Warwickshire,
United Kingdom 
i have a raised bed filled with a mix of compost / manure and top soil can i grow a tree in it raised bed is 6ft x 3ft and 18 inches deep its got good drainage and sits on top of concrete
3 Nov, 2010
Answers
I agree 18" is not enough, I also think most of the larger shrubs will not do that well as its on top of concrete nowhere for the roots to go down and find water? Perhaps herbs and heathers may work?
4 Nov, 2010
Small shrubs, Denise? LOL. Sub shrubs? ;-)
4 Nov, 2010
just so Spritz
4 Nov, 2010
LOL.
4 Nov, 2010
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That's not deep enough for most trees, I'm afraid. Their roots go a lot deeper than 18". In a storm or high wind, a tree planted there would blow over, with so little anchorage.
I'd stick to shrubs.
3 Nov, 2010