Growing watermelons requires warm soil. Don't tuck plants into the garden until soil temperature is above 70 degrees F, which typically occurs about the time peonies bloom in northern zones. To be safe, wait until at least 2 weeks past your area's last frost date. Prior to planting, cover soil with black plastic to hasten soil warming. Because watermelons are heavy feeders, prepare your planting bed by adding seaweed, compost, or rotted manure, or amend the soil with aged compost-enriched Miracle-Gro® Performance Organics® All Purpose In-Ground Soil to improve soil texture and nutrition. For best nutrient uptake, the soil pH should be between 6 and 6.8, although the plants will tolerate a pH as low as 5.
Give watermelon vines plenty of room to roam, which usually means spacing plants 3 to 5 feet apart. After planting, cover seedlings with floating row covers to keep out insects and trap warm air near plants.
Watermelon vines bear male and female flowers. Don't be alarmed when some of the male flowers, which appear first, fall off shortly after they open; they are followed by female blossoms about a week later. The female flowers, which have a small swelling at the base of the flower, stay on the vine to bear fruit. When vines start to bear both male and female flowers, remove row covers.
Tackle weeds before vines start to run because it will be difficult to move among vines at a later stage without crushing them. Mulching soil under the vines helps suppress weeds and slows moisture evaporation.
Water plays an important role in keeping vines healthy and producing delicious fruit. Vines are most sensitive to drought during the time from planting to when fruits start to form. Avoid overhead watering. Soaker hoses or drip irrigation deliver water directly to soil, helping prevent possible spread of fungal diseases among wet foliage. Keep soil consistently moist, but not waterlogged, which will kill plants. It's typical for leaves to wilt under midday sun, but they shouldn't remain wilted into evening. Water vines early in the morning so leaves can dry before sunset, which will further help prevent fungal diseases.
Watermelons take a long time to mature, so be sure your plants are getting a steady source of nutrition throughout the growing season. Starting with nutrient-rich soil is the first step, but then you'll also want to feed them regularly with a premium quality continuous-release fertilizer such as Miracle-Gro® Performance Organics® Edibles Plant Nutrition Granules allowing label directions.
Keep ripening watermelon from direct contact with soil to prevent rot and protect fruit from pests and rodents. When fruit is about the size of a softball, place it on a bed of straw or cardboard. Setting fruit on a light-reflecting surface, such as aluminum foil, will concentrate heat and speed up ripening. If large critters, such as groundhogs, discover your melons, protect ripening fruits by covering them with laundry baskets weighted down with a few bricks.
Some gardeners like to switch fertilizer during the course of the growing season. To do this, use a fertilizer with more nitrogen than phosphorus and potassium during the period between planting and when the first flowers open. Once flowering begins, use a fertilizer with less nitrogen and more phosphorus and potassium, such as African violet food or liquid seaweed.
Some believe that pinching off a vine's growing shoots as watermelons start to ripen will cause the plant to divert all its energies to fruit ripening. Recent research has shown this to be false. It's a vine's leaves that produce the sugars that sweeten fruit, so anything that reduces the total number of leaves available for sugar production actually lessens the sweetness of the melon.
In colder regions, remove any blossoms that start to develop within 50 days of your area's first average frost date. This will help ensure that remaining, larger fruits will ripen before frost.
Troubleshooting
Watermelons are in the same plant family as squash and cucumbers, but they do not cross-pollinate successfully. Your garden will depend on bees to pollinate the flowers, so cool, cloudy weather in the spring will slow down their development, as bees are less active in such conditions. Be patient until the weather warms.
Fungal diseases can multiple rapidly on melon leaves. Alternaria leaf spot, anthracnose, and gummy stem blight produce spots on leaves, while stem blight also forms bleached or tan sections on stems and rot on fruit. Downy mildew causes yellow or pale green leaf spots, while powdery mildew produces white spots on leaves. Treat fungal diseases with fungicides. Check with your local garden center or Extension Service to learn which fungicides are approved in your state for the disease you're fighting.
Also be on the lookout for pests. Melon aphids, for example, can quickly colonize a vine, so inspect leaf undersides daily. If you spot aphids, treat them with insecticidal soap. Spotted and striped cucumber beetles can attack vines, transmitting bacterial wilt disease, which causes vines to collapse without chance of recovery. Treat adult beetles with rotenone or a pyrethrum-based insecticide; apply at dusk to avoid harming honey bees.