The summer months were made for growing watermelons—at least, for those who live in warm weather climates. Although watermelons aren't difficult to grow, they need a lot of nutrients, sun, warmth, and water to thrive. The sweet, juicy fruit grows during the summer months. Those in warmer climates can sow the seeds directly outdoors, while cold-climate gardeners can start short-season varieties of watermelon indoors.
Watermelons 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 part of the Cucurbitaceae family, they're not in the same genus of Citrullus.
- Botanical Name: Citrullus lanatus
- Common Name: Watermelon
- Plant Type: Vining fruit
- Mature Size: Vines can reach 20 feet in length
- Sun Exposure: Full sun
- Soil Type: Sandy, loamy, and well-drained
- Soil pH: 6.0 to 6.8
- Bloom Time: Summer
- Flower Color: Yellow
- Hardiness Zones: 3 to 11
- Native Area: Tropical Africa
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 a hill with eight to 10 seeds per row with eight feet between each row. The watermelon seeds will germinate in four to 12 days, but it will take weeks for the plants to grow fruit large enough to harvest.
To grow to their fullest potential, watermelons need full sun for eight to 10 hours a day. Plant the fruit in a location that doesn't have its light blocked by buildings or trees. Without enough sunlight—even when there's too much cloud cover day in and day out—the fruit will be underdeveloped and have unappetizing flavor.
Not only do watermelons need a lot of warm sunlight, but they also need warm soil. Plant only when the soil temperature is above 70 F, at least two weeks past the region's last frost date. Cover the soil with black plastic before you plant, which will also increase the soil's temperature.
Watermelons can tolerate a soil pH that's as low as 5.0, but they thrive in 6 to 6.8 pH. Add composted or another type of nutrient-rich 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 that it's warmer and has plenty of nitrogen.
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 it from having direct contact with the soil. This helps prevent rotting, as well as discourage pests from damaging the fruit.
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. Water the plant with soak hoses or drip irrigation, rather than overhead watering, to get the water straight to the soil.
Temperature and Humidity
Watermelons are warm-weather plants, meaning they need temperatures around 80 F to survive. They also require bees to pollinate its flowers, so cool, cloudy weather that lessens 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 not impossible. You can start watermelon seeds indoors until the temperature is regularly above 50 F, and then sow the plants directly into the ground once the soil temperature is 65 F or above.
Watermelons need regular feeding to thrive. Add continuous-release fertilizer throughout the growing season for a steady source of nutrition. Don't use types of fertilizers that contain weed killers because they will destroy the fruits.
Varieties of Watermelon
There are four main types of watermelons grown in the United States:
- Seedless watermelons are sweet but free from annoying seeds.
- Picnic watermelons are larger—between 12 and 50 pounds—but feature those black seeds.
- Icebox watermelons fit into a refrigerator and weigh between five and 10 pounds.
- Yellow-orange watermelons are round or oblong, have flesh that is orange or yellow, and 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
A watermelon plant gives off a number of signs 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.
- 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 make the hollow sound, so if it does not sound that way, it does not necessarily mean that the watermelon is not ready to harvest.
Common Pests and Diseases
Watermelons are susceptible to a number of pests and diseases. Potential diseases include:
- Anthracnose: This fungal disease in Identifiable by small spots on the plants that grow and turn black or gray. To control, remove and destroy diseased plants. Fungicides may 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, which are all common problems. Treat aphids with insecticidal soap and beetles with rotenone or pyrethrum-based insecticide. To avoid hurting the bees that are necessary for pollination, apply the insecticide at dusk.