Standard advice says to plant watermelons 5' to 7' apart in rows 4' to 6' apart. But watermelons aren't the space hogs they are often thought to be. "Ten square feet per plant is about right," says Scott NeSmith, a professor of horticulture at the Georgia Agricultural Experiment Station at Griffin. "You can achieve that by planting in the center of a block that's 3' by 3', 2' by 5', or 3' by 4'. All will give similar results."
NeSmith has grown two varieties of watermelon at spacings of 3' by 3', 4' by 4' and 6' by 6'. At the closest spacing, total yields were always highest because more plants fit in the patch but it had no effect on fruit size or the number produced by an individual plant. The ideal, NeSmith thinks, is for the ground to be almost filled in with foliage by the time the first blossoms appear. If you want giant melons, let only one or two fruit form on a plant, rather than setting the plants farther apart.