Water hyacinth mats can double in size within one to two weeks
A pretty lavender-flowered pond plant from South America has earned the nickname terror of Bengal. Water hyacinth is among the fastest-growing plants known, cloning itself through runners so aggressively that plant numbers can rise more than a hundredfold in about 23 days, smothering lakes far from its native range.
The plant floats freely, buoyed by spongy, bulb-like swellings at the base of its stalks, with glossy leaves 10 to 20 centimetres across that can rise up to a metre above the water. Purple-black feathery roots dangle below, and a single upright spike carries 8 to 15 showy six-petalled flowers. Besides runners that bud off daughter plants, each individual can make thousands of seeds a year, which may stay viable for more than 28 years.
Its flowers reveal how it spread. In its native South American lowlands, populations have three flower forms that differ in style length, long, medium and short, and long-tongued bees pollinate them. Across its introduced range, the medium form dominates, the long form is occasional and the short form is missing entirely. That pattern suggests the global invasion began from a few founding plants, and large patches are often a single clone.
Water hyacinth prefers water at 25 to 30 degrees Celsius, stops growing below about 12, and is killed by frost. It cannot tolerate salinity above roughly 15 percent that of seawater, so harvested rafts have been floated out to sea to destroy them. Nutrient-rich reservoirs and polluted lakes are especially vulnerable. Dense mats outcompete native plants; a 2011 study at Dianchi Lake in Yunnan found they suppressed photosynthesis in phytoplankton and submerged plants. As the mats rot they strip oxygen from the water, often killing fish.
The plant soaks up heavy metals, then sinks and releases them when it dies, polluting water further, and infested waters can breed mosquitoes and snails that carry disease. It contains prickly crystals and can cause itching. On the positive side, it can feed goldfish and help oxygenate and clean water in controlled settings.
Source: Pontederia crassipes
