Kowhai are small, woody legume trees in the genus Sophora native to New Zealand. There are eight species, S. microphylla being the most common. Kowhai trees grow throughout the country and are a common feature in New Zealand gardens. Outside of New Zealand, Kowhai tend to be restricted to mild temperate maritime climates. Species in this group include S. chathamica, S. fulvida, S. godleyi, S. longicarinata, S. microphylla, S. molloyi, S. prostrata, and S. tetraptera.[1][2]
Despite having no official status as such,[3] the blooms of the kowhai are widely regarded as being New Zealand's national flower.[4]
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Most species of Kowhai grow to around 8 m high and have fairly smooth bark with small leaves. Sophora 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 Kowhai tree. Many other Kowhai trees lose most of their leaves immediately after flowering in October or November but quickly produce new leaves. Flowering of Kowhai is staggered from July through to November, meaning each tree will get attention from birds.[5] Tui are very attracted to kowhai and will fly long distances to get a sip of its nectar.
Sophora is one of the four genera of native legumes in New Zealand; the other three are Carmichaelia, Clianthus, and Montigena.[2]
Kowhai can be grown from seed or tip cuttings in spring and autumn.[6] 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.[7] Young Kowhai are quite frost tender, so cuttings or seedlings should be planted in their second year when they are 30 cm or higher.[8]
Sophora prostrata, sometimes called "Little Baby", is used as a Bonsai tree. It grows up to 2 metres high, has zigzaging stems, and sparse smallish leaves.[9]
All parts of the Kowhai, but particularly the seeds, are poisonous to humans.[10]
In traditional times, Māori used the Kowhai 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[11] or made into an infusion to treat bruising or muscular pains.[12] If someone was bitten by a seal, an infusion (wai kōwhai) was prepared from Kowhai and applied to the wounds and the patient was said to recover within days.[11]