Posted: 19/10/2015 at 07:26
Hessian rots down too quickly to use it in this way.
Use untreated timber if you are concerned about using the treated type.
I would dig out the rubbish and dispose of it safely first. Goodness knows what is down there that might affect the drainage of your beds. Also, if you are worried enough about contamnation not to use tannalised timber, I'd think you would want to ensure there was nothing potentially harmful under your beds.
Prepare properly - it's always worth it