I would like to know the answer too. I planted both watermelon and pumpkins at the same time 5/11. The pumpkins plants are substantial bigger and are flowering.

I originally had planned to start them from seed at least 6 weeks in advance in a orange mesh produce bag to allow me to put it in the ground with roots undisturbed (which seems to be standard advice not to let them become root bound or disturb roots when transplanting though other say no problem (?))

I think doing something like that plus putting a cold frame over it in early may and late august might be neccesary in MN to give them the heat they probably need.

They don't grow quickly like squash so right now they are not very big. But they will grow quickly later and produce. Did you get some last year? I had tons due to the heat. Make sure the soil is up to snuff and the plants get enough water when planted out. Also need lots of pollinators.

Do you have flowers or fruits yet?

I don't even have vines. They're just kind of sitting there. I did hand pollinate last year and it didn't seem to speed things up at all.

Do you plant any flowers attractive to bees as part of your garden?