Watermelons need a long growing season (at least 80 days) and warm ground for seeds to germinate and grow. Soil should be 70 degrees F or warmer at planting time. Sow seeds 1 inch deep and keep well watered until germination. To get a jumpstart in cooler climates, cover the planting area with black plastic to warm up the soil and start seeds indoors two or three weeks before they are to be set out in the garden. 
Don't start seeds any earlier, because large watermelon seedlings transplant poorly. Plant 3 seeds in 3- or 4-inch peat pots or large cell packs, and thin to the best plant. Sow watermelon seeds 1/2 inch deep. Place in a sunny south-facing window or under lights to germinate. Make sure the area is warm?day and night?ideally 80 degrees F. Use a Seedling Heat Mat if necessary. 
Watermelon is a space hog; vines can reach 20 feet in length. So plant where there is plenty of open ground. Amend soil with organic matter such as compost or composted cow manure. Add a balanced fertilizer that is high in nitrogen. Sow 8 to 10 watermelon seeds in a hill, and push seeds 1 inch into the soil. Space hills 3 to 4 feet apart, with at least 8 feet between rows. Thin plants to the 3 best in each hill. Keep soil free of weeds by shallow hoeing or with a layer of mulch.
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