Tom posted that he wants to kill and remove Leylandii
with a dbh of 5" and asks how to kill and remove the
roots. . . . "what is the best stuff to use and how
long will it take?"
The "stuff" most usually recommended is ammonium nitrate
and/or sulfuric or nitric acid. Talk about bad stuff!
All of these materials will kill the life of the soil
anywhere near the "treated" stumps. Sulfuric and nitric
acid are really nasty materials to handle and they just
don't belong anywhere near a garden nor are they all that
effective.
I think your best bet is to dig out the roots as best
you can, or maybe grind them out with a stump grinder,
keep the area moist to encourage the soil organisms to
get the remaining pieces and to then wait a season before
trying to replant in that area.