Urtica ferox

Trichomes along the leaf mid-vein

Buds and developing flowers

Urtica ferox (ongaonga) is a dioecious (either male or female) shrub or small tree in the nettle family Urticaceae. It can grow to a height of 3 metres (10 feet) with the base of the stem or trunk reaching 12 cm (5 in) in thickness.^[1][2] The pale green leaves are thin and membranous, and the surface of the leaves, stems, and stalks are covered in stiff trichomes (hairs) that can grow up to 6 mm (0.24 in) long. These trichomes are prominent along the salient mid-vein and leaf margin. The leaves range in size from 3–5 cm (1.2–2.0 in) in width and 8–12 cm (3.1–4.7 in) in length. They are arranged in opposite pairs.^[3][4][5] Juvenile leaves can reach up to 15 cm (5.9 in) long.^[4] The leaf shape is triangular, ovate-triangular, to lanceolate-triangular with coarsely serrated leaf margins.^[3][4][6]

Flowering occurs from November to March.^[1] The flowers are produced in panicles (branched flower clusters), which can reach 8 cm (3.1 in) long.^[6][2] The densely packed stigmas in the flowers receive the pollen grains.^[7][8] Fruiting occurs from December to May; the fruits are found in achenes (dry one-seeded fruits), each containing a single seed, which is 1.5 mm (0.059 in) long. These seeds take about one month to germinate.^[7][1][8] Burrows (1996) found that U. ferox is capable of building up large seed banks (dormant soil seeds) in the soil which may survive for several years, like European species in the genus Urtica which also show seed bank dormancy.^[9] U. ferox has a diploid chromosome count of 48.^[1]

Urtica ferox was first scientifically described by German naturalist Georg Forster in 1786.^[10][11] The plant was collected during the second voyage of James Cook.^[12] As of 2026^[update], there are 69 species of the Urtica genus accepted by the Plants of the World Online taxonomic database, which are found throughout the world.^[13] Among the six species of Urtica native to New Zealand,^[14] U. ferox is unique owing to it being tall and woody.^[15][16]

Vittacus mansoni, a species of gall mite that induces galls on U. ferox.

A female New Zealand red admiral (Vanessa gonerilla) laying eggs on U. ferox.

Urtica ferox seeds are dispersed by gravity and the wind.^[25] Like all Urtica species, U. ferox is wind-pollinated,^[26][27] although bees may visit the flowers to collect nectar.^[5][28] During winter in some southern localities, U. ferox can be semideciduous (losing some of its leaves) or entirely deciduous (losing all of its leaves). This characteristic is uncommon in New Zealand's native plants, only being found in about 5% of the country's native woody species.^[29][30][31]

The plant is the primary host for the larvae of the endemic mainland New Zealand red admiral (Vanessa gonerilla subsp. gonerilla),^[32][33] which begin laying eggs in early spring (September).^[34][33] The butterflies lay their eggs on the new growth of U. ferox leaves, typically at the tip and often on the side with the trichomes.^[35] After hatching, the caterpillar larvae then spend up to six weeks feeding on those leaves. During this vulnerable stage, the caterpillars use the spiny leaves for protection, wrapping the leaves around themselves like a blanket or building a small tent to hide from potential predators such as birds and insects. Populations of V. gonerilla have had a perceived decline, possibly in part because of people treating U. ferox as a weed and removing it.^[34]

Several moth species feed on the plant as larvae, including Mnesictena flavidalis, Diarsia intermixta, Meterana inchoata, Pasiphila urticae, Epichorista crypsidora, two species of Pseudocoremia, and species of Udea.^[5][36] A rare moth species, Pseudocoremia flava, uses U. ferox as a host plant.^[37] The beetle Oemona hirta can also be found on the plant.^[38] A species of gall mite, Vittacus mansoni, uses U. ferox as a host plant, and a leaf miner species of fly Liriomyza urticae.^[39][40] Rust fungi of the genus Puccinia and the leaf spot Ramualria urticae can also be present on the plant.^[41][42] Deer, sheep, and goats eat U. ferox's leaves in spite of the trichomes (all introduced mammal species).^[5][26][43] The fruits are also eaten by introduced common brushtail possums.^[5][44][43]

Urtica ferox is endemic to New Zealand where it is found throughout the North and South Islands.^[1] Otago was thought to be the species' southern limit by botanist Thomas Cheeseman in his 1906 publication, the Manual of the New Zealand Flora.^[52] It was later recorded on the Tītī / Muttonbird Islands in 1915 by Dugald Poppelwell.^[53] A 2011 book of New Zealand flora notes that the plant occurs as far south as Stewart Island.^[54] The plant varies in abundance throughout its distribution area, being especially common near Wellington and Christchurch,^[5] whilst they are uncommon or absent north of Auckland (as noted by botanist Lucy Moore in 1978).^[55] The 2023 assessment of U. ferox in the New Zealand Threat Classification System was "Not Threatened",^[56] but in the Northland Region, it has been assessed as "Regionally Critical" with an estimated 250 mature U. ferox plants in the region.^[57]

Urtica ferox is a plant usually avoided by people, but it did have some traditional and medicinal uses for the indigenous Māori people.^[59] In traditional medicinal practices, the bark of U. ferox and the leaves of kawakawa (Piper excelsum), boiled together, were used to treat eczema and sexually transmitted infections.^[20] The plant was also a traditional Māori food source. The inner stems were consumed after the leaves and outer bark had been removed. The thin film that makes up the inner bark was also eaten raw and is said to have a sweet taste.^[34][24] In Māori mythology, the plant was placed in forests to prevent people moving freely and to irritate people. The Polynesian explorer Kupe was said to have placed several obstacles, including this plant, to slow down those chasing him.^[45]