Grown for its red, juicy, sweet fruit, a watermelon plant (Citrullus lanatus) requires regular maintenance to ensure it remains healthy and productive. Watermelon seeds grow best when sown in the garden after the last average spring frost date, when soil temperatures reach at least 70 degrees Fahrenheit. These warm season plants readily grow when planted in old tires filled with potting soil. The black rubber of the tire collects heat from the sun, warming the soil for the watermelon vines and increasing their growth rate.
Fill a bucket half full of warm soapy water. Splash the water over the outside and inside of the tire. Scrub the entire surface of the tire with a scrub brush to remove any dirt, debris or stains. Rinse the tire off thoroughly with water from a garden hose. Place the tire on a clean surface in direct sunlight and leave it to sit for one hour to dry. Measure the width of the tire with a tape measure.
Remove weeds, rocks and debris from a 6-to-8-foot-square planting site that receives full sunlight and contains well-draining soil with a pH of 6.5 to 7.5. Spread a 4-inch deep layer of compost over the planting site with a rake. Mix the compost into the top 10 to 12 inches of soil with a garden fork or tiller. Smooth the soil's surface with the rake.
Dig a 2-inch-deep circle in the center of the planting site with a shovel or trowel. Make the circle equal to the width of the tire. Place the tire in the circle, with its cut side facing upward. Tamp the soil against the outside edge of the tire to secure it in place.
Fill the tire to 1-inch below its top with potting soil. Tamp the potting soil down firmly to fill in any spaces and remove excess pockets of air.
Spread 2 to 3 teaspoons of 10-10-10 nitrogen, phosphorous, potassium slow-release fertilizer per square foot of soil evenly over the potting soil in the tire. Mix the fertilizer into the top 3 to 4 inches of soil with a trowel. Tamp the potting soil back in place, smoothing its surface. Water the soil with a garden hose, moistening it to a depth of 10 to 12 inches.
Make a 3-inch-wide and 1/2-inch-deep hole in the center of the potting soil with your fingers. Place 6 watermelon seeds into the hole. Fill in the hole with soil, tamping it over the seeds.
Water the soil when the top 1/2 inch begins to dry. Keep the soil evenly moist but never soggy. Watch for germination 10 days after planting the seeds.
Pull the three or four weakest seedlings out of the soil when they reach 1 to 2 inches tall or develop two sets of true leaves. Cover the remaining seedlings with a fabric cover, removing it during watering. Remove the fabric cover permanently when daytime temperatures reach 90 degrees Fahrenheit or when the plants produce flowers.
Water the watermelon seedlings when the top 1 to 2 inches of soil becomes dry, applying 1 to 2 inches of water at a time. Pull weeds from the potting soil and around the tire as they appear. Allow the watermelon vines to spread outward and over the sides of the tire naturally.
Dig a 2- to 3-inch-deep trench in the potting soil around the watermelon plants after the vines produce runners. Space the trench 6 inches away from the plants' centers. Spread 1 to 2 tablespoons of 21-0-0 nitrogen, phosphorous, potassium slow-release fertilizer evenly in the bottom of the trench. Backfill the trench with soil. Water the area immediately.
Cut off all but the two or three largest fruits from each watermelon plant when they reach the size of tennis balls. Place a 12-inch-square plywood board under each fruit to protect it from soil-borne pathogens and insects.
Spray neem oil mixed with water at a rate of 2 tablespoons per 1 gallon of water on foliage infested with whiteflies, green aphids, web-spinning spider mites or waxy mealybugs. Apply the neem oil during the morning hours, coating both sides of the plants' leaves completely. Repeat applications every 7 to 14 days until the insects disappear.
Harvest the watermelons when they reach full size and turn dark green in color. Cut through the fruit's stem with a knife, 1/4 inch above the melon.