I am not sure if it will kill it, I've been gone for the past 6 months for work and came home to an overgrowth of hydrilla and had to try to kill it. It is still very green. I made a make shift sub-surface spray mechanism to get the poison down to the roots, added a little pond dye to see where I was spraying and I am hoping I can kill a lot of it before we get cold down here in Louisiana. I will hopefully be able to catch it in the spring when it starts to grow and knock it down. I'm getting a permit to buy some grass carp so I can hopefully keep it under control without chemicals again.