Among the most serious introduced aquatic weeds are the oxygen weeds egeria (Egeria densa) and hydrilla (Hydrilla verticillata). These form dense floating mats which block pump- and power-station intakes, clog drains and irrigation races, interfere with recreational activities, and impair water flows in rivers and streams. They grow in water where fertiliser runoff from farmland has raised nutrient levels.
Managing water weeds is costly. Methods include building screens to prevent turbines from clogging, cutting and harvesting weeds in large open waterways, dredging, using mechanical diggers in drains and irrigation races, and applying herbicides.
About 75% of land weeds and over 50% of freshwater weeds were originally grown in gardens or home aquariums. The places with most weeds are often close to towns. On average, eight species of garden plant each year become naturalised in the wild. They threaten the survival of native plants and the long-term existence of some native animal species. Weeds that seriously threaten New Zealand’s native species, ecosystems and conservation lands are old man's beard (Clematis vitalba), wild ginger (Hedychium gardnerianum and Hedychium flavescens), pampas grass (Cortaderia selloana and Cortaderia jubata) and contorta pine (Pinus contorta).
The Department of Conservation (DOC) manages about 8 million hectares of native forests, tussocklands, alpine areas, wetlands, dunelands, estuaries, lakes and islands – about 30% of New Zealand's land area. DOC is responsible for preserving and protecting these areas, including managing invasive weeds. The cost is considerable – DOC spends over $14 million a year on weed control, and regional and district councils also spend large sums.