A summer treat for kids and moms alike, watermelons (Citrullus lanatus) grow on annual vines and are related to cucumbers (Cucumis sativus), summer squash (Cucurbita pepo var. malopepo) and other cucurbits. Watermelons grow best spaced 4 to 6 feet apart in well-drained, moist soil with a pH between 6 and 6.8. Soil temperature should be at least 70 degrees Fahrenheit, so you should plant watermelon two weeks after the last expected frost date. A soaker hose will keep the soil moist but not soggy for these water-loving plants.
Watermelon plants send out trailing, hairy stems that can spread 15 feet or more. The 2- to 10-inch-long leaves have deep lobes and grow along the stems on 1- to 5-inch stalks. Greenish yellow, trumpet-shaped male and female flowers measure 1 inch wide. Insects pollinate the flowers, which open early in the morning after sunrise. When a watermelon is ripe, the rind turns from bright green to dull green, and the part that touches the ground turns creamy yellow. Watermelon plants produce fruit irregularly throughout the summer, and the fruits generally are ready for harvest in 70 to 85 days, says the University of Illinois Extension.
Most watermelon varieties, including “All Sweet” and “Royal Sweet,” have fully developed seeds that you can plant directly in the ground. Each watermelon variety has a distinctive shape and color. For example, “All Sweet” watermelons are oblong fruits that have bright red flesh and thin, dark green rinds with light green stripes. They weigh an average of 21 to 26 pounds, according to the South Carolina Department of Agriculture. “Royal Sweet” watermelons weigh 21 to 28 pounds and have an oblong shape with red flesh and light green rinds with dark green stripes. Icebox types have smaller fruits that weigh an average of 5 to 12 pounds.
So-called seedless watermelons actually have soft, white, edible seeds. Because the seeds are not fully developed, you should grow these varieties from transplants. You will have to interplant seeded varieties to promote pollination. Seedless fruits have oblong or round shapes with medium-thickness rinds and brightly colored flesh in a variety of colors. “Orange Julius” fruits have orange flesh and weigh an average of 11 pounds, and “Amarillo” has yellow flesh and weighs about 8 pounds. Both varieties have light green rinds with narrow, dark green stripes.
Watermelon plants are susceptible to diseases including fusarium wilt, which infects plant roots and causes wilting and stunting, and anthracnose, which causes sunken spots on leaves in cool, rainy weather. You can plant Fusarium-resistant watermelon cultivars such as sugary, red-fleshed “Crimson Sweet” or pink-fleshed “Starbrite.” To control anthracnose, remove and destroy dead vines each year. The following year, grow non-cucurbit plants in the site where the watermelons grew.
Poor soil drainage and improper fertilizing can lead to disorders such as sunscald, which causes gray or white patches on watermelon fruits in hot, sunny conditions. Too much nitrogen fertilizer promotes leaf growth but reduces fruit production. If you do not have soil test results, work in 3 pounds of 5-10-10 fertilizer per 100 square feet of soil before planting, recommends Clemson University Cooperative Extension Service. Spread 1 pound of 33-0-0 fertilizer per 100 row feet around the vines before runners develop, and again when the fruit is developing. Do not allow children or pets near chemical fertilizers. Wear gloves to avoid contamination from chemicals or soil bacteria, particularly if you are pregnant or have small children.