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Aquatic plants, like all other plants, may be weeds in one location and a source of income and therefore coveted in another location. Introduction of exotic aquatic plants to the United States has always proceeded at a rapid rate. Many plants were brought in for horticultural or agricultural purposes. A greater number of aquatic species were brought in as aquarium plants and then accidentally or purposely introduced into the wild as a future source of income. A much lesser number have been introduced into natural waters from ballast pumpage. Most are of tropical or semi-tropical origin and initially were confined to waters in Hawaii, Florida, or California. Expansion of their range to other states has progressed until many exotic aquatic plants have become both a problem to water managers and a source of profit to the aquarium industry.
As a rule exotic plants are not considered pests unless they are highly invasive. It is generally recognized that the nonindigenous aquatic plant species which are most invasive include hydrilla (Hydrilla verticillata), Eurasian watermilfoil (Myriophyllum spicatum), waterlettuce (Pistia stratiotes), alligatorweed (Alternanthera philoxeroides), parrot-feather (Myriophyllum aquaticum), egeria (Egeria densa), and waterhyacinth (Eichhornia crassipes). The latter is recognized as the world's worst aquatic weed and in many areas the most prolific.
Very few aquatic plant species are reported to have been accidentally introduced. These are usually from ship's ballast and include waterlettuce, alligatorweed, and salvinia (Salvinia minima). Horticulturists are credited with the introduction of the waterhyacinth for its showy flowers. Hydrilla, egeria, parrotfeather, Eurasian watermilfoil, limnophila (Limnophila sessiflora), and hygrophila (Hygrophila polysperma) were all introduced by the aquarium trade and often sold as oxygenators. In areas where water conditions are favorable and the native vegetation is disturbed, these invasive species rapidly become the dominant species.