Watermelon is a $215 million industry in the southeastern United States. This region produces 50% of U.S. conventional watermelon but only 7% of U.S. organic watermelon (U.S. Department of Agriculture, 2014, 2016a). Because organic sales have increased more than 80% since 2007, wholesale prices of organic watermelon are twice that of conventional watermelon, and 80% of organic products are sold within 500 miles of the farm (USDA, 2014). Therefore, there is unmet market potential for organic watermelon in the southeastern United States. However, research to support and improve organic watermelon production in the region is lacking. Although there have not been many studies of organic watermelon production, one study evaluated the impact of reduced tillage on watermelon production and found no statistically significant differences between reduced tillage and tillage treatments (Butler et al., 2016).
Weeds are severe in the humid, subtropical southeastern United States, and they are a major threat to watermelon yield of both conventional and organic systems. Weeds compete with the crop for space, light, and nutrients, can promote disease and insect damage, and impede pollination and harvest (MacDonald, 2000). Typically, watermelons are transplanted 3 to 4 weeks after sowing and are harvested 8 to 10 weeks later. Approximately 1 to 2 weeks after transplanting, vines begin to enter row middles, making chemical or mechanical weed control nearly impossible. Although vigorously vining in nature, watermelon plants are poor weed competitors early during their growth cycle; plants must be sown or transplanted ≈2 m2 apart and require 4 to 6 weeks to create a closed canopy with adjacent plants, thus allowing ample space and time for weeds to emerge. Various weeds, even at low densities, have been shown to impact watermelon yield throughout the growing season. Large crabgrass (Digitaria sanguinalis) was shown to impact watermelon yield for up to 6 WAT (Monks and Schultheis, 1998), smooth pigweed (Amaranthus hybridus) impacted the yield up to 3 WAT (Terry et al., 1997), and american black nightshade (Solanum americanum) impacted the yield up to 4 WAT (Adkins et al., 2010). Densities as low as 2 weeds/m2 of yellow nutsedge [Cyperus esculentus (Buker et al., 2003)], goosegrass [Eleusine indica (Wallinder and Talbert, 1983)], and american black nightshade (Gilbert et al., 2008) have been shown to reduce watermelon yield.
Conventional watermelon production in the southeastern United States relies on synthetic preemergence and postemergence herbicides (Culpepper and Smith, 2016), which are not permitted in organic production (USDA, 2016b). Organic weed control must use an integrated approach that may include the following: cover cropping, crop rotation, stale seedbed preparation, competitive crop genotype selection, tillage, and mechanical weeding (Bàrberi, 2002). When watermelon plants begin to vine at ≈1 to 2 weeks after transplanting, cultivation is limited, with hand-pulling and mulches being an organic grower’s few options for weed control.
Organic weed-control for other vining crops has been investigated. For example, in Washington, pumpkin (Cucurbita pepo) and butternut squash (C. moschata) yields comprised 80% of conventional production when a no-till roller crimper system was used (Luna et al., 2012). A variety of mulches, including shredded newspaper, shredded newspaper and grass clippings, hardwood bark chips, and black polyethylene plastic, were evaluated in another study and were shown to reduce the total weed biomass by 78% or more for pumpkin production (Splawski et al., 2016).
Weed control during organic production is a serious barrier to economic sustainability because mechanical weeding, including hand-weeding, is estimated to cost growers up to 20-times more than conventional herbicide programs (Gianessi and Reigner, 2007). Klonsky (2012) reported 192%, 168%, and 110% increases in weed-control costs for organic vs. conventional production of tomato (Solanum lycopersicum), broccoli (Brassica oleracea var. italica), and lettuce (Lactuca sativa), respectively, in California. Identifying cost-effective organic weed-control strategies would greatly benefit organic growers. To our knowledge, this study is the first to determine the cost of weed-control for organic watermelon production.
A more long-term approach to sustainable weed management for organic production may be the breeding and selection of competitive crop varieties. To breed weed competitiveness, the qualities that allow high watermelon yields in weedy conditions or under a specific weeding regime must be determined (Pester et al., 1999). At this time, no studies relating the watermelon plant architecture to weed competitiveness are available. We hypothesized that short-internode plants, which have a compact growth habit, are well-suited for weed management in an organic system. Compact plants develop a denser leaf canopy that may shade competing weeds. Nonsprawling vines may be easier to hoe and hand-weed than a traditional vine-type variety. Although the vine length of commercial watermelon varieties can exceed 4 m, the compact variety selected for this study, called Companion, has vines that grow to only 1 m in length. Vine-type watermelon varieties are typically grown 1 m apart, and compact-type watermelon are grown 0.5 m apart (Fig. 1). Although ‘Companion’ was bred to be a nonharvested pollinizer for seedless watermelon production and is not grown commercially, it was included in this study to determine if its unique compact growth habit trait conferred improved weed management in an organic system that could be exploited in plant breeding.
The objectives of this study were to determine the optimal weed control regime to reduce hand-weeding costs while maintaining the yield in an organic system and to evaluate the compact growth habit, a nontypical watermelon trait, for weed competitiveness and improved hand-weeding efficiency.
AdkinsJ.I.OlsonS.M.FerrellJ.A.StallW.M.SantosB.M.2010Critical period of interference between american black nightshade and triploid watermelonWeed Technol.24397400
BoyhanG.E.GranberryD.M.KelleyW.T.2000Soils and fertility management p. 3-5. In: D.M. Granberry and W.T. Kelley (eds.). Commercial watermelon production. Univ. Georgia Coop. Ext. Bul. 996
BukerR.S.IIIStallW.M.OlsonS.M.SchillingD.G.2003Season-long interference of yellow nutsedge (Cyperus esculentus) with direct-seeded and transplanted watermelon (Citrullus lanatus)Weed Technol.17751754
ButlerD.M.BatesG.E.Eichler InwoodS.E.2016Tillage system and cover crop management impacts on soil quality and vegetable crop performance in organically managed production in TennesseeHortScience5110381044
CulpepperA.S.SmithJ.C.2016UGA weed control programs for watermelon in 2016. Univ. Georgia Circ. 1080
FakeC.KlonskyK.DeMouraR.L.2009Sample costs to produce mixed vegetables: Tomatoes winter squash melons. Univ. California Coop. Ext. Bul. VM-IR-09
GilbertC.A.StallW.M.ChaseC.A.CharudattanR.2008Season-long interference of american black nightshade with watermelonWeed Technol.22186189
LunaJ.M.MitchellJ.P.ShresthaA.2012Conservation tillage for organic agriculture: Evolution toward hybrid systems in the western USARenew. Agr. Food Syst.272130
MacDonaldG.2000Weed control in watermelons p. 22. In: D.M. Granberry and W.T. Kelley (eds.). Commercial watermelon production. Univ. Georgia Coop. Ext. Bul. 996
MonksD.W.SchultheisJ.R.1998Critical weed-free period for large crabgrass (Digitaria sanguinalis) in transplanted watermelon (Citrullus lanatus)Weed Sci.46530532
NortonJ.D.CosperR.D.SmithD.A.RymalK.S.1985AU-Jubilant & AU-Producer quality disease-resistant watermelon varieties for the south. Auburn Univ. Agr. Expt. Sta. Circ. 280
SplawskiC.E.RegnierE.E.HarrisonS.K.BennettM.A.MetzgerJ.D.2016Weed suppression in pumpkin by mulches composed of organic municipal waste materialsHortScience51720726
TerryE.R.StallW.M.ShillingD.G.BewickT.A.KostewiczS.R.1997Smooth amaranth interference with watermelon and muskmelon productionHortScience32630632
U.S. Department of Agriculture2014Census of Agriculture: Organic survey 2014. U.S. Dept. Agr. Rpt. AC-12-SS-4
U.S. Department of Agriculture2016aVegetables: 2015 Summary. U.S. Dept. Agr. Washington DC
U.S. Department of Agriculture2016bPart 205 - National Organic Program. 10 Nov. 2016. <http://www.ecfr.gov/cgi-bin/text-idx?c=ecfr&sid=3f34f4c22f9aa8e6d9864cc2683cea02&tpl=/ecfrbrowse/Title07/7cfr205_main_02.tpl>
WallinderC.J.TalbertR.E.1983Goosegrass interference with watermelon growth. Proc. Southern Weed Sci. Soc. p. 158