Pontederia crassipes
Chemical control

Chemical control is the least used of the three controls of water hyacinth, because of its long-term effects on the environment and human health. The use of herbicides requires strict approval from governmental protection agencies and skilled technicians to handle and spray the affected areas. The use of chemical herbicides is only used in case of severe infiltration of water hyacinth. However, the most successful use of herbicides is when it is used for smaller areas of infestation, because in larger areas, more mats of water hyacinths are likely to survive the herbicides and can fragment to further propagate a large area of water hyacinth mats. In addition, it is more cost-effective and less laborious than mechanical control, yet it can lead to environmental effects, as it can penetrate into the ground water system and can affect not only the hydrological cycle within an ecosystem, but also negatively affect the local water system and human health. Also of note, the use of herbicides is not strictly selective of water hyacinths; keystone species and vital organisms such as microalgae can perish from the toxins and can disrupt fragile food webs.

The chemical regulation of water hyacinths can be done using common herbicides such as 2,4-D, glyphosate, and diquat. The herbicides are sprayed on the water hyacinth leaves and leads to direct changes to the physiology of the plant. The use of the herbicide known as 2,4-D leads to the death of water hyacinth through inhibition of cell growth of new tissue and cellular apoptosis. Almost a two-week period may be needed before mats of water hyacinth are destroyed with 2, 4-D. Between 75,000 and 150,000 acres (30,000 and 61,000 ha) of water hyacinth and alligator weed are treated annually in Louisiana.

The herbicide known as diquat is a liquid bromide salt that can rapidly penetrate the leaves of the water hyacinth and lead to immediate inactivity of plant cells and cellular processes. The herbicide glyphosate has a lower toxicity than the other herbicides, so takes longer for the water hyacinth mats to be destroyed (about three weeks). The symptoms include steady wilting of the plants and a yellow discoloration of the plant leaves that eventually leads to plant decay.
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