Harvest watermelons by using a sharp knife to cut the ripe fruit from the vine. Do not pull or tear ripe watermelons away from the vine, as doing so creates an opening through which pathogens can enter both the fruit and the vine. Ensure a long shelf life by storing harvested watermelons in dry conditions at between 50 and 60 degrees Fahrenheit.
The most difficult step in harvesting watermelons is determining when the fruits are actually ripe. Unlike most other melons, watermelons do not give many clear indications of ideal harvest time, though there are a few indicators gardeners use to judge when these fruits are ripe.
One test experienced gardeners with good senses of hearing use in determining ripeness is gently knocking on the watermelon with a knuckle. Most healthy, ripe melons produce a deeper, hollow sound, while unripe melons produce a sharper, shallower sound.
Many watermelon varieties are ready for harvest when the fruits stop growing in size. This method is not foolproof, as some varieties continue ripening on the vine even after they stop growing. Harvesting unripe watermelons still produces edible fruits, as melons continue softening when taken off the vine. Unripe watermelons never grow any sweeter once they are removed from the vine, however.
Pay attention to the contrast between different color bands on striped watermelon varieties. When the stripes are roughly the same color, the melons are likely ripe enough to harvest.