Watermelons are a warm-season crop prized for their sweet, juicy, bright pink fruit. Because watermelons need warm weather to both germinate and thrive, southern growers generally have more success with them. Likewise, because they require a long growing season of warm weather, getting your plants into the ground as early as possible will ensure a healthy crop. If you're a northern gardener or getting a late start, you can use techniques that will help you grow watermelons in less-than-ideal conditions.
Time to Harvest
Like most fruit and vegetable plants, watermelon varieties vary in the amount of time you need to allot between planting and harvest. Early varieties such as Sugar Baby and Golden Crown require between 70 to 75 days between germination and harvest. Most watermelons -- including the seedless varieties -- need 80 to 85 days to fully ripen before harvest. Check the seed packet or plant tag for information on harvest time.
Watermelons are a tender crop, meaning that they cannot survive frosts. While you can use tools and techniques to give your plants some protection from the cold, watermelons grow best in warm weather, so you should anticipate harvesting them before the first frost. To determine the last possible date when you can plant watermelons in your area, you first need to determine the first frost date for your region. The National Climatic Data Center maintains this information, and you can also find it from a cooperative extension agent or experienced nursery employee. Subtract the number of days needed until harvest, adding a week if you plan to start the plants from seed. This date is the last possible date you can plant watermelons in your area, although ideally, you should have them in the ground well before then.
Extending the Season
If you got a later start planting your watermelons than you intended, plant transplants rather than seeds, if possible. Watermelons grown from transplants are ready to harvest two weeks before those grown from seed.
If you find temperatures dropping before your melons are ready for harvest, you can use several techniques to keep both the soil and the air around them warm. Black plastic mulch spread between the rows raises the soil temperature. To protect plants from cool weather and early frosts, cover plants with floating row covers. These lightweight covers allow full penetration of light and water while raising temperatures a few degrees around the plants.
Tips for Cooler Climates
Gardeners in cool, northern climates have less room for error when planting watermelons. If you find yourself getting a late start because springtime conditions aren't warm enough for watermelons, you can jumpstart the melon season and better ensure a good harvest. Start seeds indoors four weeks before the last spring frost date in your area so that plants can go directly into the ground without waiting for germination. To warm the soil for your plants, use black plastic mulch but remove it or spray it white once summer temperatures heat up. Floating row covers can keep the occasional chilly spring day from damaging your plants.