I've tried to grow watermelons several times but never get more than a few baseball-sized fruits. I'm trying to decide whether to make room for them this year. It sounds like you may have a selection that is not maturing quickly enough for your garden spot, says David Maas, University of Idaho extension educator.
Watermelons require a long, hot season to mature.
Choose a variety that matures in 80-100 days after planting and make sure the soil temperature is above 65 degrees Fahrenheit when you transplant or sow. While the ideal soil pH for melons is 6.0 to 7.5, they will do fine in more alkaline Idaho soils, Maas says. To reach their full potential, watermelons need full sun and six to eight feet between hills. Hold off additional fertilizer unless a soil test indicates its necessity. A recent study in the Journal of the American Dietetic Association suggests that, by the age of five, children will eat more food than they actually crave if they are given larger portions.