Download Article 107 BioControl 7 - Botanical Society of South Africa

Weeds in our Area (Part One Hundred and Seven)
By Bob and Ena McIntyre – Garden Route
BIOLOGICAL CONTROL CONTINUED
More Water Weeds – Eichhornia crassipes (Water hyacinth)
We stay with aquatic weeds and the option of biological control of these invasive alien plants. Our
subject today is Eichhornia crassipes (Water hyacinth, Nile Lily), considered by experts to be the
number one water weed in the world. Native to South America, it has spread to all the continents
of the world except Europe. Introduced into South Africa around 1910 as an ornamental for ponds
it has since spread to dams and slow flowing rivers throughout the country. The stunning flowers
without any doubt contributed hugely to its popularity for domestic ponds.
The Water hyacinth is generally free-floating with trailing feathery roots and can become
anchored in shallow water. When anchored it produces flowers and seeds. Reproduction is mainly
vegetative which means that plants bud and form daughter plants that break off, float away, get
trapped and become established elsewhere. One flower spike can produce over 5000 seeds with a
viability of about 20 years. Vigorous growers, Water hyacinths can double their mass every 18
days.
Impacts: Infested lakes, dams and rivers become choked
inhibiting boating and fishing activities. The floating mass
clogs irrigation canals and pumps and threatens
hydroelectric schemes. The plants contribute to silting by
impeding water flow. Instances of cattle drowning after
stepping onto an apparently solid surface of water
hyacinth have been recorded. The thick vegetative cover
reduces light penetration which in turn results in
ecosystem changes. The quality of drinking water is
adversely affected while the mats of vegetation harbour
mosquito larvae and bilharzia-carrying snails both of which
have severe negative health implications.
Identification: Eichhornia crassipes is perennial with shiny
dark green leaves arranged in rosettes. The leaf stalks have
bladder-like bases that aid flotation. Flowering from
November to April, the flowers are pale violet or blue with
up to ten individual flowers per flower spike, each flower measuring about 50 mm in diameter. The
flowers have six petals of which the centre upper petal is the largest and displays a prominent dark
blue area surrounding a yellow-centre patch. The fruits are capsules with very fine seeds.
Control: Several biological control agents ranging from a fungus causing leaf spots, two stem
borers, a leafstalk borer and a leaf miner have been released over the years with considerable
success. Two weevils (the water hyacinth moth and the Neochetina weevil) released on the White
Nile during the 1970’s have been particularly successful and have since been released in other
countries in Africa. Adverse climatic conditions in some areas have affected the overall success of
the bio-control agents released in South Africa, while in other areas of the country the method has
been very successful. Research is ongoing. Six herbicides with the active ingredient glyphosate are
registered for this weed. Should you still have water hyacinths in your pond, please bear in mind
that Eichhornia crassipes is a category one invader that have to be removed.
References: Alien Weeds and Invasive Plants by Lesley Henderson, Copyright © 2001 Agricultural
Research Council
Global Invasive Species Programme © 2004 (including illustration), Problem Plants of South Africa
by Clive Bromilow 2001. http://www.wikipedia.org and http://www.arc.agric.za.