Organic weed control in two watermelon variety trials

Authors: Webber Iii, Charles; Davis, Angela; SHREFLER, JAMES - OSU LANE, OK; Perkins Veazie, Penelope; Russo, Vincent; EDELSON, JONATHAN - OKLAHOMA STATE UNIV.

Submitted to: Oklahoma Agriculture Experiment Station Departmental Publication
Publication Type: Experiment Station
Publication Acceptance Date: 12/30/2005
Publication Date: 3/28/2006
Citation: Webber III, C.L., Davis, A.R., Shrefler, J.W., Perkins Veazie, P.M., Russo, V.M., Edelson, J.V. 2006. Organic weed control in two watermelon variety trials. 2005 Vegetable Weed Control Studies, Oklahoma State University, Division of Agricultural Sciences and Natural Resources, Department of Horticulture and Landscape Architecture. Stillwater, OK. MP-162, p. 31-33.

Interpretive Summary: Weeds are often cited as the number one pest problem in organic vegetable production systems. The weed control methods employed must be organically approved and fully integrated into the organic production system. The objective of these experiments was to investigate the impact of different weed control systems on weed control in organically produced watermelons (Citrullus lanatus var. lanatus). Six watermelon varieties were transplanted at two locations (Lane, OK and Center Point, OK). The varieties included three seeded varieties ('Early Moonbeam,' 'Sugar Baby,' and 'Allsweet') and three seedless varieties ('Triple Crown,' 'Triple Prize,' and 'Triple Star'). The weed control system at Lane utilized black plastic mulch on the crop row, while the area between rows was cultivated to control weeds. The no-till organic system at Center Point used a mowed rye and vetch cover crop, hand weeding, and vinegar (5% acetic acid) for weed control. The plastic mulch and cultivation between crop rows was a successful method of weed control at the Lane location. Spiny amaranth (Amaranthus spinosus) was the primary weed at Lane, and only became established between rows once the extension of the watermelon vines restricted cultivation between rows. The plastic mulch at Lane provided a more satisfactory barrier to prevent weed growth than the cover crop mulch at Center Point. Although the initial plant mulch at Center Point was dense and heavy at transplanting, averaging 1.9 tons per acre, weed control was necessary 6 days after transplanting. The weeds present then consisted of two types, those that survived the cover crop mowing process [cutleaf evening primrose, (Oenothera laciniata )] and newly emerged weeds [common ragweed (Ambrosia artemisiifolia)], large crabgrass (Digitaria sanguinalis), and sandburs (Cenchrus pauciflorus)]. The cutleaf evening primrose's prostate growth habit provided sufficient plant survival during mowing and the plant's maturity helped overcome the dense plant mulch, while the newly germinated weeds developed primarily in the disturbed soil in close proximity to the transplanted watermelons. Hand removal of the germinating weeds next to the watermelon plants was a labor-intensive process, but effective. Spraying vinegar between watermelon plants and rows controlled the scattered small germinating weeds, but the vinegar was ineffective in controlling the established cutleaf evening primroses, although these plants were somewhat stunted. As watermelon vining increased, crop safety of vinegar applications decreased and weed control also decreased. When sprayed with vinegar the sandburs at the 2-4 leaf stage were controlled, while larger sandburs were only stunted. At harvest, sandburs were a serious weed problem at Center Point, not only because of the weed competition, but also because of the painful contact with the burs from the sandburs at harvest. Further research will specifically investigate the impact of weed competition on watermelon fruit quality.

Technical Abstract: Weeds are often cited as the number one pest problem in organic vegetable production systems. The weed control methods employed must be organically approved and fully integrated into the organic production system. The objective of these experiments was to investigate the impact of different weed control systems on weed control in organically produced watermelons (Citrullus lanatus var. lanatus). Six watermelon varieties were transplanted at two locations (Lane, OK and Center Point, OK). The varieties included three seeded varieties ('Early Moonbeam,' 'Sugar Baby,' and 'Allsweet') and three seedless varieties ('Triple Crown,' 'Triple Prize,' and 'Triple Star'). The weed control system at Lane utilized black plastic mulch on the crop row, while the area between rows was cultivated to control weeds. The no-till organic system at Center Point used a mowed rye and vetch cover crop, hand weeding, and vinegar (5% acetic acid) for weed control. The plastic mulch and cultivation between crop rows was a successful method of weed control at the Lane location. Spiny amaranth (Amaranthus spinosus) was the primary weed at Lane, and only became established between rows once the extension of the watermelon vines restricted cultivation between rows. The plastic mulch at Lane provided a more satisfactory barrier to prevent weed growth than the cover crop mulch at Center Point. Although the initial plant mulch at Center Point was dense and heavy at transplanting, averaging 1.9 tons per acre, weed control was necessary 6 days after transplanting. The weeds present then consisted of two types, those that survived the cover crop mowing process [cutleaf evening primrose, (Oenothera laciniata )] and newly emerged weeds [common ragweed (Ambrosia artemisiifolia)], large crabgrass (Digitaria sanguinalis), and sandburs (Cenchrus pauciflorus)]. The cutleaf evening primrose's prostate growth habit provided sufficient plant survival during mowing and the plant's maturity helped overcome the dense plant mulch, while the newly germinated weeds developed primarily in the disturbed soil in close proximity to the transplanted watermelons. Hand removal of the germinating weeds next to the watermelon plants was a labor-intensive process, but effective. Spraying vinegar between watermelon plants and rows controlled the scattered small germinating weeds, but the vinegar was ineffective in controlling the established cutleaf evening primroses, although these plants were somewhat stunted. As watermelon vining increased, crop safety of vinegar applications decreased and weed control also decreased. When sprayed with vinegar the sandburs at the 2-4 leaf stage were controlled, while larger sandburs were only stunted. At harvest, sandburs were a serious weed problem at Center Point, not only because of the weed competition, but also because of the painful contact with the burs from the sandburs at harvest. Further research will specifically investigate the impact of weed competition on watermelon fruit quality.