Apart from adding boron, as we have suggested above, there are some things that you can try if you're looking to have sweet jubilee watermelons for your enjoyment. The first is to lay off on the watering. When your jubilee watermelons are young, you will want to give them a lot of water. Doing so will help the plant grow more vines.  But once your jubilee watermelon produces fruit, give it less water. This will force your plant to concentrate on producing sugars for the fruits. Your jubilee watermelon will not only be sweeter but will also have better flavors. Before planting, you can also give the soil about four to six inches (10 to 41 centimeters) of composted manure. Further, if you want more fruits, you can put plastic mulch around your jubilee watermelon. Plastic mulch will help prevent weeds from growing and competing with your watermelon, but this material will also help heat up the soil.