Watermelon

Soils and Fertilization

Watermelons tolerate acid soil (pH as low as 5.5) but require micronutrients which may be unavailable in some soils. Either low Mg or high ratios of potassium and calcium to magnesium can cause leaves to fall off. This deficiency is often mistaken for a disease. Too little calcium is also a problem, however. A darkened, leathery area develops on the blossom end of the watermelon if water is in short supply and adequate calcium for fruit development is not available. Field preparation with dolomitic limestone and irrigation can usually prevent this disorder, called blossom end rot. Fertilizer recommendations based on soil tests are given below. See Soil Management for organic sources of these materials.

Planting

Watermelons are planted on 80-inch wide beds with 2 to 4 feet between plants. Seeds are drilled 1.5 inches deep for large and .75 to 1 inch deep, for small-seeded cultivars. In the absence of intensive management practices such as container-grown transplants, drip irrigation and black plastic, a maximum density of 900 to 1000 plants per acre is generally recommended. Seedless watermelons are less hardy and grow more slowly than conventional cultivars. They are generally transplanted because of the high seed cost. With both types of watermelons, Florida growers are being urged to plant more deeply up to the seed leaves rather than the top of the root ball in order to increase vigor. Deeper planting seems to protect seedlings from wind damage and promotes high early yields.

Water Requirements and Mulching

Without adequate water, starting early in the fruit development stage and continuing through harvest, the incidence of misshapen fruit and fruit with blossom end rot increases.

Pollination

One to two hives per acre are recommended to improve quality and size of watermelon fruit. Watermelons are monoecious (male flowers, then female) like cucumber and squash, but unlike muskmelons, which are andromonoecious.

Physiological Disorders

Misshapen fruit. These can result from drought, poor pollination or other developmental problems. Watermelon vines need to be inspected weekly to remove diseased or misshapen fruit. 'Bad' fruit should be thrown in the row middles to avoid inspecting them again. In watermelons, only two 'good' fruit will usually mature on a single plant, even in the small ('icebox') cultivars.

Hollow heart. Hollow heart is a problem most likely to occur early in the season and when conditions alternate between wet and dry soil and/or hot and cold temperatures. Crown set fruit and seedless watermelons are more susceptible to hollow heart. Good pollination is important in preventing this disorder, particularly in seedless watermelon. Too much nitrogen and/or excessive water, especially during fruit set, can encourage hollow heart and bland flavor.

Melon cultivars with insect and disease resistance