Watermelon

Watermelon (Citrullus lanatus) is a warm-season annual vine in the gourd family, Cucurbitaceae, grown across hot-summer regions for its large, sweet, watery fruit.15 The genus is native to Africa, and archaeological, iconographic, and literary evidence points to northeastern Africa as the centre of origin for the dessert watermelon, where wild populations still occur.245 Watermelons have been cultivated for more than 4,000 years, with the sweet dessert forms we recognise today emerging in Mediterranean lands roughly 2,000 years ago.45 Today it is grown and eaten worldwide and has the greatest production of any cucurbit, more than 63 million tonnes a year, with China the largest producer at around 38 million tonnes.5 For the home grower it is a sprawling, single-season crop that rewards a long, hot summer with heavy, sugary fruit.

The plant grows as a prostrate, basally branched, softly hairy vine that scrambles or trails along the ground, spreading up to about 10 feet (roughly 3 metres) wide.15 Its leaves are deeply and roundly lobed, blue-grey-green, with three to five primary lobes, toothed margins, and hairy undersides, and the vines carry the branched tendrils typical of cucurbits.5 The fruit is a pepo, a large modified berry, spherical to oval, with a mottled or striped green rind.1 The juicy flesh is usually red but may be pink or yellow depending on cultivar, and fruits generally weigh anywhere from 6 to 60 pounds or more.15

Growing Watermelon

Watermelon is strictly a warm-season annual that needs a long, warm growing season to mature; it is frost-tender and should be planted only after all danger of frost has passed and the soil is warm.15 Extension guidance treats it as a summer annual rather than a perennial landscape plant, so it can be grown in a wide range of zones wherever the season is long and hot enough.1 Give it full sun: NC State Extension lists six or more hours of direct sunlight a day as the minimum, and home-garden guidance favours eight to ten hours where possible.17

Soil should be sandy, well-drained, and liberally enriched with organic matter, kept moist but never soggy.17 The crop tolerates a fairly wide pH band; NC State cites a neutral, loamy soil in the range of 6.0 to 8.0, while home-garden sources favour 6.0 to 6.5, noting tolerance down to about 5.0.17 Watermelon is propagated by seed, which it grows from easily.17 Germination is very slow when soil temperature is below 70°F, so wait for warm ground.7 For direct seeding, sow about 1 inch deep in hills, four to five seeds per hill, thinning to two strong plants per hill after they emerge.7 Seed can also be started indoors with one or two seeds per pot under supplemental light, then hardened off gradually before transplanting; transplants can be harvested as much as two weeks earlier than direct-sown plants.7

Space plants generously: home growers are advised to allow roughly 6 to 8 feet (2.4 to 3.0 metres) between plants, or about 24 square feet per plant, to give the vines room.7 Most of the root system sits in the top 12 inches of soil, and because the fruit is about 92 percent water, consistent moisture in the root zone matters, applied as several shorter cycles rather than one heavy soak.7 Watermelon flowers are insect-pollinated, and fruit set is reduced by cold, rainy weather that hinders pollination; poorly formed fruit is most often caused by a lack of pollination by bees.17 For feeding, a dilute fish-emulsion solution applied weekly suits young plants, with a kelp-based foliar spray once plants are in full flower.7 Avoid over-fertilising with nitrogen, which produces all vine and little fruit, and try not to disturb or move the vines, as that can interrupt the flow of nutrients.7 Watermelon takes a long season to finish: about 80 to 100 days from seed, or 60 to 90 days from transplants.7

Harvest and uses

Watermelon does not continue to ripen after picking, so timing the harvest matters.7 Read several ripeness signs together: the curly tendril opposite the fruit stem dries out completely, the underside of the fruit where it rests on the ground turns yellow, and a tap on the fruit returns a dull thump rather than a tight ring.7 Once cut, fruit keeps about seven to ten days at room temperature, or up to about two weeks if stored below 60°F.7

The flesh is broadly edible and non-toxic, eaten raw or pressed for juice, with only the typical food-allergy or medication-interaction cautions seen in susceptible individuals.1345 Little need go to waste: the rind can be stir-fried, stewed, or pickled, and the seeds are eaten as a snack in some cultures, for example esteemed and eaten with other seeds at Chinese New Year celebrations.7 Seedless cultivars produce only small, soft, white, under-developed seedcoats that are tasteless and can be eaten along with the flesh.17

Pollination and seedless types

Watermelon is monoecious, carrying separate male and female flowers on the same plant, with typically one flower per stem node.57 Because bee visits are essential, eight or more visits per blossom are needed for optimum fruit set, so a healthy pollinator population is worth encouraging.7 Seedless watermelons add a wrinkle: they are sterile hybrids that do not produce enough viable pollen on their own, so they must be interplanted with a standard seeded variety to act as a pollenizer.17

Common problems and pests

Watermelon is susceptible to a range of fungal diseases, insects, and mites, though insect pests are generally less damaging on watermelon than on other cucurbits.7 Reported pests and diseases include mites, aphids, squash bugs, squash vine borers, cucumber beetles, downy and powdery mildews, bacterial wilt, fusarium wilt, anthracnose, and blossom-end rot.1 Two common cultural problems are worth flagging: blossom-end rot is primarily caused by inadequate calcium reaching the fruit, and a plant that makes plenty of vine but little fruit is usually the result of over-fertilising with nitrogen or planting too close together.7

Sources

1. NC State Extension. “Citrullus lanatus (Watermelon).” North Carolina Extension Gardener Plant Toolbox.
2. U.S. Fish & Wildlife Service. “Wild Watermelon (Citrullus lanatus ssp. lanatus).”
3. National Center for Biotechnology Information. Review on Citrullus lanatus. PMC.
4. Paris, H. S. (2015). “Origin and emergence of the sweet dessert watermelon, Citrullus lanatus.” Annals of Botany.
5. ScienceDirect. “Watermelon” (Food Science topic overview).
6. iNaturalist. “Citrullus lanatus.”
7. Growables. “Watermelon (Citrullus lanatus).”
8. PictureThis. “Citrullus lanatus.”