Clear an area to plant crimson sweet watermelon plants a month or two before your last average frost date. The location should be exposed to at least eight hours of direct sun each day. It should also be large enough to accommodate the crimson sweet watermelon plant, which will stretch up to 10 feet in every direction. The soil should be well-drained, as the plants and fruit will rot if left sitting in waterlogged soil. Sandy soil or sandy loam is best for growing watermelons because it is well-drained and warms up quickly in the spring.
Test the soil to be sure it has the correct pH level and amount of nutrients necessary for optimal watermelon production. The soil pH should be between 5.5 and 6.5. For assistance with a soil test, contact your local county agricultural extension office.
Remove all competing vegetation from the chosen planting area and dig the soil with a shovel, hoe or garden fork to a depth of 8 inches. Spread a 2-inch layer of compost along with the correct amount of soil amendments recommended by the soil test over the planting area. Work the compost and amendments into the top 6 inches of soil and rake the area smooth.
Plant crimson watermelon seed 1/2-inch deep after all danger of frost has passed and soil temperatures are at least 75 degrees Fahrenheit. If you plant the seed too early, germination will be delayed, but it will sprout when the soil temperature is suitable. Place a small wooden stake where each plant will be growing and plant three seeds 2 inches apart near the stake. The stake serves as a marker so you know where the center is when the the plant gets large and you lose track of the center. That way, you can water the roots of the plant without getting all the leaves and fruit wet. Space watermelon plants 6 feet apart.
Thin the watermelon plants by cutting out the two weaker plants at each planting location when the plants are 2 feet long. Cut off the plants at ground level with a sharp cutting tool. Pulling them out may damage the roots of the desirable plant.
Water the crimson sweet watermelon plant by soaking it deeply every week until the vines begin to run, or grow vigorously in every direction. Then water deeply every 10 days only if the weather is unusually dry or the plants become stressed from drought. Do not allow water to splash onto the leaves, fruit or flowers of the plant, as this may cause the spread of fungal or mildew diseases.