Morningglories (Ipomoea spp.) are among the most troublesome weeds in cucurbits in the United States; however, little is known about Ipomoea spp. interference with horticultural crops. Two additive design field studies were conducted in 2020 at two locations in Indiana to investigate the interference of ivyleaf morningglory (Ipomoea hederacea Jacq.), entireleaf morningglory (Ipomoea hederacea Jacq. var. integriuscula A. Gray.), and pitted morningglory (Ipomoea lacunosa L.) with triploid watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai]. Immediately after watermelon was transplanted, Ipomoea spp. seedlings were transplanted into the watermelon planting holes at densities of 0 (weed-free control), 3, 6, 12, 18, and 24 plants 27 m−2. Fruit was harvested once a week for 4 wk, and each fruit was classified as marketable (≥4 kg) or non-marketable (<4 kg). At 1 wk after the final harvest, aboveground biomass samples were collected from 1 m2 per plot and oven-dried to obtain watermelon and Ipomoea spp. dry weight. Seed capsules and the number of seeds in 15 capsules were counted from the biomass sample to estimate seed production. Ipomoea spp. densities increasing from 3 to 24 plants 27 m−2 increased marketable watermelon yield loss from 58% to 99%, reduced marketable watermelon fruit number 49% to 98%, reduced individual watermelon fruit weight 17% to 45%, and reduced watermelon aboveground biomass 83% to 94%. Ipomoea spp. seed production ranged from 549 to 7,746 seeds m−2, greatly increasing the weed seedbank. Ipomoea spp. hindered harvest due to their vines wrapping around watermelon fruits. The most likely reason for watermelon yield loss was interference with light and consequently less dry matter being partitioned into fruit development due to less photosynthesis. Yield loss was attributed to fewer fruits and the weight of each fruit.

Removal and plant-back studies were conducted in North Carolina in 1991 and 1992 to determine the critical period of large crabgrass competition in transplanted triploid (seedless) watermelon. For every week that large crabgrass remained in watermelon, medium (3.6 to 7.3 kg) melon yield decreased 3,996 kg and 716 fruit ha−1. For every week that large crabgrass emergence was delayed, yield increased by 814 kg and 142 fruit ha−1. Likewise, for every week that large crabgrass remained in watermelon, marketable (3.6 kg and over) yield decreased 5,582 kg and 911 fruit ha−1. For every week that large crabgrass emergence was delayed, yield was increased 881 kg and 151 fruit ha−1. Large crabgrass emerging after 6 wk had no effect on marketable fruit or number of watermelon. To achieve the greatest quality or quantity of medium or marketable fruit, a large crabgrass-free period between 0 and 6 wk after transplanting was necessary.