It's probably temp driven....the hyacinth around my big pond has just started taking off the last 2 weeks. Prior to that it was pretty wimpy looking.
I can vouch for the filtering effect it has. My pond will go from 12-18" visibility to 36-48" in the next month or so. REALLY clears the water up. And it makes a great spot for forage to hide and grow all summer....then when it dies off it's game on for the larger predators.