Water hyacinth or Eichhoria crassipes as it is known botanically, is a loose-floating tropical-to-temperate-climate freshwater plant. It is known to be so aggressive in its growth habit that many states consider it an illegal species. The free-floating plants mass together to form a green mat over the surface of the water, choking out all oxygen and life in the water. At maturity, the plants reach 3 feet in diameter and have long dense root systems that hang down into the water. They can be cut back drastically or removed from waterways almost entirely and will quickly re-establish themselves and multiply. Reducing their coverage once established requires constant and aggressive maintenance.
Wade into the water or use a pole to reach the water hyacinth and jostle the plant mats a bit to disperse any pond creatures still alive and resting or feeding under it.
Don your gloves and lift up or pull the plant mats out of the water, grasping all of its underwater roots, and set it down on the side of the pond or on your floating skip or boat.
Rip or cut the hyacinth mats down to the desired size, understanding that the plant will grow back to its current size very rapidly. Reducing the size of the mats helps to keep a small infestation under control and improve oxygen conditions under the water for other life forms.
Remove large swaths of the hyacinth plant mats and discard in the trash to get an established infestation under control. Ruthless thinning will not stop the plant's spread, only serve to keep it in check, so brutal reduction is called for.