Question
I read your answer that a retaining wall will kill a dogwood, my dogwood is at least 25 years old and I'd like to put a ten foot wide wall around it, only about 10 inches high.  There are some thick roots above the ground, I was just wondering is there any way to put a wall around a dogwood that wouldn't kill it? Thanks

Answer
Hello, June -

According to the National Arboretum, Dogwoods have a shallow, far-reaching root system.  

http://www.usna.usda.gov/Gardens/faqs/dogwoodfaq2.html

It is like a wineglass on a dinner plate ?the aboveground part is the glass, the roots are the base and plate.  10?inches doesn't sound like much, but imagine trying to breathe through a 10?pillow over your face!

You can add a THIN layer of topsoil or mulch (2 to 4 inches), but here is an illustration of just how shallow their root structure is - see picture at the end of this article from the Virginia Ag Extension:

http://www.dof.virginia.gov/urban/landscape-manual.shtml

Imagine an imaginary line drawn down from the edges of the tree crown ?a circle on the ground around your tree. Now if the soil being placed is OUTSIDE of this drip line, or covering no more than half of the area, the tree can probably adapt ?If you intend on covering ALL the area under your tree with 10 inches of soil, I would suggest you rip the tree out, build your retaining wall, then re-plant the old one, or a NEW tree at the new grade(or wait until your old one dies, then replace it).

If you push 10 inches of dirt right up to the base of your existing tree,  I will guarantee its death ?like pulling your collar up ten inches over your head and expecting no change in your ability to function normally.  As the article above calls it: a 搗olcano?versus a 揵agel?of mulch.  

It is your tree, I just want you to be aware of the potential damage you may be doing to it.  These are general guidelines.  If it is a young or small tree (say, no more than 5 feet tall), transplanting it to the new grade may be the answer, but if it is a prized ancient giant, avoid disturbing it. A case of loving a tree to death.

Hope it helps you in your decision-making.  All the best ~M