Slices of sweet, juicy watermelon are a special treat on a hot summer day. Growing watermelons in your backyard keeps these plump beauties readily available at your fingertips all season long. However, watermelons need a long, warm growing season to flourish. Knowing when to plant watermelon will help you harvest a bounty of large ripe melons.
Watermelons can grow in USDA Hardiness Zones 3 through 11 and need 70 to 90 days to reach maturity, depending on the variety. Soil temperatures must be at 60- to 18.3 degrees Celsius at the 4-inch depth, or watermelon seeds will not germinate. Average air temperatures must be between 21.1 and 29.4 degrees Celsius. Only plant watermelons well after the danger of frost has passed.
Zones 3 to 6
In northern zones where the warm season is relatively short, start watermelon plants indoors and choose early varieties that mature quickly. In zones 3 and 4, set seedlings outside a couple of hours a day starting at the beginning of June to harden the tender plants to the weather. Plant seedlings in the middle to end of June. In zones 5 and 6, begin hardening seedlings in May for planting in mid to late May.
Zones 7 to 11
Plant seedlings or direct-sow your watermelon seeds in the southern zones 7 through 11. In zones 7 and 8, plant watermelon seeds and seedlings in April. In the subtropical zones 9 through 11, plant watermelon as early as March. Stagger plantings in the long growing season of the southern zones to enjoy watermelon for a longer period.
Early ripening watermelons grow well in areas with a short warm season or as a second crop in areas with a longer season. Early varieties mature in 70 to 75 days and include Sugar Baby, a sweet red fleshed watermelon with a green rind, and Golden Crown that has a yellow rind at maturity, and Yellow Baby and Yellow Doll that have yellow flesh. Main season and seedless varieties take 80 to 85 days to reach maturity and include Sangria, Madera, Honey Heart and Tiffany watermelons.
Watermelon likes warm, fertile soil and will drink a lot of water over the growing season. Use black plastic mulch to heat the soil before planting and to help with water retention. Ted Gastier of the Ohio State University Extension recommends applying 0.454kg. of nitrogen, 0.907kg. of phosphorus and 1.36kg. of potash per 1,000 square feet of watermelon garden. However, always take a soil sample to your county extension office for testing before adding fertilisers and other soil amendments.