This is a newly released video about when to plant watermelons. https://youtu.be/tHT2mAnNRWk
Watermelon, cantaloupe, cucumber plants are very sensitive to low temperatures. Even when frost has passed, soil temperatures below 60°F can result in transplant establishment failure. Check soil temperatures before planting. The rule of thumb is to plant watermelon, cantaloupe and cucumber transplants when soil temperatures at the root zone are stable above 60°F. Ideally, watermelon, cantaloupe, and cucumber grow well when soil temperatures are above 70°F.