If I recall correctly Navigate kills milfoil by the root, the problem is that any remaining seeds can germinate the following spring/year. In addition milfoil can be propagated by cuttings too, so any milfoil that is missed can also can help it come back next year if it gets cut up into smaller pieces. We used Navigate for several years to control eurasian milfoil and it very effective on milfoil, one of the drawbacks was that it was not very effective on coontail which then took the milfoils place. We are using a Reward/Cutrine Plus/surfactent mix now on everything which includes: milfoil, coontail, sago (or horned pond weed), chara, and FA. \:\(