Yellow 'monster' sweeps through region

Last updated 12:42 04/12/2012
PRETTY BAD: A broom-covered hill near Kawatiri Junction, viewed from State Highway 63.

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The yellow carpet covering many hillsides throughout the Nelson region may look stunning in the spring sunshine, but it is a sign of a rampant weed on the march.

Tasman District Council biosecurity officer Lindsay Grueber calls broom "a monster", and in many ways worse than gorse and other pest plants because of its ability to spread so widely and quickly.

After an ideal growing season, it was more prominent than ever, he said.

"It's been a standout year. South Island-wide, broom is on the move, filling in a lot of steep country."

Under the council's pest management strategy, broom is rated as a 7 on its infestation curve for the region, apart from the Howard-St Arnaud area, where its lower incidence has seen it assessed at 4.

As a result, it is categorised as a containment pest in the Howard-St Arnaud area, which requires landowners to control it regardless of where it is on their properties. Elsewhere, it is rated as a boundary control pest, and owners only have to clear a 10-metre strip if the neighbouring property does not have it.

Howard-St Arnaud farmers were doing a good job of keeping it down, "but they can't afford to turn their backs on it for a year", Grueber said. Maruia farmers had also formed a care group.

Around the rest of the region, little could be done to halt its spread, as the cost was prohibitive, apart from landowners keeping it clear of productive farmland or managing it by planting forests, he said.

"Unless you have got a very organised and intensive management and stocking regime, gorse and in particular broom will get away from you on those steeper slopes."

Broom was a perfect coloniser of poorer land, Grueber said, because it spread quickly, fired out huge numbers of seeds which were viable for at least 50 years, was shade and drought-tolerant, and could withstand very cold temperatures.

"You see it right up to the skyline."

Its flowers and seeds were also attractive to insects and rodents.

As well as being able to smother native species in regenerating shrubland, it invaded plantation forests and quickly covered harvested areas, which Nelson had a lot of, he said.

Now was a good time to spray broom, while it was easy to see and growing, he said. Leaving it until late summer, when its growth started to slow due to the dry weather, meant stronger chemicals needed to be used.

The council had also embarked on a long-term biocontrol programme, Grueber said, which involved the release of the broom seed beetle and the broom gall mite. There was also a psyllid, and the council had a broom leaf beetle in its nurseries which it had yet to harvest.

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However, given the "sheer abundance" of plants and seeds, a limited budget and the large areas of difficult land where it was not economic to spray, it would be decades before the insects had an impact, he said. The council spent $20,000 a year on biocontrol of all its pests.

In the end, it was up to individual landowners to be good neighbours and take responsibility for controlling weeds, Mr Grueber said.

"You have just got to keep at it."

Peter Colombus, of Vegetation Services in Motueka, said the wet winter followed by a warm, dry spring had seen many weeds take off. "There is a lot of yellow flower around."

There had been increasing demand from smaller block holders in particular who wanted their broom, gorse and blackberry sprayed. Some tried to do it themselves but ended up calling him in because he could do it a lot quicker, he said.

- Nelson

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