Last summer, a couple out exercising their dog took their usual walk around a local public reserve in Pukekohe.
As they passed a stormwater pond, something caught their eye: a vibrant green plant with purple flowers, floating on the water’s surface. It looked healthy, almost ornamental. Only a few months later, that seemingly innocent plant had exploded across three-quarters of the water’s surface, forming an impenetrable mat.
Cloverlea Reserve pond covered in pest water lilies.
Water hyacinth (Pontederia crassipes) is a serious water weed and is infamous for its invasive, fast-growing nature. Given the right conditions, a single small infestation can double in size in just one to two weeks.
At first glance, water hyacinth may appear too pretty to be a problem with its stunning lilac-coloured flowers from mid-summer to early autumn, each stalk boasting up to 20 delicate blooms. But beneath its beauty lurks an ecological nightmare.
Water hyacinth doesn’t just spread outward - it grows downward, with roots extending up to a metre deep. It forms dense mats that can reduce water quality, change water flows and increase sediment, crowding out native aquatic plants and animals, altering ecosystems, destroying habitats, and blocking irrigation systems.
It is also an expert at long-term survival. Each water hyacinth flower, once fertilized, produces hundreds of tiny seeds that sink to the bottom of waterways where they can remain viable for three decades, waiting for the perfect conditions to sprout.
Water hyacinth is an unwanted organism in New Zealand, meaning it is illegal to sell, grow, display or distribute it. It is also a notifiable organism - so if you do spot it, you must report it immediately to Biosecurity New Zealand.
Thanks to the sharp eyes of the Pukekohe couple, the outbreak in their local park was identified before it could spread further. Biosecurity officers were able to remove the plants, and the pond will be monitored for the next 30 years.
Cloverlea Reserve after treatment.
What Can You Do?
Be on the lookout: Water hyacinth has round, glossy green leaves and distinctive lilac flowers.
Help spread awareness: This invasive water weed may seem like a good choice for aquariums or garden ponds, so many people don’t realize they are illegal and harmful.
Never release aquatic plants into the wild: Many invasive species start as discarded garden or pond plants. Biosecurity New Zealand can dispose of these pest plants safely.
New Zealand’s biosecurity depends on all of us. If you think you’ve found water hyacinth, report it to Biosecurity New Zealand at 0800 80 99 66 or https://report.mpi.govt.nz/
