In my square foot garden, I have a trellis beside one edge. Am I being too ambitious to try to put a square for pumpkins, then an empty square, then a square for watermelon along the edge?
Melons and squash are vigorous growers. You are trying to use your vertical space and try to try to trellis them upwards. Yes, this is ambitious. ;) It's a little bit harder than it sounds, since they have a little bit of a mind of their own about where they want to grow. If you are gentle and patient, you should be able to coax them into growing up the trellis and then gently bind them to the trellis. Even with this approach, their leaves will probably spread out so much that they'll take at least two square feet.
It would be better if you gave them two square feet from each other, but with only a single empty square they'll just compete with each other for sunlight in the middle. Try to spread them a bit further apart on the trellis and make sure they aren't shading out the plants in front of them too badly.
Some other options that would work in your square foot garden:
- Give them (almost) all the space they want. Although they will easily be able to take over a 4x4 foot area, you should be able to guide and prune them into a 3x3 area. The space isn't necessarily lost to the squash though; you can companion plant them with anything else that grows higher and faster than they do. For example, corn and sunflowers will both grow high above squash, and the squash will keep the ground "mulched" for their companion. "Three sisters" is a famous companion planting technique that combines corn, beans, and squash.
- Think outside the box... literally. Root them on the very corner of your raised bed, and coax them to climb over the edge and down into your yard. If they don't take to the trellis for whatever reason, this is a good runner-up option.
Don't underestimate how much watermelons and the like spread. I've tried growing small pumpkins and mini watermelons before, and they can vine out easily many feet. I've never tried growing them up anything like a trellis, but my sense is that even small pumpkins might be too heavy for much upward growth without potential breakage of the vines. I had a wire fence around the edge of my garden, and I don't remember them growing very far up it, so I would expect them to grow more outward than upward.
In short, give them a wide berth (honestly, probably at least 2-3 empty squares between them) or be prepared to chop away at them to keep them from tangling up with each other and overtaking any other plants.