By Georgeadair
Cheshire,
United Kingdom 
I'm thinking of putting a small raised bed (9' x 9') in my garden for herbs, small vegs/fruits etc. I'm thinking of using old railway sleepers to act as the border. What depth should the side walls be?
Also, if the bed is going to be on top of existing soil is it advisable to line the bed with a membrane to stop weeds/grasses coming through? If so, what about drainage through a plastic membrane? Would the bed become water-logged?
It is advisable to put a layer of hardcore at the bottom of the bed (either with or without a membrane) to assist the drainage? If so, how deep?
Many thanks
George
8 Aug, 2011
Answers
Some companies make "railway sleepers" for this purpose and they will probably be creosote free, so be careful where you source them.
8 Aug, 2011
If they are greenish, though, they may be preserved with arsenic salts, which are far worse than creosote, in my opinion.
8 Aug, 2011
It goes from bad to worse...
9 Aug, 2011
The general advice is that you should not use old railway sleepers because they are impregnated with creosote. Personally, I am not convinced that this will pass along into the food chain. The bed for small plants can be as deep or shallow as you wish. I would suggest cleaning and digging the ground that the raised bed will sit on and dig some compost into it. Do not se any membrane or hardcore. The roots of the herbs etc can then travell below the depth of the raised bed. This is better for the plants and the bed can be shallower, using less compost.
8 Aug, 2011