I do recommend that you refer to The Square Foot Gardening Book by Mel Bartholomew for good information on how many of each plant to place in a square foot area and some good set up ideas for vertical frames and covering cool season plants. His plant by plant info is general enough that it is ok too- I do not use the same mix that he recommends in his book for my
square foot garden ( I have tried it and it isn’t bad but it is hard to mix by yourself and not my favorite)– I prefer
to use Lady Bug Premixed Square Foot Garden Soil with some additions.
For every 4 bags of Lady Bug premix –I added 2 bags of Lady Bug revitalizer compost ( or you could use 3 cubic feet of your own compost) + 6 cups of lava sand and 2 cups of Sul-po-mag or K-mag.I planted from plants and put a handful of 50/50 worm castings and rock phosphate mixture in the holes—if you are doing seeds—work a cup of each product in each square. When planting cool season plants- make that mixture equal parts of worm castings, rock phosphate and alfalfa meal . ( The alfalfa meal really picks them up after our usual on & off cool spells)
The average square foot gardens that are shown in the Square Foot Gardening Book and also most of the ones sold as prefabricated kits in garden centers -are made with boards about 6 inches high and laid on top of soil or even concrete—with all sorts of ideas to make dividers to separate the 1 foot squares. If this suits you –I have seen this work . ( although I can’t believe that deeper soil wouldn’t be best)
I wanted to make my beds raised high enough to afford me a sitting area along with a planting area inside the frame – so I designed my own. My square foot garden is 2 cinder blocks high-or about 16 inches deep — and my planting space inside the cinder blocks is 4 feet wide and 8 feet long–( the cinder blocks are 8 inches wide)– so over all– the whole space takes up 5 feet 4 inches wide by 9 feet 4 inches long –you would need 42 Cinder Blocks to do one like mine. You will see different ideas in the book –those are fine too–do what suits your needs best. You can buy fancy cinder blocks at building supplies in all natural brick and stone colors and textures.
I did exactly as follows-and had great luck with mine…
Remove or cut short the grass and weeds in the area that your garden will cover —-sprinkle DE ( garden grade Diatomaceous Earth that you buy in a garden center-not the kind that goes in the pool) on top of the soil. It doesn’t
have to be a real thick layer – I used about a cup. Using the DE will keep fire ants from invading your bed.
Another good tip is to use hardware cloth on the floor of your beds to keep moles from coming in to shop for worms. You can buy hardware cloth- cut to fit -off of big rolls in many garden centers and hardware stores. Lay down pieces of overlapped cardboard-wet it down good- and walk on it—build your garden on top of this.
If you are making it as tall as mine you can fill the bottom portion of the garden (up to the first cinder block ) with an inexpensive potting soil —I used 10- 2 cubic foot bags of living Earth potting soil to cover the bottom of my raised garden and then added 8 bags of the Lady Bug mix with 4 bags of Revitalizer compost mixed in with it to fill my inside garden area of 4’X8’-and I amended as mentioned.
Since I planned to use the holes in the exposed cinderblocks for perennial herbs-I used potting soil in those all the way to the top-it took about 8 bags of soil. If you choose to do a lower square foot garden bed – it will take 5 -2 cubic foot bags of potting soil for a 4X4 square X 1 cinder block high planter—but if you are only doing a 6” high board-like in the book-you will need only the planting mix on top of the cardboard—you need 4 bags of the Lady Bug for every 4X4 space. Add the other amendments too for best results ( compost, worm castings rock phosphate, Sul-po-mac or K-mag– and in the fall alfalfa meal)
The other thing you need to do -is mark your 1’X1’ squares off—as I mentioned you will see all kinds of things used- like string- vine tape-old blinds etc—I wanted something easier to handle and something that would last forever and look nicer-so we make light weight metal grids that we make to order –just e-mail me.