Question
Hello..

We recently built a small retaining wall around several dogwoods in our front yard and also on one Japanese Maple.  The walls are two blocks high and have been filled with top soil.  One dogwood is over 15 years old and the remaining trees were planted within the past two to three years.  

Our question is will this extra soil hurt the growth of the newly established trees. Someone told us that the additional soil placed on top of the trees root system would cause problems.  However, we have seen several landscaping photos of where retaining walls were built around trees with plants and flowers planted in the new soil.

This is our first outside project like this and we wanted to make sure we were not going to cause damage to the trees by adding this new feature.

Many Thanks...

Answer
The unfortunate answer is YES, the extra soil will kill your trees.  The tree's root system in in the top layers of soil, and adding to it smothers it, plus, extra dirt promotes rot at base of the trunk.  As little as 2 inches can kill some trees (Oaks, especially).  Sometimes just planting them in a "hole", a few inches below the finish grade is enough to kill a new tree.  And, I've killed plenty!  16" is a death-sentence.  

Photos of trees in planters with flowers is a great idea, IF you put them in all at the same time.

For more detailed discussion, try this link:

http://forums2.gardenweb.com/forums/load/trees/msg040038379315.html

My suggestion is to DIG UP the younger trees, since they are relatively small (and light), fill your area with soil, then re-plant them at the new level. Its a pain, but so is buying a new tree!  

Do this in the fall or early spring (depending on your area).  The 15-year-old Dogwood...I wouldn't try it, unless you use a hydraulic tree spade and an operator, which is not a cheap option!  Usually $100-$200, with a 50:50 chance of killing the tree.  

Sorry to be the bearer of bad news. -Marc