Make watermelon mounds 4 to 8 feet apart to give vines room to spread. Each low, broad mound should be a minimum of 3 to 4 inches high and ideally 8 to 10 inches high and 12 to 18 inches wide. If you live in a cooler climate and need to warm the soil, make a 3- to 6-inch trench around the mound, then place black plastic over the mound and anchor the edges by filling in soil over the trench.
Sow four to six seeds in a circle on top of the mound. When the seeds have developed one or two true leaves, remove all but two or three of the strongest plants. The seeds will first develop cotyledons, or embryonic leaves. True leaves are those that look like the leaves on an adult plant.
You can grow watermelons in U.S. Department of Agriculture plant hardiness zones 3 through 11, but they require weather up to 80 degrees Fahrenheit and a growing season of 70 to 90 days, depending on the cultivar. If you live in a colder climate, you may have to plant seeds indoors or buy seedlings from a nursery. Mounds hold the heat better, an advantage if you live in a cooler climate. However, if you live in a very hot climate, mounds may dry out quickly, and it may be to your advantage to grow them on level ground using the same spacing and number of plants that you would for a mound.
Soil and Draining
Roots of watermelons planted in mounds spread out from a central point, enabling them to get more water and nutrients. Watermelons do not like to sit in wet soil. If you have heavy, clay soil, mounds have the advantage of draining more quickly than level soil. However, if you have sandy soil, mounds may drain too quickly, hampering the germination of seeds.