On Saturday, August 30, 2014 11:29:14 AM UTC-7, Brooklyn1 wrote [in part]:

Most tree roots don't go more than two feet deep and most feeder roots

are just below the surface.

While feeder roots might be near the soil surface, roots taking up water
can be much deeper. Walnut trees can send their roots down 20 feet.
Oaks can send their roots even deeper seeking water. Roots only 2 feet
deep could not support a giant redwood or a "Blue Gum" eucalyptus; the
first moderate wind would topple such trees. Even tomato plants can
send their roots down 10 feet.

--
David E. Ross
Climate: California Mediterranean, see
http://www.rossde.com/garden/climate.html
Gardening diary at http://www.rossde.com/garden/diary