Kōwhai are small, woody legume trees in the genus Sophora native to New Zealand. There are eight species, Sophora microphylla and S. tetraptera being the most recognised as large trees. Their natural habitat is beside streams and on the edges of forest, in lowland or mountain open areas. Kōwhai trees grow throughout the country and are a common feature in New Zealand gardens. Outside of New Zealand, kōwhai tend to be restricted to mild temperate maritime climates.
The name kōwhai comes from the Māori word for yellow—a reference to the colour of the flower.  The name is usually spelled as kōwhai but frequently without the macron, and is sometimes spelled kōhai in some areas.
The 8 species of the kōwhai are:
- Sophora chathamica, Coastal kōwhai
- Sophora fulvida, Waitakere kōwhai
- Sophora godleyi, Godley's kōwhai
- Sophora longicarinata, Limestone kōwhai
- Sophora microphylla, Small-leaved kōwhai
- Sophora molloyi, Cook Strait kōwhai
- Sophora prostrata, Prostrate kōwhai
- Sophora tetraptera, Large-leaved kōwhai
Description and ecology
Most species of kōwhai grow to around 8 m high and have fairly smooth bark with small leaves. S. microphylla has smaller leaves (5–7 mm long by 3–4 mm wide) and flowers (2.5-3.5 cm long) than S. tetraptera which has leaves of 1–2 cm long and flowers that are 3 cm-5 cm long.
The very distinctive, almost segmented pods, which appear after flowering each contain six or more smooth, hard, yellow seeds. These seeds can be very numerous and the presence of many hundreds of these distinctively yellow seeds on the ground quickly identifies the presence of a nearby kōwhai tree. Many species of kōwhai are semi-deciduous and lose most of their leaves immediately after flowering in October or November, but quickly produce new leaves. Flowering of kōwhai is staggered from July through to November, meaning each tree will get attention from birds such as tui, wood pigeon and bellbird. Tui are very attracted to kōwhai and will fly long distances to get a sip of its nectar.
The wood of kōwhai is dense and strong and has been used in the past for tools and machinery.
Kōwhai can be grown from seed or tip cuttings in spring and autumn. The hard, dark or bright yellow seeds germinate best after chitting and being soaked in water for several hours. They can also benefit from a several minute submersion in boiling water to soften the hard shell and then being kept in the same water, taken off boil, for several hours to soak up the water. Young kōwhai are quite frost tender, so cuttings or seedlings should be planted in their second year when they are 30 cm or higher.
If grown from seed kōwhai can take many years to flower, the number of years varies depending on the species.
All parts of the kōwhai, but particularly the seeds, are poisonous to humans. There, however, do not appear to have been any confirmed cases in humans of severe poisoning following ingestion of kōwhai in New Zealand.
Traditional Māori use
Māori would also use the kōwhai tree as medicine. The bark was heated in a calabash with hot stones, and made into a poultice to treat wounds or rubbed on a sore back or made into an infusion to treat bruising or muscular pains. If someone was bitten by a seal, an infusion (wai kōwhai) was prepared from kōwhai and applied to the wounds and the patient was said to recover within days.
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