I don’t like to use redwood lumber as garden bed edging any more than necessary, and especially not 2-by-12 lumber, which needs to come from larger trees. The solution I’ve found is straw wattles. Normally used for erosion control, they come in 25-foot lengths and are 8 inches thick. They’re infinitely flexible and eventually compostable. I expect them to last about three years. Each 25-foot wattle costs me about $17, and they are available at construction and irrigation supply stores.
In my garden, I put gopher wire and some cardboard under my raised beds for weed suppression. Then I use the wood stakes that come with the wattles to hold the wattles in place.
Santa Rosa, California