I'm tearing down a shed, and considering the painted particle board walls for putting at the bottom of some tall raised beds. Is that a bad idea? Can/should I use the other structural wood from the shed?
Particleboard is an engineered wood product composed of wood fibres and chunks mixed in with a binder and pressed flat. The wood fibre is just cellulose and will rot down, however the binder is the equivalent of putting a bunch of plastic into the pile. It does not rot. There have evidently been some experiments to grind up and compost such board and as the piles are turned in the composting process the gases have a chance to escape; in a hugelkultur bed the pile is not turned so anything buried is trapped.
The fact that these panels are painted just makes the situation more problematic. At some point someone should ask what is in the paint. Some quite toxic materials have been used in the past to make paint.
Check with your local recycling authority and see what they recommend for disposal. In the majority of cases it seems that the landfill is the solution.
"Out of sight, out of mind" does not really work for many gardeners. Knowledge that that material is there, whether the end use of the bed is flowers or vegetables, becomes a long term issue.