Watermelon vines can reach 20 feet (6 m) in length. [1] X Research source

The ideal pH for watermelons is between 6 and 6. 8.

You can either use large sheets of black plastic or landscaping fabric from a gardening store, or you can cut open black garbage bags and use those. Landscaping fabric is a great option if you want to reuse the material. It also allows water to trickle down to the soil below, unlike plastic. You can easily cut slits in the fabric to allow your plants to grow up through it.

If you live in a cooler climate, you can start your seeds indoors and transplant them outside once the soil warms up. Start them about two weeks before the last frost date and plant them two weeks after the last frost date.

If you live in a cooler climate, black plastic mulch or landscaping fabric will help keep the soil warmest as the watermelon seeds grow.

Once the fruit has formed, you can cut back on watering. Only water the plants during dry spells when there’s no rain. Make sure that the plants do get watered once a week, either through rain or irrigation.

Make sure there’s an opening in the lid for the stem, and that there’s a lid that opens, closes, and latches. Glass and acrylic boxes are ideal because they allow sunlight to pass through. You can find special molds designed to grow watermelons, which are available from internet-suppliers. You can also use a wooden or metal box with lots of holes in it. Alternatively, you can make your own mold from wood, metal, or another material. If you’re stuck for molds, you can also use a concrete or cinder block for the mold, but this will not yield a perfectly square watermelon.

Place the box on the ground with the lid and stem at the top.

As the watermelon grows, the fruit will get bigger and will eventually take on the shape of the mold. Monitor the growth of the watermelon. If it appears to be too big for the box, take it out and allow it to finish maturing outside the box.

A watermelon is ready for harvest when the tendrils around the stem dry out, a light patch develops on the bottom of the fruit, and the fruit sounds dull and hollow when you tap on it. [10] X Research source