. 7). Melon counts and weight were much lower in the Rolldown treatment than in the Plastic treatment (Figs. 6 & 7). Yields did not differ significantly between the Newsprint and Hay treatments, or between the Plastic and Newsprint treatments.



Figure 4. The four treatments after harvest. Clockwise from upper left: Plastic, Rolldown with Hay, Rolldown without hay, and Newspaper with hay.

Figure 5. Length (+S.E.) of longest watermelon vine on each plant three weeks after transplanting. Bars labeled with the same letter do not differ significantly (Tukey's test, alpha=0.05).

Figure 6. Watermelon yield, expressed as fruit count (left) and weight (right). Bars of the same color labeled with the same letter do not differ significantly (Tukey's test, alpha=0.05).

Figure 7. Relationship between weed density and watermelon yield, as determined by data collected from four different mulch treatments and two watermelon varieties (n=3).

Conclusion

The Newspaper mulch treatment slowed early season vine growth relative to plastic mulch, but still offered the most promising alternative to plastic. Watermelon yield was depressed by approximately 33, 50 and 98%, relative to plastic, by the Newspaper, Hay, and Roll-down treatments respectively. All of the alternative treatments offer environmental benefits over plastic: They do not require mulch removal or disposal after harvest; they are more likely to build soil organic matter; and they require relatively little energy input. The yield reductions, relative to plastic, represent trade-offs between yield and environmental impact. This trade-off was least pronounced for the Newspaper treatment, making it the most likely compromise for sustainable growers seeking a balance between economic return and environmental impact.

References

Roger Fay, Graham Treloar and Usha Iyer-Raniga. 2000. Life-cycle energy analysis of buildings: A case study. Building Research and Information 28: 31-41.
USDA. 1999. National Organic Program Regulations (Standards) and Guidelines. http://www.ams.usda.gov/nop/NOP/NOPhome.html

Acknowledgements

Bryant Harvey completed his work on this project as a student intern with Kentucky State University's Research and Extension Apprenticeship Program. Watermelon seed was donated by US Seedless, LLC. Evans-Allen funds were used for all other salaries, materials, and supplies. We thank Eddie Reed and Ken Andries for help with study setup. The study will be repeated in 2007.