"It takes up a lot of space. It grows in ditches and marshes, streams, lakes, shorelines and it spreads quite rapidly, and it replaces and outcompetes native plants."
Milfoil can establish itself in water as deep as 10 metres, and forms thick mats that grow up to the surface, blocking out light for other vegetation. It can become so thick that even fish have trouble getting through it, and it can be a hazard for swimmers who might get caught up in it.

It also creates pools of stagnant water that are ideal for mosquitoes.