Slices of sweet, juicy watermelon are a special treat on a hot summer day. If growing watermelon is a new world to you, it's important to know what month to plant and how long it will take. As soon as the temperature hits 60-65 degrees Fahrenheit or there is no chance of frost again for the season, you can begin to plant watermelon.
Watermelons can grow as annuals in USDA plant 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 65-degrees Fahrenheit at the 4-inch depth, or watermelon seeds will not germinate. Average air temperatures must be between 70 and 85 degrees Fahrenheit. Only plant watermelons well after the danger of frost has passed.
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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, sow watermelon seeds and plant seedlings in mid-April. In the subtropical zones 9 through 11, plant watermelon as early as mid-March. Stagger plantings in the long growing season of the southern zones to enjoy watermelon for a longer period.
Varieties of Watermelon
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 that mature in 70 to 75 days 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 1 lb. of nitrogen, 2 lbs. of phosphorus and 3 lbs. 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 fertilizers and other soil amendments.
Watermelon takes up a lot of space. The vines alone can reach up to 20 feet in length. If you plan on planting multiple watermelons, make sure to have plenty of room. Grow the vines in raised rows or hills which allow efficient drainage and hold heat from the sun longer. Plant the watermelons at least two feet apart in a five-foot-wide hill. However, keep them six feet apart if you plant in rows.
You can tell when a watermelon is ripe as it will break easily from the vine upon twisting. An additional sign of ripeness is when the bottom of the watermelon changes to a rich yellow color from white.