I use plain old rock salt a 25 pound bag dissloved in hot water in the
wash tub a few times a year, then leave house for day, it doesnt hurt
the tree, but kills the roots and is near free. my underground pipe is
all terracota, I dont know if it would hurt cast iron?
tree and bush roots can travel a long way, I removed a tree but it
try the rocksalt its cheap and if done in early spring before stuff
leafs out works wonderful, Its time tested been over 10 years.