Black Diamond Watermelon is an open-pollinated heirloom variety known for its dark rind and enormous fruits, which can weigh up to 50 pounds! These melons have a red flesh and a high water content, making them juicy and refreshing. They require a long growing season to reach such a gargantuan size with vines as long as 20 feet. About 90 days is what they should take to mature. They need at least 6-8 hours of full sun per day. For the biggest melons, you should limit the vines to a single fruit per plant. Black Diamond watermelon is suited for zones 3-9, but they enjoy hot and humid temperatures. They require plenty of water, but avoid soggy soil. Their ideal soil is a slightly acidic, well-draining soil rich in organic matter.. These melons are too big for trellising, so you will need to devote ample space for them to spread. You can use an old pallet or something similar to keep the melons off the ground. Even a grate or grill propped up on some bricks or a 2×4 frame with hardware cloth can work similarly. If not that, then a bed of straw will work. You can intercrop with fast-growing tall plants that will raise above the patch before your watermelon vines spread out if you want to utilize the space where the leaves spread out to. The watermelon leaves can help act as a groundcover if you choose the right companion. You can also try planting at the base of young fruit trees as long as they will not shade the watermelon too much.
Use nitrogen fertilizers during vegetative growth, then switch to phosphorous and potassium-based fertilizers when flowering begins to help support better blooming and fruiting. You can use bonemeal or rock phosphate for phosphorous, but these release rather slowly, especially the rock phosphate, so you may want to consider blending these in when you do your soil preparation. You can use wood ash, burnt cucumber peels or banana peels for potassium. Powdery mildew is common as with all plants of the cucumber, melon, squash family.. You can also use a copper fungicide or neem oil on the leaves as a preventative, but it is important to increase airflow. Proper removal of affected leaves early on can help with controlling the spread. You should also avoid planting anything in this family in the same location two years in a row. Reduce watering about 2 weeks before harvest time to increase the sugar content.
Sow your black diamond watermelon seeds ¼” deep in a well-draining fertile soil. You can start in seed-starting cells or containers, but it is also easy to direct sow as long as the daytime temperatures are consistently above 70 degrees F and nights above 50 degrees F. Starting indoors may be recommended in areas with shorter seasons. Otherwise, direct sowing is usually easier and faster. For direct sowing, you can build up mounds of soil about 6 to 12 inches high and 5 feet across. Sow about 6 seeds across the mound and thin out the best two or three seedlings. Mounds should be spaced about 5 feet apart from each other. You can use 20-gallong containers if you wanted to grow in containers, but the ground will always be better. A drip system for watering is the best as it provides even moisture and does not get any water on the plants. Moisture on the leaves is what allows fungus to thrive. So you should water early in the morning letting your hose soak into the soil below the leaves. Avoid spraying as that can kick up dirt and pathogens and provide them moisture to thrive. Watering in the morning ensures you have the whole day for the plant to dry out, and it is also prepared with a fresh supply of water for the days sun, kinda like having a balanced breakfast in the morning
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