Watermelons might be the most challenging plant when it comes to knowing when to harvest. The best practice is to keep track of the date you planted and the days to maturity based on the variety of seeds. This way you can mark on your calendar a date to know when to start looking for the following clues:
On the East Coast, Mark McOmber who was featured in the documentary Back to Eden is growing the most epic watermelons. Mark's favorite heirloom varieties of watermelon is Crimson Sweet. After 10 years of Back to Eden gardening with wood chips, Mark harvested a 56 pound Crimson Sweet watermelon! Watch Mark's video to learn how to harvest watermelons, cantaloupes, and honeydew.