No trellising? 3-5' square each then assuming you need to have some access to the bed. ;) Closer together can increase numbers of fruit but reduced size. Larger spacing increases fruit size. Rather than alternating I would suggest a half and half bed with each variety grouped on it's own half.

Dave

I'd keep the loupes and watermelons in separate plots.
One of my melon plots is equal to your 20'x50' plot and I just planted 15 watermelon plants within that area (3 rows @ 20' long, 5 plants each). This has served me well in the past though it's a tight squeeze when thumping and picking by August. Right now I have bush beans around the outer perimeter of this plot which will be harvested and yanked out by the time the melon vines over take them. Early Broccoli also works well in this manner to use space that will be open later on for the melons to spread out.

bart1

How do you pin down your black plastic? Soil, pins, other weights?

anney -
I used some old boards from a collapsed deck, big rocks (my garden has tons) and unused fire wood. I bring that stuff in by the wheelbarrow load and spread it around the edges.

I'm in a pretty windy area and this works well.

Bart

B1

Thanks. I'm thinking of using rocks or pieces of lumber we have waiting for the fireplace next fall.

Thanks to all for the spacing advice.

Dave - I don't do trellising (yet!). I can only visit my garden on the weekends so I barely have time to plant let alone build a trellis and train the plants.

Is there a "proper way" to navigate the melon patch come harvest time? Everytime I go out there to check on ripeness I feel like a bull in a china shop trampling vines.

Thanks again,
Bart