Posted: 13/04/2013 at 22:59
Hi Lorraine, That is an old article. You can no longer buy pressure-treated wood containing arsenic and chromium as they have been banned for such use. It doesn't help that authors of articles on the web rarely put a date on them! Here's a more up to date one:
Just about everyone (including me) uses pressure-treated softwood timber for building raised beds and planters as untreated wood will rot in very short order, unless it is one of the more rot-resistant hardwoods, such as oak, cedar or redwood etc. Those are all prohibitively expensive for many of us though.
Your idea of lining it with polythene is a good one. I would also recommend painting it with a water-based wood treatment such as fence paint. Look for a waterproof one as that will help seal it against any possible leaching of the miniscule amounts of copper which are used in the pressure treatment.