Question
I was recently cleaning a planting bed behind the house. It is not well maintained presently. I notice a trunk growing in amongst the stems of this bush and the leafs sprouting from this trunk were broad and not at all like the small leafs of the bush itself. To my uneducated eye it looked like the beginning of a tree seedling. I subsequently found a root from a neighbor's oak tree at the rear corner of the house. I removed a seven foot section of the root running from the fence to the corner of the house. I have two questions. If a planting bed is setup properly in a scenario as outlined above and you lay down a moisture barrier to retard the growth of weeds would that also deter anything growing from these meandering tree roots? If answer is no, what do you treat the roots with to prevent it from growing back and yet is environmentally friendly? Thank you for your time.

Answer
There are two things you are confusing, one a moisture barrier and two a weed barrier. A moisture barrier is something you apply to the walls of your house or in your basement to retard or resist the movement of moisture vapors into your home. Often this is a simple sheet of plastic. A weed barrier is a geo-textile that allows water to penetrate but stops weeds from coming up from beneath the barrier. Often times people use plastic as a weed barrier but its not a good idea as is stops water from getting to the soil which is bad for many reasons.
No weed barrier or plastic sheet will stop a tree root. However, roots tend to grow more where they can find water, so I guess you could argue that if you use plastic and stop the water from penetrating the ground, maybe the root will be less likely to grow there. However, I would stop short of saying it would prohibit the roots growth.
There is nothing you can treat tree roots with that would not also be bad for other plants in the area and or possibly kill the tree. There are also now laws in many places that make it criminal for you do do anything that would or could purposefully kill a neighbors tree. But this varies from state and city. Your best bet fro an environmentally sensitive solution is a heavy plastic or metal root barrier. Try this site and others like it:
http://www.deeproot.com

Best Of Luck,

Sean J Murphy, LA, ISA
Amenity Architects, LLC
SEI - Southeastern Engineering, Inc.

www.seengineering.com
www.amenityarchitects.com