Growing watermelon in containers allows this big, sweet and juicy fruit to grow in smallest of spaces. Watermelon has long taproot and it doesn’t transplant well that’s why it is better to sow the seeds directly in a pot. Sow 3-4 seeds directly in a pot once the temperature starts to reach 65 F (19 C) and above in the spring. The germination takes place within 6 to 10 days. Thin out and leave only one of the strongest seedlings per pot. As watermelon has long taproot choosing a deep pot is essential. A large pot or bucket that is at least 2 feet deep and half wide is required. Watermelons should be grown in a sunny position. Keep reading how to grow them in containers here.
SOURCE:http://www.gardenoholic.com/how-to-grow-watermelon-in-pot-vertically/