Battle on to weed out invader from north

Field horsetail forces its way through asphalt in Wanganui's Wilson St. Photo/File

The Horizons Regional Council's website has extensive information to help farmers fight field horsetail -- a Northern Hemisphere weed that's rapidly gaining ground.

Field or common horsetail is dormant at the moment and starts growing again in August/September. Reading up on the information now will give landowners time to identify the weed before it takes hold, a council spokeswoman said.

The rush-like plant has become an issue in Manawatu and Rangitikei in particular. It's a perennial weed, difficult to control and has an extensive root system that can quickly dominate surrounding pasture. It is commonly spread by rhizome and root fragments when gravel, sand and soil is moved for road construction. Its spores can also spread over great distances.

Recently the weed spread to other parts of the region, with large infestations already in the Whanganui River system.

Field horsetail has become such a concern that in 2012 the Rangitikei Field Horsetail group was formed, with support from the Landcare Trust and Horizons Regional Council.

A meeting hosted by the group this year attracted more than 80 people, highlighting the increasing numbers of farmers and landowners who are worried.

The Horizons website, horizons.govt.nz, has been updated to include extensive information on field horsetail, details on the Rangitikei Field Horsetail Group and methods to contain its spread and manage it.

It has video clips and photos of the weed and links to promotional material such as a recent Rural Delivery episode discussing the threat field horsetail poses.

If more direct information is needed, Horizons staff can be contacted on 0508 800 800.

Horizons environmental co-ordinator Craig Davey said there was no simple way to manage field horsetail. Instead, a number of actions needed to be taken.

"Field horsetail is best contained by using a combination of chemical, pathway and biological interventions," Mr Davey said.

"Landowners need to be determined and persistent, spraying the weed three to four times per year if sites are small and the root system isn't too deep."

Plants often spread from roadsides to paddocks, and landowners were encouraged to assess their road frontage in order to keep field horsetail at bay, he said.

Gravel extraction companies had also been made aware of the problem and were being encouraged to use best-practice methods.

Chemical and pathway solutions are unlikely to be the whole answer to this scary weed.

"In the long term, biological control is likely to be the only cost effective and sustainable option for field horsetail and this is the approach that Horizons is encouraging," Mr Davey said.

The Sustainable Farming Fund gave the Rangitikei Field Horsetail Group $316,150 last year for an initiative to import insects and diseases to fight the weed.

These will be brought from the Northern Hemisphere, where they prevent field horsetail becoming a problem.

The project began in July and the group now has one possible biological control agent, an insect, in New Zealand. It's being assessed in Christchurch.

The project has also surveyed the whole country, looking for pests or diseases that limit the weed's spread in New Zealand. The survey found nothing here -- hence the reliance on Northern Hemisphere imports.

The group aims to import and assess at least three or four possible control agents -- either diseases or insects. It hopes to gain approval to release at least one of them and be ready to begin mass rearing and distributing it.

After that, it's likely to take up to five years for the agent's population to build to a level where its impact can be assessed.

In the meantime, Horizons will review and develop its Regional Pest Management Strategy early next year. The strategy will need to align with new legislation under the Biosecurity Law Reform Act 2012 and will cover major pest plants such as field horsetail.