I have heard not to use anything chemically treated in case the chemical leaks into the vegetable bed. That used to be true when CCA treated lumber was arsenic based. But that is no longer the case (since 2003). Many of the new chemically treated lumbers available are USDA approved for use around the gardens. Some still object to it but many do use it. Your choice. Concrete blocks is another option often recommended in the many previous discussions of this question as is the corrugated metal Len mentioned. Old railroad ties is another. Decorative stone is another. There are also several new plastic composite boards available - but expensive. Dave

This is our 3rd season for concrete block square foot beds. We use the heavier (for stability)concrete ones which cost $1.00 apiece last year from a local supplier (Home depot and Lowe's were a little more expensive). First year we made 4x8 beds, 2 blocks high. To get 4x8' interior space it took 40 blocks (seven each end and 13 each side, overlapped). Also used 18 flat blocks to cover the holes at $.70 each, so a 4 by 8 bed was $52.60 total, which was definitely cheaper than plastic wood. You may only need 1 level high, depending on your existing soil. Top flat blocks are nice to sit on and easily removed to plant in holes, stick in a trellis, or set on the ground when muddy. Grew marigolds and herbs successfully in holes. You can carry a few in your car, so just depends on how many you need and how many trips you want to make. Last year we expanded the beds to 4x16 interior so had them delivered. We just hand leveled (well, fairly level!) as we set the blocks ourselves, slow and heavy, but you only have to do it once! Also rolled landscape fabric around the inside edge of bed before filling with dirt, helps keep moisture in. First year filled with bags of compost & topsoil, but last year ordered 'topmix' of mushroom compost, topsoil & sand delivered & shoveled in, mixing in a few big bags of vermiculite from Atwoods. After initial labor it's wonderful to add a little more soil, measure & plant as I'm doing now that our icy winter has finally thawed. I can email photos if interested.

The type of grass may still send runners that will show up in your garden. Typically after you have tilled, you can at least notice the new grass more quickly. You may just need to spend time weeding through that. At the spots where the concrete blocks are lieing, you may also want to remove the grass there, and then lay down weed blocker.

If you buy used bricks, then you might be concerned about where they were used and if it was in an industrial process that deposited some heavy metals onto their surface. Assuming new brick, it is probably less toxic than treated wood. I think it would be a good idea to get a soil lab to test for heavy metals in your soil a few months after you build a new raised bed for growing food, just to be sure. Heavy metals are more common than people understand. In the Bay Area, a lot of the soil is contaminated with nickel, and guess what: my own heavy metals test shows very high levels of nickel in my blood and hair.