Building a raised bed over tree roots
Is it ok to build a bottomless timber raised bed over existing tree roots? I mean so that the tree is not inside the bed, the raised bed is next to it but the bed is over the tree's roots on 2 sides (an L-shaped bed).
It's probably not a good idea. We've had dreadful problems with tree roots coming up inside our three raised beds, they suck all the moisture and nutrients out. We've solved the problem we hope by putting Terram (a very tough professional quality root barrier) down underneath all three, bringing it up the sides a little and then stapling it all round. Our beds are extra deep so we can dig for potatoes and stick bean poles in - we don't want to puncture the fabric. It was quite expensive at over £100 a roll, but there's lots left so should last us years!
It's not ideal for the tree either.
Girona, Catalunya, Northen Spain
Agree with the other replies, the fine feeder roots of trees (not the big chunky ones near the trunk that anchor the tree to the ground) can be much nearer the surface than you think, and extend a good distance beyond the trunk, up to and sometimes beyond the outer leaf canopy. By piling soil on top of these roots, you are depriving the tree of the essential water and oxygen it needs. In a bid to survive it will send feeder roots upwards into the raised beds and compete with whatever you plant there. The tree will not be happy - it may slowly starve, you may not notice for some years, if its a large tree it may fall and damage people or property.
I found out all of the above AFTER I had raised the soil in front of a small olive tree that was on sloping ground, to level it off and incorporate it into a large raised flower bed! However, I have undemanding drought-tolerant perennial flowers there that don't compete too much, the tree benefits from additional water from a land drainage outlet, plus I'm not really that bothered about the tree, its been badly hacked around in the past, I have larger, nicer specimens elsewhere, if it dies I won’t be in mourning.