Don't worry. If you run out of room, just pull out the under-performing plants and forge ahead. One zuke is more than enough for a family of four for fresh eating and it'll start producing long before it gets huge. You can always succession plant -- so when the one zuke gets bigger and production starts to taper off, the replacement is getting big enough to flower. There's also a climbing zuke that likes growing in containers, but I haven't tried it. When I lived in California and had a small patio, I gardened square foot style. Most everything grew with precision and exactly as the books and web sites say. In Alabama? I think there's steroids in the dirt. I get monsters here. I experimented with small melon plants last year, and Minnesota Midget really does live up to the description but all the melons ripened at once and then the plant died. I got 5 4-5" melons off a tiny little plant with a couple of 4' vines, though. This year I am trying a reportedly smaller zuke plant (cocozelle) and smaller winter squash varieties. I have lots of room but the BF doesn't eat many veggies and I need smaller portions!

Oh brother, I scewed this garden up badly. Last fall I decided I was never going to like it in iris after not waiting long enought to see how they do. I moved a batch of them iris and started the iris rows upstairs. Then I planted daylilies. This spring I see that I should have left it all alone and it was fine. Moved the daylilies out of there. So now that the iris there are finished blooming I am trying to fix that garden. Its going to be all TB iris as originally planned for the big square. I moved that rose in the picture last year and put 4 iris in the little square. Today I moved the four iris out of the little higher square and I put them back along the edges. Will continue to move iris from the second row back to the edges. But meanwhile I had planted a vitecila summer blooming clematis in the tower to give summer color. Today I put more plants in the small middle square. I put Agastache (hummingbird mint) in each corner, then yellow columbine in the middle along the edge between the Agastache. Around the tower I put common orange Butterfly Weed all around. So the small square is done. I did this today. Then I will continue to fix the iris along the edges. I just make so much work for myself by messing things up but at last I am getting this right.

I don't think there's a need to raised the sides. I had a similar situation and added Mel's mix on top of what was there. Unless you have problems with bermuda grass coming up in the bed in the past, the weed barrier isn't really necessary. The photo above is a 4X7 garden with pine sides. Here is a link that might be useful: My raised bed garden

Not sure about composite decking. Is it treated wood? You don't want chemicals leaching into your food supply. Why not used Cedar?? 1 x whatevers are fine, but do you really need to make them 20 feet long? I'd use cedar and use 12" wide boards rather than two 6" wide boards. Just seems like it would be cheaper and a lot less hassle. Cedar will last years and it will not put chemicals into you food. I'd recommend splitting your 20 ft long beds into 2 ten foot long beds, especially if you were to use cedar. I made a 12 x 3 bed out of 1x12x12 cedar last year. I buried the lower 1/2" to keep the bottom from bowing and then just ran a 1x1 cedar post between both sides right at the midpoint. This sucks both sides in a the top and aesthetically, creates separation in the box. If you are dead set on using composite decking material, then just run a few 1x1 railings posts between the sides. I know, picture quality not so good, but it was through a screen with my phone, so didn't expect much. Anyway, you get the idea. And yes, it is snowing in Minnesota today!!