Put a price on it

DH is somewhat bemused at my rather obsessive (to him) gardening...I told him that he should be thankful I was increasing our property value. And then I started thinking, how much DOES landscaping increase property value? Does anyone have any ideas? Am I throwing money away on an expensive hobby or could I theoretically receive some return on all the "sweat equity"? (Of course, despite the expense, it's still cheaper than professional therapy. :) )

pam