Model Magic works just fine, but you can't just shove a stick in it and slap some paint on.
I used the clay to make a good sized base (usually no less then about 4"x4"). I stuck (and then removed) some styrene tubing in it to make a dimple where I could later glue the tube in. Once the clay cured, I superglued styrene "stumps" into place and added the usual basing materials to give it texture. Then I painted and flocked as usual.
For Trees, all I did was get some plastic plants on sale at the big box hobby store and cut them up to size. I wrapped the stems of the plants with brown floral tape for color and texture.
Then, I took the plastic/floral tape trees and stuck them into the trunks on the base.
End result: A wooded terrain model with trees that can be easily removed for storage or to just get them out of a models way.