You can trim back melon (canatloupe and watermelon) vines to somewhat control the size and cause more stem shoots, but the melon will get to be a very large plant regardless of how much you tip the runners. Melons are planted in early spring and again in August in hot climates. Plant after danger of frost in areas that receive freezing winters. Trying to mature melons in the heat of summer causes poor fruit set and development. Be sure to allow plenty of bees and pollinating insects around when both male (flower only) and female (flower with small fruit attached) flowers are present. If you are not getting both types of flowers, you may have planted too late or be growing in too much shade - melons must have eight to 10 hours of direct sun daily.
