Glenn-
You might have to do two applications. Do first one at the very first sign of growth, typically early March to April depending on climate. Being in Ga. I'd say March. Follow up with second spot treatments in May or June. The copper sulphate will run about $50 per bag but it is the only stuff we found that will effectively control the hydrilla. The worst part is that hydrilla in not indegenous to the U.S., it's another one of those lovely Asian imports like fruit flies, Japanese beetles, the Asian flu ............. :unsure: