Watermelon is a tender, warm-season vegetable.
Watermelon is truly one of summertime's sweetest treats. It is fun to eat, and good for you. Watermelon seeds were brought to this country by African slaves. Today there are more than 100 different varieties of watermelons. The flesh may be red, pink, orange, or yellow. There are seedless varieties and super-sweet round ones that fit nicely into the refrigerator.
Watermelons can be grown in all parts of the country, but the warmer temperatures and longer growing season of southern areas especially favor this vegetable. Gardeners in northern areas should choose early varieties and use transplants.
Mulching with black plastic film also promotes earliness by warming the soil beneath the plastic. Floating row covers moderate temperatures around the young plants, providing some frost protection in unseasonable cold spells.
Seedless watermelons are self-sterile hybrids that develop normal-looking fruits but no fully developed seeds. The seeds for growing them are produced by crossing a normal diploid watermelon with one that has been changed genetically into the tetraploid state. The seeds from this cross produce plants that, when pollinated by normal plants, produce seedless melons.
In seedless watermelons (genetic triploids), rudimentary seed structures form but remain small, soft, white, tasteless, and undeveloped tiny seedcoats that are eaten virtually undetected along with the flesh of the melon. Seed production for these seedless types is an extremely labor-intensive process that makes the seeds relatively expensive. Because germination of these types is often less vigorous than normal types, it is recommended that they be started in peat pots or other transplantable containers, where the germinating conditions can be closely controlled Once transplanted, cultivation is similar to that for regular watermelons.
For pollination necessary to set fruit, normal seed types must be interplanted with seedless melons. The pollinator should be distinct from the seedless cultivar in color, shape, or type so that the seedless and seeded melons in the patch can be separated at harvest. Because seedless types do not put energy into seed production, the flesh is often sweeter than normal types and the vines are noticeably more vigorous as the season progresses.
When to Plant
Plant after the soil is warm and when all danger of frost is past. Watermelons grow best on a sandy loam soil, although yields on clay soils can be increased significantly by mulching raised planting rows with black plastic film.
Spacing & Depth
Watermelon vines require considerable space.
- Plant 2 to 3 seeds one inch deep in hills spaced 6 feet apart.
- Allow 7 to 10 feet between rows.
- After the seedlings are established, thin to the strongest and healthiest looking plant per hill.
- Plant single transplants 2 to 3 feet apart or double transplants 4 to 5 feet apart in the rows.
- Sowing watermelon seed too early in cold soils can lead to seed rot and other plant diseases.
- Use a soil thermometer (or meat thermometer) to ensure the soil has warmed above 65 degrees Fahrenheit. Or, spare the meat thermometer and go online to find your nearest Illinois soil monitoring station at the Water and Atmospheric, Resources Monitoring Program.
Start the seeds inside 3 weeks before they are to be set out in the garden.
- Plant 2 or 3 seeds in peat pellets, peat pots, or cell packs and thin to the best one or two plants.
- For expensive seedless types, plant one seed to a pot or cell and discard those that do not germinate.
- Do not start too early - large watermelon seedlings transplant poorly.
- Growing transplants inside requires a warm temperature, ideally between 80 and 85°F.
- Place black plastic film over the row before planting.
- Use a starter fertilizer when transplanting.
- If you grow seedless melons, you must plant a standard seeded variety alongside. The seedless melon varieties do not have the fertile pollen necessary to pollinate and set the fruit.
Watermelons should be kept free from weeds by shallow hoeing and cultivation. The plants have moderately deep roots and watering is seldom necessary unless the weather turns dry for a prolonged period. In cooler areas, experienced gardeners may find floating row covers, drip irrigation, and black plastic mulch advantageous in producing a good crop in a short season.
Cucumber beetles attack watermelon plants.
- Apply a suggested insecticide for control.
- If row covers are used in the early season for temperature moderation, early-season insect pests may also be excluded if the covers are applied so that the pests cannot penetrate to the crop below.
- These covers may be left in place until the plants start to bloom, at which time pollinating insects must be allowed to reach the flowers.
Many home gardeners experience difficulty in determining when watermelons are ripe. Use a combination of the following indicators:
- light green, curly tendrils on the stem near the point of attachment of the melon usually turn brown and dry;
- the surface color of the fruit turns dull;
- the skin becomes resistant to penetration by the thumbnail and is rough to the touch; and
- the bottom of the melon (where it lies on the soil) turns from light green to a yellowish color.
These indicators for choosing a ripe watermelon are much more reliable than "thumping" the melon with a knuckle. Many watermelons do not emit the proverbial "dull thud" when ripe. For these, the dull thud may indicate an over-ripe, mushy melon.
Selection & Storage
Producing a good watermelon is a bit tricky in the short northern season. The sweetest watermelons grow during long hot summers. Harvesting is particularly critical because watermelons do not continue to ripen after they have been removed from the vine. They should be picked at full maturity. No amount of thumping, taping, sniffing, or shaking can actually give a clue to ripeness.
Look for melons that are very heavy and have a hard rind. Ninety percent of watermelon is water.
- The rind color should be right for the variety with a waxy bloom.
- Probably the most important indicator of ripeness is the underside which sets on the ground. Turn the melon over. It should be yellow or creamy colored on the underside. If it is white or pale green the melon is not ready to harvest.
- The flesh should be deep colored with mature seeds. Most watermelons have dark brown or black seeds. The seedless variety produces a few white seeds.
Once picked, uncut watermelon can be stored for about 2 weeks at room temperature especially if the temperature is about 45 to 50°. Uncut watermelons have a shorter refrigerator life, so store at room temperature until ready to chill and eat. Tightly cover cut pieces in plastic wrap and store in the refrigerator for 2 to 3 days.
Questions & Answers
Q. My watermelons are not very sweet or flavorful. Is the low sugar content caused by the watermelons crossing with other vine crops in the garden?
A. No. Although watermelon varieties cross with one another, cross-pollination is not apparent unless seeds are saved and planted the following year. Watermelons do not cross with muskmelons, squash, pumpkins, or cucumbers. The poor quality of your melons may result from wilting vines, high rainfall, cool weather, or a short growing season in extreme northern areas.
Q. What can I do to prevent my watermelons from developing poorly and rotting on the ends?
A. This condition is probably caused by an extended period of extremely dry weather when the melons were maturing. It may be aggravated by continued deep hoeing or close cultivation. Mulching the plants with black plastic film helps to reduce this problem.
Q. What causes deep holes in the tops of my watermelons?
A. The holes were probably made by pheasants or other wildlife searching for water during dry weather.