Just cutting down the shoots as they are about to branch out is enough
to kill the bamboo. Clear cut the grove. As it shoots, let it get to
the point where branches are beginning to emerge and then cut those
shoots out. You will exhaust the stored energy in the rhizomes
quickly and kill the grove. No chemicals, no money, no salt.

With the roundup you are only killing a very tiny portion of the
grove, the grove will isolate that section and continue growing in a
very happy and healthy way.