Question
Hi Mark,
I have a tree right next to an exterior wall which was always clipped neatly, and I decided months ago to let it grow fully (and it has.) But I've been wondering if the roots will grow larger now that the top (branches) are much bigger. Could this endanger the wall /structure?
I'm in Southern California, and the tree is evergreen with small deciduous-like leaves. (Do you know a good website where I can find the name of this tree?)
Thanks,
Greg  

Answer
Greg,

Yes, there is a connection between the growth you can see and root growth.  If I remember right, the new growth produces a hormone that drops down the plant and stimulates root growth.  So there is a rough balance there. .. the bigger the top, the bigger the roots.

About damage, It's a possibility but it depends on the plant.  First, how big does the plant actually want to be? 12 ft? 120ft?  Also, some send roots down, some out nearer the surface, some seek water (pipes, drainage fields), and some will just  stay put or work around obstacles.  So the plant id is important.

Some websites that might help:
Oregon State's Landscape Plants (Go Beavs!)
     http://oregonstate.edu/dept/ldplants/
Arbor Day Foundation
     http://www.arborday.org/trees/treeID.cfm

My first guesses would be some kind of laurel, or myrtle, but you're far enough south that plants will be growing there I've never seen.

For what it's worth,
Mark in Portland