Hydrilla verticillata is the only member of its genus. It is part of the Hydrocharitaceae family, known as “Frog-Bits”. The species was first described by Linneaus in 1782 from specimens found in Europe. By that time it was already found in most of the warm parts of Africa and Asia, and was so widespread that today no one is even certain where it originally evolved (most guesses center around southern Asia). The plant prefers warm tropical conditions, but it also capable of surviving and reproducing in cooler temperate areas. In addition, it can tolerate brackish water and survive in tidal estuaries.
In appearance, Hydrilla consists of long jointed stems, often multi-branched, with a series of 7 or 8 small leaflets arranged in circular whorls along their lengths and a dense growth bud at the tip. The plant is entirely aquatic, capable of thriving in any permanent body of freshwater, even from just a few inches deep. The plant is widely variable and has several different patterns of growth. In many cases, Hydrilla forms small tubers in the bottom mud, up to a foot deep, from which the stems can reach as long as 25 feet to the surface. When rooted, the bare stems reach up to the surface, where they then branch out extensively, forming a thick layer and putting over 80% of the plant’s mass at the water surface. The tubers also put out subterranean runners to make new stems.
In many cases, the tubers can be dispensed with entirely, and the long branched stems simply float freely at the surface. Any fragments of stem that break off will grow into an entirely new plant. And finally, some forms of Hydrilla can also reproduce from seed–its flowers are tiny and inconspicuous. Two different forms of Hydrilla are found in the US–the most common form contains only female flowers and cannot produce seeds. But an isolated population in North Carolina contains both male and female flowers–it is assumed that this variety has a different source than the more common type, and that there have been invasions by this plant from at least two different geographic areas.
In good environmental conditions, Hydrilla can grow as fast as a full inch per day. Even just a few fragments of stem are capable of completely overgrowing a body of water within a few months, forming a tangled mat floating on the surface. The thick mats choke off water flow, and can be so heavy that they stop boat traffic. Growths of Hydrilla also shade out other plants, deplete the oxygen level in the water, and rapidly fill in ponds and lakes with thick sediment layers of dead plant material.
Hydrilla’s attractive feathery appearance and deep green color, and its ease of propagation, made it a favorite with home aquarium hobbyists, and in the 1950’s and 1960’s the plant was widely sold in pet stores and specialist aquarium shops, often labelled as “Elodea” or “Anacharis”. As a result, it quickly became established in Australia, South America, and the US. Its first appearance in the US happened in Florida in 1960, when wild colonies were found in the Crystal River area near Tampa and in a canal near Miami. It is thought that these were escapees from shipments that entered the country for the pet trade (Miami and Tampa are the two primary ports of entry for most imported exotic pets and aquarium plants). By the early 1990’s Hydrilla had infested almost half of the lakes in the entire state and covered over 140,000 acres of water surface, and had spread throughout the southeast up to Maryland, and in the west-coast states of California and Washington. It was even found in ponds and lakes in Arizona, presumably released by aquarium keepers. In the US, the Hydrilla invasion has so far not spread to most of the northern states, but in other countries like Russia and Poland, the plant has established itself in conditions similar to those at the US-Canada border. It is therefore likely that Hydrilla will continue to spread until it occupies the entire US.
Florida promptly banned import or possession of the plant, but it was already too late. Once Hydrilla establishes itself in a body of water, it can produce several thousand submerged tubers per acre, which are impossible to eradicate. The state estimates that at least 40,000 acres of wetland are still occupied by Hydrilla growth. Florida spends around $20 million a year to try to control the plant, mostly with herbicides. Grass Carp eat Hydrilla in large quantities–but Grass Carp are themselves an invasive species and are illegal to introduce in most states (including Florida). State officials have carried out some experiments in using sterilized Grass Carp as a method of controlling Hydrilla in individual bodies of water.
Hydrilla does have one potential bright side, though. It is very efficient at extracting nutrients, and it has been seriously considered as a form of bio-remediation for filtering and cleaning toxic levels of heavy metals from polluted bodies of water.