T.E.R:R.A.I.N - Taranaki Educational Resource: Research, Analysis and Information Network

Tree nettle (Urtica ferox)

Kingdom: Plantae
Division: Magnoliophyta
Class: Magnoliopsida
Order: Rosales
Family: Urticaceae
Genus: Urtica
Species: U. ferox
Binomial name: Urtica ferox
Common name: Ongaonga, Bush nettle, Tree nettle.

The tree nettle is one of New Zealand's most poisonous native plants due to its stinging action.  See last paragraph below.

Urtica ferox is a nettle that is endemic to New Zealand. Sometimes have woody stems and unusually large stinging spines, and can grow 5 metres tall. Even the lightest touch can result in a painful sting that lasts several days. 
There has been one recorded human death from contact—a lightly clad hunter who died five hours after walking through a dense patch. Acute polyneuropathy can occur due to U. ferox stings. Polyneuropathy is a neurological disorder that occurs when many nerves throughout the body malfunction simultaneously.
In Maori folklore, Kupe was said to have placed several obstacles to hinder pursuers whose wives he had stolen, one of which was the Urtica ferox.
Urtica ferox is a dioecious plant with pale green leaves which are arranged opposite each other. Their shape is ovate-triangular to lanceolate-triangular with pointed ends. The edges are deeply and coarsely toothed. The teeth can be up to one centimetre long. The leaves are about eight to twelve centimetres long and three to five centimetres wide. Stipules grow at each leaf axis, which is like small leaves without a stalk.
The veins, edges and stalks of the leaves, as well as the branches and flowers, are covered in numerous stiff stinging hairs. The stinging hairs are about six millimetres long and have sharp pointed ends. When touched the stinging hairs break and release a toxic substance which causes a prickling-burning sensation.
Urtica ferox has during November to March small pale greenish flowers which grow together on spikes up to eight centimetres long and are coming from the leaf axis. Pollination happens through the wind.
The fruits develop and ripen between December and May and are a one and a half millimetre long ovoid, brown nut.

Urtica ferox grows on the North and South Island of New Zealand as well as on Stewart Island. The tree nettle is in temperate bush in lowland areas, which includes stock-damaged bush, scrubland and forest margins. This plant can resist temperatures down to minus eight degree Celsius. In colder areas, it will drop its leaves during winter.

Urtica ferox is the main food plant for larvae of the New Zealand Red Admiral butterfly Bassaris gonerilla and The Yellow Admiral butterfly (Vanessa itea)

The tree nettle is one of New Zealand's most poisonous native plants. Standing about two metres tall, it’s coarsely toothed leaves have numerous white stinging hairs (trichomes), up to 6mm long, at the tip of each tooth as well as on young stalks and leaf veins. These are hollow cylinders with tapered points, which break after piercing the skin, injecting toxins into the tissues, giving rise to pain and rash. There have been cases of dogs and horses developing neurological problems, with respiratory distress and convulsions within minutes of exposure, often dying within hours, although some do recover. There are also reports of human poisoning in botanical references or the press. Connor, in his book, The Poisonous Plants in New Zealand, mentions a group of trampers who developed loss of coordination for three days after being stung. In another instance, a typist developed tingling numbness in the hand after grasping a nettle bush, preventing her from typing for five days. There are also reports of severe headaches, blurred vision and extreme fatigue. A fatal poisoning was described in 1961 when a young man died of paralysis and respiratory problems several hours after walking through a patch of tree nettles. The above text thanks to the New Zealand Plant Conservation Network.

A young plant. photographed at Otari Wilton Bush Reserve, Wellington

Topside of the leaf

The underside of the leaf

Thanks to Wikipedia for text and information: https://creativecommons.org/licenses/by-sa/3.0/