Recommend an OP watermelon for raised beds
 

I have a raised bed that is about 18 inches deep and is 5 feet wide by 10 feet long. Can anyone recommend an open pollinated or heirloom watermelon that can be grown in this sized bed, please? I am not interested in trying to trellis a melon so these will be lying on straw in the beds. Another thing I'm wondering is how many vines will fit into a bed this size? I'm guessing only two but I don't really know. Thanks in advance for any advice.

Sugar Baby is a very popular one that does fine in raised beds (even if there's black plastic on the bottom a foot and a half to two feet down or so). We've done that. I don't remember how many vines we had, but I'm guessing maybe four. Our beds are square (about eight feet by eight feet).

Yellow Doll is a good one here. Sweet Dakota Red is another. I will never recommend blacktail mountain though.

I've grown Sugar Baby this year for the first time, in a new raised no-dig bed. I did two plants in the same hole.

I'm three months into the growing season now (Australia), the vines have grown well but still very manageable (the leaves are tiny!), had two fruit so far and hope to get a few more.

How many per bed sort of depends on whether you need to keep them within the bed or have room for them to trail over the edges. The bed I am using is only around 5 feet by 5 feet, and they are sharing with a HUGE butternut squash plant, but I planted them in diagonally opposite ends of the bed and have trained the squash to grow over the edge of the bed and down the path while the melons get the bed surface to run around in.

For a home garden anything over 2 watermelon plants is nuts (IMO) unless you can eat 20 pounds of melon a day or are growing it to give away. I would allow a 4 ft x 4 ft area per plant.
After you harvest your 3rd melon with more on the way, you'll probably start running out of storage for them unless your family is a watermelon eating machine.

My next door neighbor planted a Sugar Baby in 10" plastic pot and got 2 melons off the vine. He just let the vines wander around the ground beside his 2 small raised beds. He would move them out of the way for the lawn mower. I didn't even know it was possible to grow them in a pot. But I'm going to try it myself this summer.

I bought a close-out sale Sugar Baby from the garden center last summer and couldn't find room for it in my garden. So I planted it in a clearing in the woods near one of my compost piles thinking it would at least get nutrients from the compost. It survived for a couple of months but finally died of dehyration. It was a brutal summer and, unfortunately, my garden hose wouldn't reach that far. If I had known, I would have given the poor Sugar Baby a plastic pot. :bummer: