Question
I have a mature maple tree which I would like to put a ring of landscaping blocks
around.  However, I have run into trouble because the root system causes the
blocks to be uneven.  What can I do to prevent this from happening?  Or, if it is
impossible, what alternative landscaping ideas do you have?  To give you a
better idea, I have a large corner garden which I wanted to expand to the tree.)

Answer
Although it is commonly done, its better not to do this at all.

The problem is the roots are constantly growing and as long as you are building the wall in the structural root plate (within about 10 to 15' of the tree) the roots will continue to expand and push your wall up. The only way you might do it by "bridging" the roots and allowing several inches for the root to expand. And I do mean several. However, Even if you do this, inevitably a new root that you didn't plan for will come up where you have no bridge in your wall.

What you can do is just plan to annually re-build the wall if you must have one.

You might note that you never see a wall built of these materials in a commercial landscape or one done by a knowledgeable professional around a tree...its because it is just not really a good idea.
Sorry to be the bearer of bad news.

Best of luck
Sean J Murphy, LA,ISA, LEED AP
Please check out my blogs and articles on landscape topics like this on my websites.
http://www.seanjmurphy.com
http://amenityarchitects.com