Watermelon Plant Profile

Dozens of watermelons in a pile.

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The summer months were made for growing watermelons—at least, for those who live in warm climates. Although sweet, juicy watermelons aren't difficult to grow, they need a lot of nutrients, sun, warmth, and water to thrive. Those in warmer climates can sow the seeds directly outdoors in spring, while cold-climate gardeners can start short-season varieties of watermelon indoors until the ground warms sufficiently. Watermelon plants grow quickly, but the fruits need 70 to 90 days mature, depending on the type.

Watermelons (Citrullus lanatus) are part of the plant family Cucurbitaceae, which also includes cucumbers and squashes such as pumpkin and zucchini. Although other types of melons, including honeydews and cantaloupes, are also part of the Cucurbitaceae family, they're not in the same genus of Citrullus.

Watermelons are generally planted from seeds when the ground has warmed to at least 70 degrees Fahrenheit. On average, it takes about 80 days for the seedlings to mature and produce ripe fruit.

Botanical Name Citrullus lanatus
Common Name Watermelon
Plant Type Vining fruit
Size Vines can reach 20 feet in length
Sun Exposure Full sun
Soil Type Sandy, loamy, well-drained
Soil pH Slightly acidic (6.0 to 6.8)
Hardiness Zones 3 to 11 (USDA)
Native Area Tropical Africa
Toxicity Non-toxic

How to Plant Watermelons

When you're growing watermelon from seed, plant them in a space that provides plenty of room to roam, as the vines can grow to be up to 20 feet long. Plant the melons on slightly elevated hills each containing 8 to 10 seeds, with 3 to 4 feet between hills. If planting rows of hills, allow about 8 feet between the rows. Drive the seeds down about 1 inch deep. The watermelon seeds will germinate in 4 to 12 days, but it will take about 80 days for the plants to grow fruit large enough to harvest.

Once the plant grows fruit that's about the size of a softball, add a layer of straw or cardboard on top of the soil to keep the fruit from having direct contact with the soil. This helps prevent rotting, and it discourages pests from damaging the fruit.

Watermelon Care

Light

To grow to their fullest potential, watermelons need full sun for 8 to 10 hours a day. Without enough sunlight—even if it's a matter of too much cloud cover day in and day out—the fruit will be underdeveloped and have an unappetizing flavor.

Soil

Ideally, plant watermelon seeds when the soil temperature is above 70 degrees Fahrenheit, and at least two weeks past the region's last frost date. If desired, you can cover the soil with black plastic before you plant, to raise the soil temperature.

Watermelons can tolerate a soil pH that's as low as 5.0, but they thrive in 6.0 to 6.8 pH. Add compost or another type of amendment to your soil to give the watermelons plenty to feed on. If you have the opportunity to plant the melons in a compost pile or have access to fresh manure, using that as your soil will help guarantee warmth and plenty of nitrogen.

Water

The sweetness of a watermelon depends, for the most part, on how much water it gets. Watermelon vines are sensitive to drought, so the soil needs to be consistently moist. It's best to water with soaker hoses or drip irrigation, rather than overhead watering, to deliver the water straight to the soil and to avoid fungal problems on the leaves.

Temperature and Humidity

Watermelons are warm-weather plants that need temperatures around 80 degrees Fahrenheit to thrive. They also require bees to pollinate their flowers, so cool, cloudy weather that limits bee activity means the watermelons won't grow as quickly.

Because watermelons require such warm weather, it's difficult to grow them in some northern regions—but it's not impossible. You can start watermelon seeds indoors until the air temperature is regularly above 50 degrees Fahrenheit, and then sow the plants directly into the ground once the soil temperature is 65 degrees or above.

Fertilizer

Watermelons need regular feeding to thrive. Add continuous-release fertilizer throughout the growing season for a steady source of nutrition.

Watermelon Varieties

There are four main types of watermelons grown in the United States:

  • Seedless watermelons are sweet and free from annoying seeds.
  • Picnic watermelons are larger—between 12 and 50 pounds—but feature those black seeds (which can be fun to spit out at a picnic).
  • Icebox watermelons fit into a refrigerator and weigh between 5 and 10 pounds.
  • Yellow-orange watermelons are round or oblong, with flesh that is orange or yellow; they can weigh between 10 and 30 pounds.

There are also a number of interesting or uncommon varieties of watermelon that you won't normally see at grocery stores, including:

  • 'Golden Midget': yellow rind with pink flesh
  • 'Orangeglo': striped green rind with orange-yellow flesh
  • 'Moons and Stars': purple rind with yellow dots and pink or red flesh
  • 'Melitopolski': traditional green rind and pink flesh; native to Russia
  • 'Densuke': black rind with pink flesh; native to Hokkaido, Japan

Harvesting Watermelons

A watermelon plant provides a number of clues that it's ready to harvest. Watermelon should be ready about 80 days after it is planted. At about the 75-day mark, start keeping your eye on it to see if it is ripe, looking for the following clues:

  • The tendrils that are normally bright green near where the watermelon meets the stem will turn brown.
  • The surface of the watermelon goes from shiny to dull.
  • The side of the melon that rests on the soil will turn from green to yellow.
  • It gives off a dull, hollow sound when knocked on. However, not all melons make the hollow sound, so if it does not sound that way, that doesn't necessarily mean that the watermelon is not ready to harvest.

Common Pests and Diseases

Watermelons are susceptible to a number of pests and diseases, including:

  • Anthracnose: This fungal disease in Identifiable by small spots on the plants that grow and turn black or gray. To control it, remove and destroy diseased plants. Apply fungicide to help prevent the disease.
  • Bacterial fruit blotch: Symptoms of this disease include water-soaked spots that spread and become necrotic on seedlings and young plants or fruit. The fruit's rind will crack and ooze yellow liquid. Remove and destroy affected plants, or spray with copper hydroxide fungicide.
  • Downy and powdery mildew: Downy mildew creates angular leaf spots that start as yellow but turn brown with purple spores, while powdery mildew showcases as a powdery white substance on leaves, which will then turn brown and die. These fungal diseases usually do not kill plants, and they are more likely to occur where air circulation is poor; give plants plenty of space to grow.
  • Gummy stem blight: This fungal disease appears as black wrinkled spots on leaves and dark, sunken areas on stems and fruit. Applying fungicides may control the disease.

To help prevent diseases, water the plant at the ground level only instead of watering the leaves. Additionally, garden pests love watermelon as much as you do. Watch out for cucumber beetles, vine borers, and melon aphids.

Vine borers can be prevented by using row covers to prevent the vine borer moth from laying eggs at the base of the plant (though the covers will need to be removed to allow the flowers to be pollinated).

Treat aphids with insecticidal soap and beetles with rotenone or pyrethrum-based insecticide. To prevent hurting the bees that are necessary for pollination, apply the insecticide at dusk.