According to horticulturists at the University of Illinois, watermelons grow best in soil that is nutrient-rich (loamy) and somewhat sandy. High humidity will promote disease and insect infestation; but the soil should never be allowed to completely dry out. A temperature range between 70 and 85 degrees F is ideal for all stages of a watermelon plant's life cycle. Per the Texas A&M Horticultural Extension, the total time between seed planting and first harvest is approximately 85 to 95 days.
Seeds can be sown directly in the ground or in seedling pots. They sprout within just a few days. Viable seedlings will have four to five leaves and can be transplanted after 3 weeks. Strong plants will grow quickly and begin to form a runner along the ground within a few weeks. The watermelon vine develops large, deeply lobed leaves near the base. These initial leaves promote a healthy rate of growth.
Dense foliage will begin growing when the first vine branch reaches 12 to 18 inches long. With proper care, the plant can then double in size within a few days. The growth spurt will continue at a fast pace until the first runner reaches about 6 feet in length. Some species grow vines up to 12 to 15 feet, so rows of watermelon plants should be widely spaced.
Plants form additional runners on the ground radiating out from the center. This stage sometimes occur early, within 30 days of planting. Multiple vines from the same plant are desirable as long as these are limited to three to five at a time. This offers maximum potential for healthy fruit production on a single plant. Each vine branch can support the growth of a single melon. Branching may take 30 to 45 days to complete.
The first flowers will be male and generally begin forming during the branching phase. These blossoms provide pollen but do not produce fruit themselves. Each male flower withers after 24 hours. The female blossoms mark the beginning of the fruiting stage and will form near the tip of each mature vine branch. These feature a furry, marble sized melon just below the flower. Bees and other insects pollinate the female blossoms.
Once pollinated, the miniature fruits swell up to form rapidly growing watermelons. They are considered viable after they reach the size of a golf ball. This usually takes a day or two. Watermelons will continue to grow for 30 to 35 days as long as they remain undisturbed. They must be protected from animals, insects and rot. The fruit is ready to harvest if it produces a dull, hollow sound when thumped.