Growing watermelon on a trellis makes use of free vertical space in a garden. It eliminates the space that a sprawling watermelon plant takes up on the soil and walkway surfaces. It allows you to plant watermelon plants closer together, and makes harvesting fruit easier because you do not have to bend over to cut the fruit off the vine.
Anchor your trellis firmly to a tall fence or wall. Watermelon vines can be very heavy so use large, heavy-duty nuts and bolts. It can also be anchored to the ground or in a large bucket of concrete. Your trellis can also be a series of heavy-duty wires attached to a garden wall.
Create a 12-inch high hill of mounded potting soil at the foot of the trellis. Plant 6 seeds in the hill 1/2 inch deep 1 to 2 weeks after the last frost. Water the hill gently to settle the soil.
Thin the seedlings to three plants per hill once they are 2 inches tall. Pull up the weak or slow growing plants by hand. Grasp the base of the weak plant and pull upward to dislodge the seedling. Keep only the three strongest seedlings that will provide you with the best growth.
Mulch the hill with bark, shredded newspaper or dry straw. Spread it on top of the soil around the plants either by hand or by using a shovel. Keep the mulch level between 2 to 3 inches. Mulch will keep the soil warm, preserve water and suppress the growth of weeds.
Train the watermelon vines up the trellis as it grows. Weave the vines in and out of the trellis slates by hand. Carefully pull the end of the vine through a hole in the trellis and out the back. Pull the end through the next hole from the back to the front once the vine is tall enough to reach. Repeat this process until the vine reaches the top. Once at the top, either pinch off the ends of the vines or train them to go down the other side.
Tie strips of stretchy fabric to the trellis to form slings as the watermelon sets its fruit. Use old T-shirts or pantyhose. Stretchy fabric will allow the fruit to grow while acting as a support. Place the watermelons in the fabric slings.