Watermelons begin to ripen in the summer approximately 80 days after planting the seeds. The plant tendrils and the outside color of the fruit’s rind are both good indicators of the proper time to pick a watermelon.
When the tendrils around the fruit begin to die off, nutrients are no longer reaching the fruit and ripening is complete. A ripe watermelon has a duller surface, and any parts that were in contact with the ground turn a pale green or yellow. Gardeners can also test for ripeness by smacking the fruit with their palms. If the fruit makes a hollow sound, it is probably ripe. Not all ripe fruits make this sound, however.