Question
QUESTION: We have a 4-foot retaining wall made of boulders in our backyard. Since our house is new construction, we do not have any plants yet. I wanted to plant a tree or two or maybe Thuja Green Giants along the top of the retaining wall. Will the roots destroy the wall over time? Is there anything that I have to be careful of when planting near the top of the wall?

ANSWER: If the wall is built well and according to proper engineering techniques, it should survive the roots of a Thuja spp. Make sure to plant at least thirty six inches away from the wall with this tree. Five feet would be even better, if you have room. I would avoid planting any trees that are known to have root crowns that will get too large for the space you have above/behind the wall. If you don't have 36" inches then you need to get something smaller than a Green Giant...like an Emerald Green Arborvitae or a Leyland Cypress.
Best of Luck

---------- FOLLOW-UP ----------

QUESTION: Thanks, your answer was very helpful. How close to the retaining wall can we plant the Emerald Green Arborvitae? (We are at the top of the wall). Also, I had been looking for information on the size of root crowns, with no luck. Do you know of any good websites that would provide this information?


Answer
Emerald Green Arborvitae is a small thuja and will do fine. Plant the center of this tree at least 36" from the wall.

Regarding root crows, I doubt you will find any books describing this aspect. Knowing if a tree has a substantial root crown is something you just know form experience with certain trees. For example a large oak tree when it gets 50 years old or more may have a very large root crown of say ten feet in diameter, you see root crowns tearing up sidewalks all the time. So you should steer away from planting a tree like that close to the wall.


Sean J Murphy, LA,ISA, LEED AP
Please check out my blogs and articles on landscape topics like this on my websites.
http://www.seanjmurphy.com
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