Posted: 03/07/2014 at 17:58
For veggies it is probably best to use untreated wood, so it will rot eventually.
My solution was to use old joists, which may or may not have been treated but unlikely to have any residue to leach out now. Untreated 6x2 joists should take a few years to rot out!
If you have some kind of reasonable soil underneath (it can be quite thin under a lawn - I get about 1.5 spade depths before I hit rocks and clay) I would mark out the footprint of the bed, dig it 2 spades deep (if possible, less if the soil is not so good), temporary lift out the soil to a couple of inches deep and level the shallow pit, then position the bed so that when you refill it, it will be sunk a couple of inches below the turf level. No liner required.
Soil cannot escape underneath, worms can get in, long roots can get out, excess water will seep out between planks, the sun will warm the bed quicker than the ground and you can work you patch at a comfy height.
Every few years you may have to replace or repair a bed, but you might be surprised, where the soil sits tight against the timber lack of air may just prevent rot from living there and attacking the timber - my beds are too young to say from experience!
If you are going to fit a plastic liner in a raised bed, you might as well use growbags and fit skirting boards around them for appearance Now there's an idea - I had a spare bag of compost I decided to cut the side off and sow some carrots in (for baby carrots, hopefully there is enough depth for that, they certainly won't grow full size!), it looks a bit ugly, maybe I could clad it to improve the appearance....