First cultivated in northern Africa, the watermelon is a warm weather crop, requiring hot days and warm nights to fully mature, most particularly in the case of the larger varieties. Start seed indoors at 85°F. soil temperature, 3-4 weeks before setting out. When is a watermelon ripe? There are several schools of thought on this subject. Many believe that the surest way to know is when the fruit makes a dull, hollow sound when tapped. Others, however, recommend looking at the bottom surface of the fruit. When the “ground spot” has turned bright yellow, it’s ripe and ready for picking. Still others insist that the twisted tendril nearest the fruit must have turned brown.