Plant biology and other issues that relate to the management of water hyacinth: a global perspective with focus on Europe

Water hyacinth reproduces vegetatively and sexually and seeds are numerous and long-lived. Rates of vegetative growth are high (weight and numbers can double in less than 1 week in optimal conditions) and governed by temperature and available nutrients. The period from germination to setting seed can be less than 12 weeks. Dispersal of propagules is by flow, man and machines. Biomass of living material is high 90 to > 800 tonnes/ha and comprises around 95% water, and the weed often grows in inaccessible areas. Consequently, social, economic and environmental impacts of water hyacinth are often large and management of the problem becomes very important. However, management of water hyacinth is difficult, and utilization of this weed should not be mistaken as a control measure. Containment and eradication from a catchment may only be accomplished if the invasion is very young, small, isolated and accessible, and if the short-term resource commitment is high. Most infestations are non-eradicable because once noticed reaction time is too long. However, in Europe, as the weed invades new areas on the edges of its ecological range, eradication of new infestations could (should) be attempted as this is the most cost effective way to deal with the weed. In addition, pathway shut-down and awareness campaigns should seek to reduce further new introductions, which, for spread to new regions, are mostly anthropogenic. For established populations the removal of accessible floating biomass can be accomplished using herbicides (if permitted) and by mechanical means. Both are expensive because re-growth ensures repeated action is necessary. Biological control is a useful tool in warmer climates, less so in cooler climates. It may be more useful in cooler areas if integrated with other management techniques. Additional biological control agents are being studied to improve and broaden the value of biological control. The lack of policies to permit the use of biological control for weeds in Europe limits the ability to manage this weed.