Hydroponic gardens use a soilless medium and liquid nutrient solutions to grow fruits, vegetables and other plants. Though you might think that the size of watermelons would make them hard to grow hydroponically, they actually grow quite well in such systems. The biggest problem with growing watermelons hydroponically is making sure that they have sufficient light and support for their weight because watermelons can become quite heavy.
Watermelons typically grow well with general-purpose fertilizers. Most general-purpose nutrient solutions contain healthy amounts of major nutrition, secondary nutrients and micronutrients, allowing hydroponically grown watermelons to grow and prosper. A homemade hydroponic solution involves 1 teaspoon Epsom salts and 2 tablespoon general-purpose fertilizer mixed into 1 gallon of normal water. Any nutrient solution used when growing watermelon in a hydroponic garden should be drained and must be replaced in every two to 3 weeks to ensure that it contains sufficient nutrient levels