Watermelon is an annual fruiting vine that requires a relatively long, hot growing season to produce the iconic summertime treat familiar to picnickers everywhere. Watermelon is one member of the very large Cucurbitaceae family of plants, a group that includes most of the vining vegetables, including cucumbers, squash, and pumpkins. The watermelons grown in domestic gardens are all cultivars of the Citrillus lunatus species. Watermelon has been in cultivation for many centuries, thought to have originated almost 5,000 years ago in the Kalahari Desert of Africa. Seeds were brought to the U.S. by enslaved Africans. Thanks to hybridizing, there are over 100 cultivated varieties of watermelons, in assorted sizes, shapes, and colors.
Watermelon is a sprawling, prostate vining plant with large, lobed leaves with a hairy, coarse texture. They bloom with yellow flowers in mid- to late-summer. The deliciously edible fruits with thick rinds grow and ripen quickly for harvest in late summer and early fall. Each plant produces two to four watermelon fruits.
Modern watermelon cultivars are categorized into four groups:
- Picnic melons are jumbo-sized, 15 to 50 pounds.
- Icebox melons are smaller family-sized fruits, 5 to 15 pounds.
- Seedless melons are hybrids bred to be self-sterile. They weigh 10 to 20 pounds.
- Yellow/orange-fleshed melons are often considered to be sweeter than red and pink watermelons.
Watermelons are generally planted from seeds sown directly into the garden as soon as the soil warms to 70 degrees Fahrenheit. In cool regions with short growing seasons, seeds are started indoors two to four weeks before last frost date. Most watermelons take 80 to 90 days to mature, although there are short-season varieties are ready to harvest about 70 days after the seedlings sprout.
|Botanical Name||Citrillus lunatus|
|Plant Type||Annual fruiting vine|
|Sun Exposure||Full sun|
|Size||9 to 18 inches tall; 10- to 15-foot vine spread|
|Soil Needs||Loamy, sandy, well-drained|
|Soil pH||Slightly acidic to neutral (6.0 to 6.8)|
|Native Area||Western Africa|
|Hardiness Zones||Grown as an annual in zones 2 to 11|
How to Plant Watermelons
Watermelons can either be direct sown in the garden after danger of frost has passed, or started indoors in paper or peat pots, two to four weeks before your last frost date. Don't rush to plant watermelons—wait until air temperatures remain steady at 70 to 80 degrees, about the time the peonies begin to bloom. Seeds need warm soil to germinate. Outdoor soil can be warmed to the necessary 70 degrees by covering it with black plastic.
Watermelons are usually planted in slightly mounded hills spaced 4 to 6 feet apart; these are large plants that need room to sprawl. Plant four to five seeds in the center of the hill, about 1 inch deep. When the seeds sprout, thin them out, leaving two or three plants per hill. If planting from nursery seedlings or seeds started indoors, plant two transplants per hill.
In cooler climates, row covers can be used to keep young plants warm. Row covers will also protect the plants from the many insect pests that are attracted to watermelons, but they need to be taken off when the flowers bloom to ensure pollination.
Keep the area weed-free through the growing season. Gardeners with short seasons should prune off any flowers that form during the last six to seven weeks before the first frost date, as this will encourage the existing fruits to ripen in time. If necessary, row covers can be put in place to protect the plants late in the season.
You will generally get only two to four watermelons per vine. As the fruits get large, carefully lift and set them on pieces of cardboard or a layer of straw to protect them from rot and pests.
Watermelons require full sunlight in order to thrive. The plants can tolerate some partial shade, particularly in hotter climates, but plenty of sun is necessary to develop the sugars in the melons. Excessively shady conditions will reduce the number and size of the fruits.
Watermelons grow well in almost any well-draining rich soil. A mildly acidic to neutral soil pH of 6.0 to 6.8. is best. Heavily amending the soil with plenty of organic matter before planting is advised, as these are heavy feeders.
Watermelons need regular watering when first transplanted. Once they begin setting fruits, you can ease up on the water, unless it is an especially dry season. Their roots are relatively deep and they can withstand short dry periods. In fact, they will lose sweetness if given too much water.
Temperature and Humidity
Watermelons prefer hot growing conditions, 80 degrees Fahrenheit and higher. They will do equally well in humid and arid conditions provided soil moisture is adequate.
Watermelon plants are heavy feeders. Make sure your soil is well amended with organic matter before planting. If your soil is lacking in organic matter, add a slow-release organic fertilizer at the start of the season. To keep watermelons growing steadily, side dress at mid-season with a layer of compost.
If you choose to use chemical fertilizer, feed early with a fertilizer that contains more nitrogen than phosphorus and potassium, since this will encourage the growth of the leaves and vine. But after flowering begins, fertilize a second time with a low-nitrogen fertilizer that encourages growth of flowers and fruit.
Early/ short-season varieties mature in 70 to 75 days:
- 'Blacktail Mountain' has red flesh with a dark green rind. They weigh 6 to 12 pounds.
- 'Faerie' produces watermelons with red flesh and a yellow rind. They are relatively small fruits, at 5 to 6 pounds.
- 'Golden Crown' watermelons have red flesh and yellow rinds. They weigh 4 to 7 pounds.
- 'Sugar Baby' is a great red-fleshed cultivar. Melons weigh 6 to 10 pounds.
Long-season varieties mature in 80 to 85 days:
- 'Ali Baba' has oblong-shaped melons with red flesh. Melons weigh 12 to 30 pounds.
- 'Moon and Stars' is a hybrid plant with beautiful melons. Both red- and yellow-fleshed cultivars are available.
- 'New Queen' has orange-fleshed melons with few seeds and a high sugar content. They weigh 5 to 6 pounds.
Seedless varieties: So-called seedless varieties are not truly seedless, but the seeds are small, white, and edible. These are self-sterile hybrids. Seedless watermelons have a lower germination rate and they are more expensive than seeded watermelons, so starting them in peat or paper pots will give you a slight edge over starting them in the ground.
- 'Revolution' is a red-fleshed melon that matures in 80 days. Melons are 20 to 26 pounds.
- 'Superseedless' has red-fleshed melons that are ready to harvest 90 to 95 days after sprouting. Melons are 16 to 20 pounds.
- 'Sweet Bite' has red-fleshed melons that are ready to harvest in about 75 days. Melons are 5 to 8 pounds.
- 'Triple Gold' is a yellow-fleshed melon that matures in about 75 days. Fruits way 8 to 10 pounds.
Judging the ripeness of a watermelon requires some experience, but there are a few signs that can help you learn how to determine when they are ready:
- The watermelon rind will start to become dull.
- The curly tendrils on the vine, near where the melon attaches, will turn brown.
- You should not be able to pierce the rind with your thumbnail.
- The part that rests on the ground will change from pale light green to a pale yellow.
Watermelons are best eaten right away. Cut watermelon should be refrigerated and only keeps for a few days. Don't store uncut melons in the refrigerator. Opt for a cool room (45 to 50 degrees Fahrenheit), if possible. In this environment, uncut melons will keep for up to two weeks.
Common Pests and Diseases
The biggest watermelon pest is the cucumber beetle. Monitor for the eggs early. If you are using row covers, you will exclude many of them from the vines. Watermelons are also prone to vine borers, aphids, and mites.
Several diseases can be a problem, including fusarium wilt, anthracnose, alternaria leaf spot, and gummy stem blight. If these diseases are prevalent, choose resistant varieties of watermelon. Watermelons can be prone to powdery mildew, but this is rarely serious—only unsightly. Ground-level watering, rather than overhead watering, will keep soil spores from splashing on the leaves and causing mildew.