Instead of wood why don't you consider using PVC boards? PVC boards look like wood but never warp, crack, rot, peel, etc. You can cut them just like wood with saws or nail, drill like wood. Plus in raised beds you won't have to worry about chemical preservatives leaching into the soil. Azek is one maker of these boards. They come in different sizes. I think you could use use the 1 x 8 or 1 x 10 boards to make raised beds although I didn't use these for this purpose. We used these to replace high maintenance trim boards on our house. You can paint them just like regular wood trim boards but they don't peel or warp like other trim boards. We are happy replaced our trim with this product.

Here is a link that might be useful: Azek Trim Boards

You can try C&S Lumber on 124 Main Street in Millbury, MA. They would be down 146 south from Worcester. Nice bunch of guys - tell them what you want to build and what dimensions and they will tell you what type of wood is good for your project. They will even cut it down for you and help load it in your car.

Azek would make for VERY pricey raised beds. It is also very flexible and would need to be staked or otherwise supported at frequent intervals. If you desire to use a plastic "board" for raised beds I think one of the plastic decking products would be a better choice (although still quite expensive).

Azek and similar PVC trimboard products do appear to offer some advantages (some drawbacks as well such as thermal expansion and environmental issues--disposal). Only time will tell whether the durability advantages outweigh the disadvantages and cost. We have done some exterior trim with both Azek and Koma PVC. This year we will be trying a new composite trim board (MiraTEC). In about 10 years we'll know how these stack up to wood.

VT
(whose raised beds are ACQ treated wood)

Here is a link that might be useful: MiraTEC

My raised beds are made out of plain old untreated pine, whitewashed before they were filled. The oldest is seven years old, and looks like it will definitely see ten. The plan is that when they wear out, we'll just start all over again. At that point, the wood was dirt cheap. The plated lag bolts holding the things together were the expensive part.

I've been looking at vinyl trim to replace the rotten mouldings around the garage door. Thermal expansion isn't a problem I was expecting, but maybe I should have been. The real problem is that the aluminum flashing directly above the moulding has a bad join which was letting water down on the wood.

Thx for all the information. I will check out C&S Lumber this weekend and see if I can build the bed within my budget.