Phormium tenax \"is a perennial rhizomatous herb (to circa 2 m tall) native to New Zealand. The stems are short and stout. The leaves are strap-like, deep green, clump forming and fibrous. The flowers are red and orange, occurring in terminal sprays\" (Csurhes and Edwards 1998 in PIER 2006). \"Plants to 5 to 6 m tall. Leaves bright orange toward base, stiff, erect, at least in lower part, 100 to 300 cm long, 5 to 12 cm wide, margins entire, apex usually splitting. Inflorescences 4 to 5 m long, peduncles dark brown, 2 to 3 cm in diameter, glabrous; tepals dull red, 2.5 to 5 cm long. Capsules dark brown with age, erect, 3-angled, 5 to 10 cm long, usually falcate, abruptly constricted at apex, not twisted. Seeds elliptic, 9 to 10 mm long, somewhat twisted\" (Wagner et al. 1999, in PIER 2006).

Flax is used in Pacific cultures for weaving baskets and other objects.

Phormium tenax competes for habitat with Elaphoglossum dimorphum in St Helena where it is found on stone steps, rocks and shaded mossy banks in tree fern thickets (IUCN 2008). It is reported to be found growing primarily near footpaths and in undisturbed vegetation on Inaccessible and Nightingale Islands (Dean et al. 1994). Surveys of the New Zealand flax communities on Inaccessible Island from 1999 to 2000 revealed a population of 1000 plants occurring mainly on the steep coastal cliff faces (Ryan & Glass 2001, in Ryan et al. 2007b). Flax was common on the island in well-drained areas of relatively open vegetation (dominated by the ferns Blechnum penna-marina and Rumohra adiantiformis) and was scarce in dense tussock grass communities (Ryan et al. 2004). Flax appeared to struggle to gain a foothold in this habitat on Inaccessible Island; it was often found at disturbed sites such as at the base of overturned Phylica arborea trees on the summit scarp or growing on boulders or rocky outcrops (Ryan et al. 2004). The apparent difficulty with which flax established in dense tussock probably helped limit the spread of the species on Inaccessible. (Note, however, that flax on Nightingale Island is confined to an area of Spartina tussock grass and Phylica woodland (Ryan et al. 2007b)).

In Hawaii the species forms dense thickets in gullies in mesic areas below 300 meters above sea level (Smith, Alien Plants of Hawaii web site, in PIER 2006).

Phormium tenax New Zealand flax colonises and converts native habitats degrading them and making them unsuitable for native species. It has the potential to alter the structure and function of vegetation communities of islands where it is introduced (Ryan & Glass 2001).
The following Saint Helena's plant species are negatively affected by the spread of flax: the 'Critically Endangered (CR)' Elaphoglossum dimorphum in IUCN Red List of Threatened Species (Eastwood 2004); the 'Endangered (EN)' small bellflower (see Wahlenbergia angustifolia in IUCN Red List of Threatened Species); the 'Critically Endangered (CR)' large bellflower (see Wahlenbergia linifolia in IUCN Red List of Threatened Species) (Cairns-Wicks 2003a; 2003b); the 'Endangered (EN)' dogwood (see Nesohedyotis arborea in IUCN Red List of Threatened Species) (Cairns-Wicks 2003c); the 'Critically Endangered (CR)' He cabbage tree (see Pladaroxylon leucadendron in IUCN Red List of Threatened Species) (Cairns-Wicks 2003d); the 'Vulnerable (VU)' black cabbage tree (see Melanodendron integrifolium in IUCN Red List of Threatened Species) (Cairns-Wicks 2003e); the 'Critically Endangered (CR)' false gumwood (see Commidendrum spurium in IUCN Red List of Threatened Species) (Cairns-Wicks 2003f) and the 'Critically Endangered (CR)' She cabbage tree (see Lachanodes arborea in IUCN Red List of Threatened Species) (Cairns-Wicks 2003g). The 'Vulnerable 'VU' Tristan bunting (see Nesospiza acunhae in IUCN Red List of Threatened Species) and the 'Vulnerable 'VU' grosbeak bunting (see Nesospiza wilkinsi in IUCN Red List of Threatened Species) are also negatively impacted by flax (BirdLife International 2008a,b).

Funding from the UK Foreign and Commonwealth Office allowed for an initial clearing and eradication programme on Inaccessible Island in September 2004. A team of four was deployed by helicopter to cut down as many large plants as possible (Ryan et al. 2007b). The follow-up for this project was planned for three to five years later and took place in October 2007 under the Managing Aliens on Outer Islands project. There was more re-growth of Phormium tenax than anticipated but only a few plants had flowered since the initial clearing in 2004. Follow-up operations are recommended.