Companion planting is a great way to grow onions and watermelon side by side. The onion needs a long growing season but does not take up much space, and the watermelon needs a long growing season and takes up a lot of space. You can plant them together and leave room for other vegetables when your garden space is limited.
Plant your onion sets as soon in the spring as you can work the ground. This is generally around March or April, depending on how far north you live. If you are lower in the temperate zone you might be able to do this in January. Plant them in rows, 1 inch deep and 3 to 4 inches apart from each other. Make sure you plant them on the outside edge of the southern side of the garden for maximum sun exposure.
Plant your watermelon seeds in May or after the last danger of frost has passed. Generally, you will only need two or three plants at most to produce enough watermelon for the average family. Form a couple of 8-inch hills in between the onion rows and plant two or three seeds to each hill. When they sprout, you can thin out the extras.
Train your watermelon to grow between the onion sets and outside your garden space. The greens of the onions will rise above the watermelon leaves, catching enough sunlight for good onion formation, and the watermelon will grow right around the onion greens.
Harvest your watermelon when the stem has browned and the melon sounds hollow. When you pull up the watermelon vines, you can pull up the onions as well to dry and store for winter.