Watermelon, known botanically as Citrullus lanatus, is a tender vegetable that is related to the cucumber and native to Africa. Watermelons are humidity- and heat-loving plants that produce the largest harvests and sweetest "fruits" when grown in a full-sun exposure and protected from cool winds. They flower in the late spring and produce fruits throughout the summer season.
Seasonal Seed Planting
Watermelon seeds should be planted outdoors in the spring well after all threats of frost have passed. The seeds can be started indoors up to three weeks before planting in the garden; do not start sooner as large watermelon seedlings do not transplant well. This will give you a small jump on the growing season. Provide your indoor seedlings with moist soil, bright light and warm temperatures between 80 and 85 degrees Fahrenheit. For outdoor seed sowing, bury seeds an inch deep into the soil at intervals of up to 6 feet depending on the varietal of seed, and place transplant seedlings at intervals of 2 to 3 feet to ensure proper air flow preventing the invitation of fungus or disease. Cover seeds or the root balls of the young transplants with displaced soil and water in deeply. Maintain moist--but not consistently wet--soil around the seeds at all times through germination and sprouting. Watermelon flowers will bloom from the plants; approximately 45 days after blooming, melons will be ready for harvesting.
You want a rich and well-tilled soil for your watermelon seeds. Make soil amendments with compost and/or well-aged manure to raise the nutrient level. Sharp sand assists with good drainage. A slow-release fertilizer tilled into the planting soil will add an ongoing boost of nutrients for the plants as well as cut down on your maintenance and labor during the growing season. Add some more fertilizer right before planting them, even if you had already placed organic matter there. Locate your watermelon patch in a site with full-sun exposure and, if at all possible, where a warm humid micro-climate might occur. This could be adjacent to the south-facing wall of a house or outbuilding. The seeds and young plants will need at least 1 inch of water per week to meet the goal of keeping the soil uniformly moist.