Written by Anne Rollwagen (Lincoln University)
Edited by Michael Hudson, Lincoln University, May 2006
- Scientific name: Urtica ferox (J. G. A. Forster)
- Popular Common names: Tree Nettle, Ongaonga
- New Zealand Status: native (endemic)
- Taxonomy sources:Hoogland and Reveal (2005); Webb et al., (1990); Webb et al., (1988).
The tree nettle is a woody shrub that grows to two meters or higher (Webb et al., 1988; Allan, 1982). The trunk can get up to twelve centimetres in diameter. The numerous branches tend to grow interweaved (Salmon, 1980).
The pale green leaves are arranged opposite each other (Connor, 1977; Allan, 1982). The shape is ovate- triangular to lanceolate-triangular with pointed ends. The edges are deeply and coarsely toothed (Connor, 1977). Those teeth can be up to one centimetre long. The leaves are about eight to twelve centimetres long and three to five centimetres wide (Allan, 1982). Stipules grow at each leaf axis, which are like small leaves without a stalk (Salmon, 1980).
The veins, edges and stalks of the leaves (Connor, 1977) as well as the branches and flowers (Salmon, 1980) are covered in numerous stiff stinging hairs (Figure 2). Those stinging hairs are about six millimetres long and have sharp pointed ends (Connor, 1977). When touched the stinging hairs break and release a toxic substance which causes a prickling-burning sensation (Connor, 1977) (further detail in ’Significance to People’ section). The Urtica ferox plant is, apart from the long stiff stinging hairs, covered with countless smaller, softer hairs (Salmon, 1980; Webb et al., 1988).
The tree nettle is a dioecious plant which means that the female and male flowers occur on separate plants (Connor, 1977). Several small pale greenish flowers (Webb et al., 1990) grow together on spikes which can get eight centimetres long and are coming from the leaf axis (Salmon, 1980). The fruit of Urtica ferox is a one and a half millimetre long ovoid, brown nut (Salmon, 1980).
These nettles, like all Urticaceae, have strong fibres and clear juice (Webb et al., 1990).
That the Urtica ferox is a woody shrub is unusual for this genus; this makes it easy to distinguish from other nettles (Urtica) which are all herbs (Webb et al., 1990; Allan, 1982). Another characteristic are the comparatively long and stiff stinging hairs found alongside the veins of the leaves (Connor, 1977).
New Zealand range
Urtica ferox grows on the North and South Island of New Zealand as well as on Stewart Island (Webb et al., 1988). In Allan (1982) the distribution is limited to the area south of latitude 35 degrees, as well as on the east side of the divide on the South Island. Salmon (1980) on the other hand states that the nettle can be found only west of the main divide. In Poole and Adams (1990) the distribution of Urtica ferox reaches as far as east Otago.
Tree Nettle plants can be found between costal and lowland regions up to 600 metres above sea level (Allan, 1982; Clark, 1993; Poole and Adams, 1990; Wardle, 1991; Webb et al., 1988; Webb et al., 1990).
Natural History in Canterbury
One typical habitat for The tree nettle is temperate bush (Wardle, 1991). The temperate broadleaf bush in lowland areas, where Urtica ferox can be found, includes stock-damaged bush, scrubland and forest margins (Allan, 1982; Wardle, 1991). The nettle also grows in bush on steep and unstable slopes (Wardle, 1991). Another habitat is open patchy vegetation on primary surfaces such as shingle beach or slope debris. On slope debris The tree nettle can grow to thickets on bush margins (Wardle, 1991).
For growing, the nettle requires fertile soils and well lit places such as tree fall gaps or forest margins. But it can also grow in more shady conditions (Burrows, 1996). Like other Urtica species The tree nettle grows well in soils with high nutrient levels, in particular nitrogen (Wardle, 1991).
The woody shrub can resist temperatures down to minus eight degree Celsius (Burrows, 1996). In winter it drops its leaves in colder districts and exposed sites (Burrows, 1996; Wardle, 1991). Another occasion when the plant can loose its leaves is during droughts, especially on shallow sites (Burrows, 1996).
Urtica ferox flowers from November till March. The pollination of the separate growing female flowers happens through wind (Webb et al., 1990). The fruits develop and ripen between December and May (Allan, 1982). Figure 3 shows what the flower/ fruit spikes look like.
The seeds of Urtica ferox germinate best when they are exposed to light and the soil around them is wet enough. Light conditions require that the seeds are not covered in leaf litter, which is the case in winter or spring after their dispersal (Burrows, 1996).
In the ecological system Urtica ferox plays an important role in the lifecycle of the red admiral (Brassaris gonerilla), an endemic butterfly (Barron et al., 2004, Webb et al., 1990). The tree nettle provides food and protection for the butterfly larvae (Barron et al. 2004). Therefore the shrubs can get defoliated by the red admiral caterpillar (Brockie, 1992). Furthermore another butterfly, the yellow admiral (Vanessa itea) also lays its eggs on the Urtica ferox (Brockie, 1992).
Several browsing mammals feed on The tree nettle including the brushtail possum (Trichosurus vulpecula) (Cowan, 1990), goats (Capra species) and deer (Cervidae family) (Brockie, 1992; scientific animal names from Allaby 1991).
Because of its dangerous stings Urtica ferox provides protection for young plants like Fuchsia (Fuchsia excorticata), a species in decline, from the reach of herbivores (Brockie, 1992).
How to find a Tree Nettle
The tree nettle can be found more or less all year round at scrub and forest margins in the lowlands (Allan, 1982; Wardle, 1991). It is easier to identify it in summer, when it has not lost its leaves (Burrows, 1996; Wardle, 1991). Altogether it has no eye-catching features as it is not colourful other than green with tiny, unattractive flowers (Webb et al., 1990). Nevertheless the wanderer should watch out not to run into the woody shrub as this can have painful consequences (mentioned in more detail in the ’Significance for people’ section) (Clark, 1993; Connor, 1977).
Abundance and Conservation Status
- New Zealand: abundant
- Canterbury: abundant
Poole and Adams (1990) state that Urtica ferox is a common plant in New Zealand. The tree nettle can resist animal browsers and is therefore able to remain as part of the forest plant community (Brockie, 1992).
I could not find any published information about the plant's status in Canterbury. It grows vigorously at places where the required habitats still exist in Canterbury, such as on Banks Peninsula (personal observation).
Significance for people
The sting of The tree nettle can be very dangerous for human as well as for animals (Connor, 1977). There are several cases known where horses and dogs have died after the contact with Urtica ferox. Connor (1977) also reports about the death of one man after being severely stung by The tree nettle. Some possible symptoms which can occur after touching the nettle, especially multiple times, are listed in table 2.
|Time after contact||Symptoms|
|immediately||painful, burning sensation
|15–20 minutes||abdominal cramps
strong burning sensation in feet
|60–90 minutes||weakness, exhaustion
loss of eye sight
problems to keep the body warm
problems to control movements of arms and legs
The effects of a Tree Nettle sting can last for about three days (Salmon, 1980).
The known toxic substances which can cause this reaction are acetylcholine, 5- hydroxytryptamine (serotonin), histamine and others (Clark, 1993; Connor, 1977).
One treatment Maori created to relive the pain from a Tree Nettle sting can be found in the article available at: http://www.rsnz.org/archives/education/science_fairs/natfair96/02.html [cited 9 September 2005].
This paper deals with the plant Urtica ferox; the commonly called Tree Nettle or Ongaonga. This is a woody shrub, unusual for the genus, and is endemic to New Zealand. The main characteristics are a mature growth height of more than two metres and the plant being covered in long stiff stinging hairs. These stinging hairs release a toxic substance which causes severe pain when touched. It can be dangerous for animals as well as for people. It grows in light and fertile conditions such as forest margins. The tree nettle can be found in forest and scrubland in the lowland regions of North and South New Zealand.
Allaby, M. (1991) The Concise Oxford Dictionary of Zoology. Oxford: Oxford University Press
Allan, H.H. (1982) Flora of New Zealand Volume 1, Wellington: Botany Division, D.S.I.R.
Barron, M.C., Wratten, S.D. and Barlow, N.D. (2004) Phenology and parasitism of the red admiral butterfly Brassaris gonerilla (Lepidoptera: Nymphalidae). New Zealand Journal of Ecology, 28/1, 105–111
Brockie, R. (1992) A living New Zealand forest. Auckland: David Batemann Ltd.
Burrows, C.J. (1996) Germination behaviour of seeds of the New Zealand woody species Melicope simplex, Myoporum laetum, Myrsine divaricata, and Urtica ferox. New Zealand Journal of Botany, 34, 205–213
Clark, F.P. (1993) Tree Nettle (Urtica ferox) poisoning. The New Zealand Medical Journal 106/957, 234
Connor, H.E. (1977) The Poisonous Plants in New Zealand, 2nd ed. Christchurch: Botany Division, D.S.I.R.
Cowan, P.E. (1990) Fruits, seeds, and flowers in the diet of brushtail possums, Trichosurus vulpecula, in lowland podocarp/mixed hardwood forest, Orongorongo Valley, New Zealand. New Zealand Journal of Zoology, 17, 549–566
Hoogland, R.D. and Reveal, J.L. (2005) Index Nominum Familiarum Plantarum Vascularium, The Botanical Review, 71(1/2), 1–291
Poole, A.L. and Adams, N.M. (1990) Trees and Shrubs of New Zealand, Wellington: D.S.I.R.
Salmon, J.T. (1980) The Native Trees of New Zealand. Wellington: Reed Ltd.
Wardle, P. (1991) Vegetation of New Zealand, Cambridge University Press, Cambridge
Webb, C.J., Sykes, W.R., Garnock-Jones, P.J. (1988) Flora of New Zealand Volume 4, Christchurch: Botany Division, D.S.I.R.
Webb, C.J., Johnson, P.R., Sykes, W.R., (1990) Flowering Plants of New Zealand, Christchurch: D.S.I.R. Botany