Question
I would like to plant 30 thuja occidentalis smaragd trees along my driveway. I am a little concerned about the root growth because my leeching field is also along my driveway, what is the extent of root growth and do you think it will cause a problem with my leeching field?

Answer
These have a spread width of about 3-4 feet so I would plant them 3 feet from the driveway or the limbs will over time rub the cars as you drive in and out.
The roots will be about 1 1/2 times the width and usually in the first 2-3 feet of soil. IF any tree is planted right on top of a drain line the roots could clog the lines.  Septic lines are usually not sealed and the roots will grow towards the water source and over time can get into the pipes.