Pontederia crassipes (formerly Eichhornia crassipes), commonly known as common water hyacinth, is an aquatic plant native to South America, naturalized throughout the world, and often invasive outside its native range. It is the sole species of the subgenus Oshunae within the genus Pontederia. Anecdotally, it is known as the "terror of Bengal" due to its invasive growth tendencies.
Description Water hyacinth is a free-floating perennial aquatic plant (or hydrophyte) native to tropical and subtropical South America. With broad, thick, glossy, ovate leaves, water hyacinth may rise above the surface of the water as much as 1 m (3 ft) in height. The leaves are 10–20 cm (4–8 inches) across on a stem, which is floating by means of buoyant bulb-like nodules at its base above the water surface. They have long, spongy, bulbous stalks. The feathery, freely hanging roots are purple-black. An erect stalk supports a single spike of 8–15 conspicuously attractive flowers, mostly lavender to pink in colour with six petals. When not in bloom, water hyacinth may be mistaken for frogbit (Limnobium spongia) or Amazon frogbit (Limnobium laevigatum). One of the fastest-growing plants known, water hyacinth reproduces primarily by way of runners or stolons, which eventually form daughter plants. Each plant additionally can produce thousands of seeds each year, and these seeds can remain viable for more than 28 years. Common water hyacinths are vigorous growers, and mats can double in size in one to two weeks. In terms of plant count rather than size, they are said to multiply by more than a hundredfold in number in a matter of 23 days. In their native range, the flowers are pollinated by long-tongued bees. These plants can reproduce both sexually and clonally. The invasiveness of the hyacinth is related to its ability to clone itself, and large patches are likely to all be part of the same genetic form. Water hyacinth has three flower morphs and is termed "tristylous". The flower morphs are named for the length of their pistils: long (L), medium (M), and short (S). Tristylous populations are, however, limited to the native lowland South American range of water hyacinth; in the introduced range, the M-morph prevails, with the L-morph occurring occasionally and the S-morph absent altogether. This geographical distribution of the floral morphs indicates that founder events have played a prominent role in the species' worldwide spread.
Habitat and ecology Its habitat ranges from tropical desert to subtropical or warm, temperate desert to rainforest zones. The temperature tolerance of the water hyacinth is:
Its minimum growth temperature is 12 °C (54 °F) Its optimum growth temperature is 25–30 °C (77–86 °F) Its maximum growth temperature is 33–35 °C (91–95 °F) Its pH tolerance is estimated at 5.0–7.5. Leaves are killed by frost and plants do not tolerate water temperatures more than 34 °C (93 °F). Water hyacinths do not grow where the average salinity is greater than 15% that of sea water (around 5 g salt per kg). In brackish water, its leaves show epinasty and chlorosis, and eventually die. Rafts of harvested water hyacinth have been floated to the sea, which kills it. Azotobacter chroococcum, a species of nitrogen-fixing bacteria, is probably concentrated around the bases of the petioles, but the bacteria do not fix nitrogen unless the plant is suffering extreme nitrogen deficiency. Fresh plants contain prickly crystals. This plant is reported to contain hydrogen cyanide, alkaloids, and triterpenoids, and may induce itching. Plants sprayed with 2,4-dichlorophenoxyacetic acid (2,4-D) may accumulate lethal doses of nitrates and other harmful elements in polluted environments.
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