Glyphosate and paraquat are effective preplant burndown herbicide options for multicrop vegetable production that uses plastic mulch, but problematic weeds such as wild radish, cutleaf evening primrose, annual morningglory, or horseweed may not be adequately controlled with these herbicides alone. The herbicides 2,4-D and dicamba could help control these troublesome weeds prior to planting if they can be removed from plastic mulch and thus avoid crop damage. Treatments included 2,4-D (1,065 and 2,130 g ae ha−1) and dicamba (560 and 1,120 g ae ha−1) applied broadcast over plastic mulch a day before transplanting. Just before transplanting, treatments received either 0.76 cm of water via overhead irrigation or no irrigation. Plastic mulch samples were collected at application and planting to determine herbicide presence using analytical techniques, and cantaloupe and zucchini squash were subsequently transplanted on the plastic beds. Analytical ultra-high performance liquid chromatography revealed that 88% to 99% of the initial herbicide concentration was present at crop planting when irrigation was not implemented. At most, a 1/50 rate of dicamba and a 1/500 rate of 2,4-D was present at planting when overhead irrigation was applied prior to transplanting. Maximum cantaloupe and squash injury from 2,4-D with irrigation was 10% and did not influence plant growth, biomass, or yield. For dicamba with overhead irrigation, cantaloupe injury was 35%, vine lengths were reduced by 24%, and maturity was delayed, whereas squash injury ranged from 9% to 12%, with no influence on growth or yield. Without irrigation to wash herbicides from the mulch prior to planting, 60% to 100% injury of both crops occurred with both herbicides. Zucchini squash was more tolerant to dicamba than cantaloupe. Results demonstrated that 2,4-D can be adequately removed from the surface of plastic mulch with irrigation, whereas a single irrigation event was not sufficient to remove dicamba.

The loss of methyl bromide led vegetable growers to rely more heavily on herbicides to control weeds. Although herbicides can be effective, limited options in vegetable production challenge growers. Identifying new, effective tools to be applied over plastic mulch before planting, for improved weed control with minimal crop injury, would be beneficial. The objective of these experiments was to evaluate the persistence of preplant applications of glyphosate (1,125 or 2,250 g ae ha−1) plus 2,4-D (1,065 or 2,130 g ae ha−1) or dicamba (560 g ae ha−1) over plastic mulch, using analytical techniques and subsequent yellow squash and watermelon response. Glyphosate and 2,4-D were not analytically detected at damaging concentrations on plastic mulch when at least 3.5 cm of rainfall was received after application and before planting. In addition, bioassay results showing less than 10% visual injury for either squash and watermelon, with no growth or yield suppression observed, supported analytical results. In contrast, dicamba concentrations on plastic mulch, regardless of rainfall amount or time between application and planting, remained at damaging levels. Squash yields were reduced by dicamba applied 1 to 30 d before planting, whereas watermelon was more resilient. 2,4-D plus glyphosate applied preplant over plastic mulch can provide an additional herbicide option for vegetable growers. More research is needed to understand the impact of residual activity of 2,4-D when transplants land directly in holes in plastic mulch at the time of application. The relationship of dicamba with plastic mulch is complex, because the herbicide cannot be easily removed by rainfall. Thus, dicamba should not be included in a weed management system in plasticulture vegetable production.

Few herbicides are available for weed control in vegetable production systems using low-density polyethylene (LDPE) plastic mulch. With the elimination of methyl bromide for pest management and subsequent use of various alternative fumigants, the need for herbicides in vegetable production systems has increased. An experiment was conducted to evaluate tolerance of transplant summer squash and tomato to carfentrazone, flumioxazin, glyphosate, halosulfuron, or paraquat applied to the mulch prior to transplanting. After applying herbicides overtop of the mulch but prior to vegetable transplant, the mulch was either irrigated with 1.0 cm of water or not irrigated. Carfentrazone did not affect either crop regardless of irrigation. Irrigation readily removed glyphosate and paraquat from the mulch, as there was no adverse crop injury in these treatments. In the absence of irrigation, glyphosate and paraquat reduced squash diameter and tomato heights 18 to 34% at 3 wk after transplanting (WAT). Squash and tomato fruit numbers and fruit biomass (yield) were reduced 17 to 37%, and 25 to 33%, respectively. Halosulfuron reduced squash diameter and yield 71 to 74% and tomato heights and yields 16 to 37% when mulch was not irrigated prior to transplanting. After irrigating, halosulfuron had no affect on tomato, but reduced squash growth and yield 40 to 44%. Flumioxazin killed both crops when the mulch was not irrigated; and reduced squash yield 56% when irrigated. With irrigation, flumioxazin did not impact tomato fruit number, but did reduce tomato weight by 25%. These studies demonstrate the safety of carfentrazone, applied on mulch prior to transplanting either squash or tomato, regardless of irrigation, and also demonstrate the safety of glyphosate and paraquat if irrigated prior to transplanting. Conversely, flumioxazin should not be applied over mulch before transplanting either crop, regardless of irrigation. Halosulfuron application over mulch should be avoided before transplanting squash, regardless of irrigation, but can be applied prior to transplanting tomato if irrigated.