Watermelon requires a long, warm growing season and plenty of water to reach peak flavor at harvest time. In areas with short summers starting the watermelon seeds indoors helps extend the growing season, but using black plastic mulch ensures you get the most out of the summer heat for your plants. Black plastic helps warm the soil while preventing weeds and conserving soil moisture. Black plastic mulch also keeps the melons from setting directly on the soil, which prevents some pest and disease issues in the fruit.
Prepare the garden bed once all danger of frost is past in spring and the soil temperature is above 65 F. Lay a 2-inch layer of compost over the bed and apply a starter fertilizer following label application recommendations. Till it into the soil to a 10-inch depth using a hoe or power tiller.
Lay a drip irrigation hose along each planting row. Space rows 7 feet apart so the watermelon vines have room to sprawl.
Dig a 2-inch trench around the perimeter of the planting bed. Set the soil removed from the trench outside the bed.
Lay the black plastic mulch over the entire bed, pushing the edges into the trench. Refill the trench with the removed soil and tamp it into place. This holds the mulch down so wind doesn't pull it up.
Cut a 3-inch diameter hole in the plastic mulch for each watermelon seedling, spacing the holes 3 feet apart. Plant the seedlings to the same depth they are at in their nursery pots and firm the soil lightly around them.
Water the watermelon seedlings immediately after planting so any air bubbles in the soil around the roots collapses. Continue to water them via the drip irrigation hoses, providing approximately 1 inch of water per plant each week.
Harvest watermelons when the small tendril-like vines near the stem turn brown and dry and the skin of the melon becomes dull. Cut the melon vine off 3 inches from where it joins to the melon to harvest.