Watermelon vines should be planted approximately two weeks after the last frost date in the spring or early summer. The vines are susceptible to frost and could suffer damage or even death if exposed to freezing temperatures. Watermelons have a long growing season, up to 80 days from seed germination.
Many gardeners in cold climates prefer to start seeds indoors in the spring and transplant the plants after the danger of frost has passed. In warmer climates, seeds can be planted directly in the ground two weeks after the last frost. Consult a USDA Hardiness Zone Map to determine the date of the last freeze in your area.