The black walnut tree manufactures a substance that is a
natural insecticide according to experts at the Texas State
University in Austin.American black walnut tress contain a tannic acid chemists
call juglone. The reddish yellow substance leaches from
leaves, and some believe exudes from roots, or transfers
from branches and foliage to the roots. Tree physiologists
agree that roots of other plants that come in contact with
those black walnut tree roots die--even other black walnut
seedlings.J.