An impenetrable mat of vegetation soon crowds out native vegetation and animals by shading and drastic alteration of water chemistry. Recreation in and on the water becomes impossible, and property values plummet. In rivers and canals infested by these weeds, water flow can be disrupted so much as to encourage flooding. Exotic nuisance species of aquatic plants have no natural control mechanisms among native species. Mechanical harvesting and dredging have been used for years, although these methods are slow, non-selective, costly and time consuming.
Harvesting can aggravate the problem by dispersing plant fragments that can form new plants.
Results rarely last more than a few months. Another problem with mechanical harvesting is the removal of sport and other aquatic organisms. Biological control includes the introduction of herbivore fish, insects or pathogens into the water. One of the most common herbivores used is Asian grass carp. The use of this exotic fish is restricted in many areas because of the long-term effects they might have on native plant species.