Urtica Ferox (Onga Onga) at Rewanui

On taking over the management of Rewanui the Trimble Foundation discovered extensive areas of Urtica Ferox in and around the native bush areas. Urtica Ferox is a native to New Zealand and is a variety of stinging nettle that can grow to three metres tall although typical plants are one to two metres. The common name is Onga Onga.

Urtica ferox or Onga Onga(click for a larger image)

Onga Onga has a viscous sting transmitted by small hairs on the stems and leaves. Brushing against the plant is sufficient to create a stinging sensation that will last up to two days and extensive contact can be life threatening. The sting is caused by formic acid and other chemicals present as a tiny drop at the end of the hairs. Contact with Onga Onga can result in extensive red rashes and blisters, and there are records of people dying in New Zealand after stumbling into dense thickets. A Christchurch student has done a project on the most effective way of countering the Onga Onga sting, her work is summarised here.


Onga Onga grows around the edges of the bush in particular but there are extensive areas growing under the tree canopy. Bushes are often found next to logs and rocks in the open but not usually in grazed pasture which suggests that animals will eat it probably while it is young. As the spread of Onga Onga on Rewanui coincides with a campaign to reduce possum numbers it seems likely that possums will eat it and keep it under control and probably goats would too.

Typical Ongaonga growth on the trial areas prior to spraying


There seems to be very little research information available in controlling Onga Onga so the Foundation is conducting informal trials as to the effectiveness of several methods. Browsing by goats or possums is ruled out because of the damage this would cause to other native plants. Following are the techniques under consideration:

  • Physical cutting of the shrubs with a long handled saw. This may be practical for scattered bushes but would be difficult to achieve in dense thickets. Several bushes have been cut at ground level to ascertain the effect and whether it kills the plants.
  • Spraying with suitable herbicides is probably the best option and so three areas have been sprayed with different treatments to ascertain the effect. All the spraying has been done in the 1ha block of remnant bush in the Picnic paddock.
  • Area 1 is at the Eastern edge of the bush and was sprayed with Tordon brush killer in a knapsack sprayer at a rate of 6ml per litre of water. This spraying was done on the 27th December 2004. The active ingredient in Tordon is Picloram.
  • Area 2 is at the Western edge of the bush and was sprayed with Grazon at a rate of 3ml per litre of water on 13th January 2005. The active chemicals in Grazon are Triclopyr and Picloram.
  • Area 3 is on the Southern edge of the bush and was sprayed with Grazon at a rate of 6ml per litre (the standard rate for most brushweeds) on 13th January 2005.
  • On the 21st November 2005 regrowth along the Totara loop track was sprayed with Versatil.

It seems that the toxicity of the plants disappears as soon as wilting takes place which has been observed at three days after spraying.


In January 2006 the plots were surveyed with these conclusions:

  • Physical cutting of bushes even at ground level results in strong regrowth.
  • Spraying with Grazon at 3ml per litre gave very good control with good growth of ferns and very little regrowth of Onga Onga.
  • Tordon plots showed a good kill but sites are invaded by thistles. Some regrowth ocurred.
  • Six weeks after spraying the Versatil treated plants are showing some wilting but are still active.

Area sprayed with Grazon showing good control and strong fern growth

The Tordon treatment


Spraying Grazon at 3ml per Litre of water by knapsack will give good control of Ongaonga at fairly low material cost but a high labour content. Some follow up spraying will be necessary.

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email: info@trimblefoundation.org.nz
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