Flora of New Zealand Series

Water Hyacinth

Free-floating aquatic with many long purple feathery roots which may anchor the plant in mud around edges of ponds, streams, etc.; stolons many, thick, fleshy, radiating in all directions, producing leafy tufts which readily separate and form new plants. Leaves bright green, smooth, leathery; laminae to 8 cm wide, ovate to orbicular, many-nerved; petiole usually inflated below, spongy and bladder-like. Flowers to 7 cm diam., bluish-purple, 8-10, spirally arranged in a terminal spike; scape to ± 20 cm high with leaf-like bract; tube green; lobes blue-purple, uppermost widest with a deeper blotch containing a bright yellow spot. Capsule membranous, 3-celled, many-seeded, enclosed in withered perianth. Seeds small, ovoid, ribbed.

N. An established escape from ponds and aquaria, now eradicated from many localities. Likely to occur in northern half - North Auckland, Auckland, Gisborne, Hawkes Bay and Taranaki

(Tropical S. America)

First record: Cheeseman 1914:8.

First collection: Muir' s Lake, Waiuku, shallow water, mud substrate, N. T. Moar 445, 23.1.1950 ().

FL. 2-3.

The swollen bladder-like petioles and large showy lilac to blue flowers make this tufted free-floating species most distinct.

E. crassipes was originally introduced as an ornamental, but it both accidentally escaped from cultivation and was deliberately planed, and by 1950 was established and troublesome in dams, ponds, swamps, slow-moving streams and river cut-offs in the Auckland Province.

In New Zealand water hyacinth is rated of major economic significance and its importation has been prohibited since 1927 by the now-repealed Introduction of Plants Act 1927; since 1956, by an amendment to the Noxious Weeds Act 1950 (Section 11A), it is illegal to have possession of the plant, and a continuing obligation is imposed to eradicate known infestations. A Government initiated and financed eradication programme has been maintained since the early 1950s, many of the 30 odd known infestations have been eliminated, but the weed appears in new localities from time to time. Notwithstanding the legal position, some householders and aquarists cultivate the plant illegally, and it passes from hand to hand, with certain prospect of new infestations from initial small deliberate plantings, or by escape from garden ponds etc.

E. crassipes reproduces vegetatively by producing daughter plants at the ends of multiple radiating stolons and a few initial plants will rapidly form mats or rafts which cover large expanses of slow-moving waters. In some localities seed is occasionally produced, the seedlings establishing on marginal mud, and reinfesting bodies of water from which all floating and mud-rooting plants had previously been destroyed.

Overseas, water hyacinth is considered one of, if not the most troublesome of aquatic weeds and has, by deliberate introduction and escape, established in immense abundance in many tropical and subtropical regions-southern United States, Africa, S.E. Asia, Ceylon, India, the Philippines, Malaysia, some Pacific Islands and parts of Australia. Establishment of Eichhornia in these countries has had major economic and social effects; it has blocked waterways, caused collapse of bridges, prevented ship navigation and timber-rafting, polluted water supplies adversely affected fish and rice culture, aquatic recreation and generation of electric power, and effectively blocked drainage and irrigation systems. In preventing the passage of ships on waterways in Florida, it gained the name "million dollar weed". A periodical-Hyacinth Control Journal - is devoted to the control and eradication of E. crassipes.

Despite the disastrous effects of water hyacinth, and the enormous and costly eradication programmes there is pressure by engineers and conservationists, both here and overseas, for the culture of this alarmingly rampant weed in large artificial ponds to depollute raw sewage from towns and cities, and even from remote extra-suburban homes.

Fig. 10. Eichhornia crassipes, water hyacinth; habit, showing leaves with swollen, bladder-like petioles. © All rights reserved. [Image: 26WV]