Hornwort is native to North America and was first recorded in Napier in 1961.
This plant has now spread to much of the North Island and was once used in the aquarium trade for its fast-growing capabilities. Both South Island populations have been eradicated.
Hornwort is designated an Unwanted Organism under the Biosecurity Act 1993.
Hornwort does not have roots and can readily create rafts of growth that spread through waterbodies with ease. These rafts are often pushed by currents and/or wind into calm bays or against shorelines where they smother and shade plants that grow in these locations.
In New Zealand, hornwort does not spread via seeds, but through horn-like growths that develop from a flattish green sheet. Hornwort growth and cover tends to decrease in winter, with peak cover in autumn.
Hornwort is common throughout the North Island and is considered to dominate the vegetation in many rivers of the Waikato and Whanganui-Manawatu regions and lakes in the Bay of Plenty region. The South Island is declared free of hornwort.
Hornwort is a submerged freshwater weed that will grow in a wide range of water bodies, from lakes to moderately flowing sections of rivers. It can grow in water up to a depth of 16 m, depending on water clarity. Hornwort will tolerate a wide range of temperature, light and nutrient levels.
There are several ways in which hornwort poses a threat to our freshwater ecosystems: