Last year my watermelons got to about tennis ball size and stopped growing. I was so dissapointed. Here they are again at tennis ball stage and I'm holding my breath. The early ones I planted have already dissapointed me or maybe I dissapointed them because I think I over watered them and they got root rot. In a moment of desperation last week I gave them all a feed with liquid horse manure which I hope will do the trick. We may need some really warm nights too.