The Water Hyacinth
Beautiful to look at but a curse to live with outside its native range. Eichhornia crassipes is commonly known as water hyacinth to English speaking people. (Note: You may also see it referred to as Pontederia crassipes)
The Misplaced
Appearance and Reproduction
Eichhornia crassipes is a fresh water flowering plant which is native to South America. It forms dense, floating mats in freshwater bodies.
The plant is very variable in size, seedlings having leaves that are only a few centimetres across or high, whereas mature plants with good nutrient supply may reach 1 m in height.
Each plant consists of a ring of shoots (up to ten) which is made of a leaf and and a stem (technically known as a petiole). The stem section widens out along the bottom and is very spongy. This enables the plant to float. The leaves are hairless and smooth.
The roots are usually 20 to 60 cm long although they may grow to 3 m depending on available nutrition.
Stolons are horizontal "stems" which arise from a growing point just above the roots. They are typically 10 to 50 cm long and can grow new cloned plants.
Typically, 6 to 15 mauve flowers form on a central stem. Individual blooms only last one day.
Asexual Reproduction
The main method of rapid reproduction is the growth of stolons which result in new plants.
Cutting a water hyacinth plant into pieces will not kill it. Fragments of the plant are often capable of regenerating a whole new plant.
Sexual Reproduction
Once the ovaries of the flower are fertilised, the resulting seed capsule contains up to 450 seeds which remain viable for up to 20 years.
Ecology
| Factor | Optimum range | Limits of tolerance |
|---|---|---|
| Temperature | 25 to 30°C | No growth > 40°C, or under 10°C |
| pH | 6 to 8 | extremes of pH (below 4.5 or above 10) can be damaging |
| Salinity | 0 to 9‰ | Killed by seawater |
| Nutrient levels | requires abundant nitrogen, phosphorus and potassium |
The Displaced
Water hyacinths compete with native plants for space and has caused displacement of native plants due to its reproductive rate. It can double in biomass every 6 to 18 days. It also disrupts water flow patterns which upsets niche availability for species depending on fast flowing water. It increases evapotranspiration so that less water is available for other species. Creating havoc for plant species also threatens animal species which rely on a diet which has evolved in the absence of E. crassipes domination.
The Consequences
Water hyacinth has invaded freshwater systems in over 50 countries on five continents and, according to recent climate change models, its distribution may expand into higher latitudes as temperatures rise. Dispersal is due to both deliberate and accidental means. The attractiveness of the flowers has resulted in deliberate planting in various parts of the world for both private and public water features.
E. crassipes has caused major disruptions to a number of ecosystems and also to human communities.
Environmental problems associated with the water hyacinth are most prominent in warm areas where the weed grows throughout the year and develops into dense large, free-floating, monospecific islands or mats which compete with other aquatic species for light, nutrients and oxygen.
Lakes that are overrun by water hyacinths undergo dramatic transformations. Submerged native plants became shaded and often die. The resulting decay processes depletes dissolved oxygen in the water and leads to fish kills and reduction in water quality from decomposing plants.
As an invasive species, water hyacinth can also bring with it new pathogens which may have a deleterious effect on ecosystems.
In a lake with strongly entrenched water hyacinth, plants interlock into such dense masses that they are sturdy enough to hold people walking on them. Conversely, boat travel can become impossible with severe infestations.
Case study - Australia
Water hyacinth was brought to Australia in the 1890s as an ornamental plant. The first record of water hyacinth in New South Wales (NSW) was in 1895. In 1897, the government botanist Mr J. H. Maiden noted that it had spread rapidly in the ponds in the Royal Botanic Gardens in Sydney. At that time, he warned that the plant should be kept away from the northern rivers where it ‘may very rapidly become a serious pest’. Unfortunately, this warning went unheeded and by the early 1900s it had spread along the east coast of Queensland and the north-eastern regions of NSW.
Water hyacinth was introduced to the Brisbane metropolitan area as an ornamental pond specimen in the early 1900s. Valued for its floral presentation, it was released into ponds and lagoons in public parks throughout Queensland.
Eichhornia crassipes currently occurs along the east coast of Australia from about 100km south of Sydney in NSW to southern Cape York Peninsula in Northern Queensland.
Australian legal regulations concerning water hyacinth
Water hyacinth is a declared weed in all states and territories in Australia. It is illegal to sell, distribute and possess water hyacinth and in most cases landholders are required by law to control it when it occurs on their property.
Controlling the invaders
Water hyacinth is extremely difficult to eradicate completely once established. Therefore, the goal of most management efforts is to minimise economic costs and ecological damage.
Prevention
Laws have been put in place to ban the sale and transport of this species in many countries. However, given that we know that E. crassipes thrives in nutrient rich water, prevention of fertiliser spills in one way to reduce the likelihood of infections.
Physical control
On a small scale manual removal may work, but it must be monitored as the plant can grow back from any material which has been overlooked.
On a larger scale, mechanical harvest is possible but expensive and the results must consistently monitored for re-infection.
Chemical control
Widely used weedicides such 2,4-D, diquat and glyphosphate have been used to attempt eradication of the weed, but they each have down sides in terms of polluting water and being health hazards. They are not selective in their actions, therefore they may remove other organisms in the ecosystem and lead to its collapse.
Biocontrol
In the regions where E. crassipes is a native plant (primarily Brazil and Argentina), its population density is controlled by over 100 natural enemies such as various bacteria, insects and fish. Biocontrol agents have been selectively released in some situations to control the proliferation of water hyacinth, with various degrees of success. South Africa has emerged as leader in this field.
Other factors influencing control measures
As it is considered very attractive plant, it is still sold as an ornamental plant, although this practice is forbidden by law in many countries. Water hyacinth might escape its ornamental planting as the seeds remain viable for a long time and may be dispersed by water birds.
In some countries, water hyacinth is regarded as a resource to used.
The speed of growth of E. crassipes has been viewed as favourable by some people in some situations. It is a quick way to generate a lot of biomass in a short time.
The weed biomass can be used for
- bioremediation and bioadsorption of metals and pollutants
- biogas and biofuel production
- composting and vermicomposting
- as feed for animals and fish
- as a raw material for weaving items such as baskets and for making paper
- as carbon source for microbial growth
- as a source of a range of chemical compounds including several for pharmacology usage
It should be noted that ongoing exploitation of water hyacinth as a resource will most likely result in major disruption of local ecosystems. The longer term implications of this practice must be weighted against this damage as it likely to be unsustainable.