ledger board
n.
The top railing of a fence or balustrade.
A narrow horizontal board attached to a row of studs to support the ends of floor or ceiling joists. Also called ribbon.

If you can get them, and don't mind the informal look, try using untreated logs, and let them slowly decay as part of the garden.

You might also consider trough gardening with enough room under them to park a stool so you can sit and potter... I find them great for succulents and sempervivums, and for small, precious alpines.

For regular gardening, just make the rows wide enough for hitching along if your mobility is compromised. I know of two people, both amputees, who garden in this way. Old pants, obviously.