Purple nut sedge or nutgrass (Cyperus rotundus) is a native of India and was recorded in New Zealand in 1883. It has been described as the `world’s worst weed’ due to its aggressive nature, persistent growth and resistance to control.
On this page:what it looks like, where you can find it, responsibility for control, controlling nutgrass, further information, related publications
Nutgrass can smother crops and all other plants, removing large amounts of nutrients and moisture from the soil. In dense colonies, over 500 plants and 40 tonnes of roots have been recorded. The rhizomes can penetrate through vegetable crops, asphalt paths and plastic sheeting. Without disturbance, normal rhizome and tuber growth can extend infestations by more than a metre each year.
What it looks like
Nutgrass is an erect, perennial herb growing between 20-50 cm high. It has a deep and extensive root system of tubers and bulbs. The stems are triangular with no branches.
Leaves: Grass-like and dark green with an obvious vein along the underside.
Flowers: The flower head appears from mid January to March and stands 10 – 60 cm in height on a three sided upright stem. It is reddish to purple brown forming a cluster of flattened spikes.
Nutgrass is spread primarily by attaching itself to cultivation equipment, including discs, ploughs and rotary hoes.
Where you can find it
Nutgrass has only recently started infesting cropping land and occurs in parts of the North Waikato, Waipa and Matamata-Piako Districts. Little accurate information is available on the plants’ distribution in these, or other parts of the region.
Responsibility for control
Our Regional Pest Management Strategy aims to restrict further spread of nutgrass in the region using agricultural machinery.
The sale, propagation, or distribution of nutgrass is prohibited. Waikato Regional Council may take enforcement action against contractors who move contaminated machinery from infested parts of the region in contravention of section 130 of the Biosecurity Act 1993.
Control of nutgrass is extremely difficult. Your local plant pest contractor can assist in developing a management programme.
Control methods must aim to exhaust the perennial tubers. Cultivation is effective, but only if it is very deep, thorough and persistent. Depth is important to bring as much of the root system as possible to the surface. The process must be thorough to ensure that all tuber chains are broken. Persistence is needed to kill all the shoots at the time of most active growth.
Cultivation is most effective when the soil is dry as the plants require moisture to grow. If cultivation is not sustained it will stimulate growth, further increasing the problem and may spread the plant pest to clean areas.
The search for suitable chemical control measures has been carried out for many years. However only limited success has been achieved in localised situations. Your plant pest contractor will provide you with information on the most up-to-date chemical control methods available.
To prevent the further spread of nutgrass, it is essential that all cropping equipment is thoroughly cleaned down on site before being transported elsewhere. Ensure that you and your neighbours are aware of the problems this plant pest presents and insist that harvesters coming onto your properties have been cleaned down.
For further information and advice contact your local plant pest contractor.
- Visit our Waikato Regional Pest Management Strategy.
- 'What makes a pest a pest? - A guide to Waikatos pest management future' .Pick up, download or order for free from our offices.
- 'Plant me instead - Plants to use in place of common pest plants' Pick up or order for free from our offices.
- Download the National Pest Plant Accord.
- 'Poisonous plants and fungi in New Zealand - A guide for parents, schools and child minders'. Pick up or order for $15 from our offices.