Stopping the freshwater wild rice invader

Manchurian wild rice herbicide trial site: haloxyfop (left), control (green), and imazapyr (right).

Flowering Manchurian wild rice.

Manchurian wild rice or Manchurian rice grass (Zizania latifolia) is a giant semi-aquatic grass that has smothered riverbanks, invaded pastures, and run rampant through drainage channels in parts of the North Island from Northland to the Kapiti Coast. NIWA’s National Centre for Aquatic Biodiversity & Biosecurity has been researching methods to control this species.

Manchurian wild rice was first introduced to New Zealand from Asia around the turn of the last century. It arrived in the ballast water carried by timber ships, and was discarded on the banks of the northern Wairoa River near Dargaville. Although introduced accidentally, it was later deliberately planted in the Hauraki Plains area to supposedly stabilise stopbanks. However, rather than stabilise banks, Manchurian wild rice can, in the long term, cause them to slump, encouraging erosion.

This grass causes other problems too. It invades drainage channels, which prevents access, impedes water flow, and increases the likelihood of flooding. Unless intensive grazing is maintained in pastures adjacent to drains filled with Manchurian wild rice, it will invade these areas too. This plant dramatically reduces the diversity of native vegetation by displacing small species and enveloping taller vegetation. The result: long-term monocultures of Manchurian wild rice.

There are no reports of such nuisance growths in the plant’s native Asia (i.e., Taiwan, eastern China, and Southeast Asia), which could be due to the intensive landuse practices associated with its cultivation as a source of food. This giant grass is grown for its edible seed, rhizomes, young shoots, and stem bases. Galls induced by a smut fungus on the rice are also cultivated and used as a summer and autumn vegetable.

In New Zealand, Manchurian wild rice is typically found on the berm of waterways where it can tolerate both fresh and brackish water, and along the tidal reaches of rivers. It forms dense stands about 3 to 4 m high, and has a strong, deep root system and bulky rhizomes that spread several metres down into soft sediment. It is dispersed when water transports seeds and pieces of rhizome to new locations. Contaminated drainage machinery is also a major factor in its spread between catchments.

The biggest infestation of Manchurian wild rice is in the Kaipara District of Northland, especially around the site of its introduction – the northern Wairoa River and associated waterways. Smaller infestations occur in the Whangarei and Far North Districts, as well as in Rodney and Waitakere Districts (Auckland), Hauraki Plains (Waikato), and Kapiti Coast (Wellington). Manchurian wild rice could potentially infest any lowland wetland, especially the margins of still or flowing water bodies.

NIWA has been investigating a combination of physical and chemical control options to stem the plant’s progress. Mechanical diggers have commonly been used to remove the plant, but there is the risk of transferring rhizome fragments to new sites. The Northland Regional Council (NRC) has identified this as the main method of dispersal, and actively promotes cleaning drainage machinery before it is used in areas not infested. Mowing, grazing, burning, and a combination of these methods have been used to control the plant where it has spread to pastures. However, because stock will graze only on new shoots, the pastures must be constantly maintained to prevent plants from becoming large and unpalatable.

Herbicide trials in New Zealand have evaluated sodium chlorate, sodium TCA, paraquat, glyphosate, and dalapon (2,2-dichloropropionic acid) in combination with amitrole. Although none of these products will eradicate this grass, some do reduce its height or cover (or both), preventing it from flowering and dispersing seed. The recent use of grass-specific herbicides also shows promise.

NIWA has used its Aquatic Weed Risk Assessment Model to assess the weediness of Manchurian wild rice, and is evaluating new tools to control and manage it for concerned managers of water bodies. We trialled three herbicides previously used with some success in New Zealand for the control of nuisance rhizomatous marginal grasses, including Manchurian wild rice, phragmites, and spartina. Haloxyfop (Gallant®) and quizalofop (Targa®) were used because they are grass-selective, and imazapyr (Arsenal®), a broad-spectrum herbicide, was trialled because of its success controlling phragmites.

We conducted the trials in containers at NIWA’s experimental facility at Ruakura, and in field plots near Dargaville in collaboration with NRC. The trials were monitored for more than a year and each product was evaluated at several different rates. Haloxyfop and imazapyr significantly reduced the leaf biomass of the grass in containers. The best results for the grass in field plots were also achieved with haloxyfop – rates as low as 0.5 kg/ha were gained by using very high water rates (1600 L/ha), reducing cover to less than 10% for more than a year. This rate is equivalent to a 40% reduction in the amount of haloxyfop previously recommended by NRC to control Manchurian wild rice.

From the information resulting from these trials, and ecotoxicological studies carried out at NIWA, NRC have obtained a consent to control all Manchurian wild rice within their region. They have also set up a programme to progressively control the grass, beginning with isolated areas outside the main infestation zone.

We acknowledge the Foundation for Research, Science & Technology, Northland Regional Council (who co-funded the programme), and Peter Joynt who kindly assisted with organisation of the field trial.