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September - Kōwhai - Native Plant of the month


Genus Sophora

Along with pōhukutawa, kōwhai are arguably our most well-loved and well-recognised native flowering plants. A tree in full bloom is a striking sight and the inevitable addition of tūī squabbling and warbling among the flowers can keep a watcher captivated for hours. September is the time kōwhai are usually at their peak flowering and so we thought it would only be fitting to celebrate kōwhai as our plant of the month.

People have loved kōwhai throughout the history of Aotearoa. Māori grew the trees near pā and other sites of significance and brought seeds with them as they moved to new areas, including the Chatham Islands. Kōwhai were used to lure in birds to eat, for their durable wood, to make dyes and medicine. Early European explorers to Aotearoa collected kowhai seeds that were then grown in Kew Gardens. Kōwhai have been admired and used as an icon of kiwiana for many decades.

The name kōwhai is well known, referring both to the trees and the yellow colour of their flowers. However, there are more kōwhai than you might expect. We have a total of eight native species of kōwhai (all of which are endemic), along with a couple of introduced species, which are also commonly referred to by the same name within New Zealand. Of the eight native species, three are naturally occurring in the Auckland area and only two are naturally found in Kaipātiki. Coastal kōwhai (Sophora chathamica), as the name suggests, typically grows in coastal habitats and along the margins of large rivers and lakes. It can be found throughout the country (including the Chatham islands), but it is thought to have been originally native to only the top half of the North Island and is therefore introduced to the remainder of its current habitats. Small-leaved kōwhai (Sophora microphylla) is the second native species found in Kaipātiki. These plants are notable for having divaricating (tangled, wide-angled branching) juvenile plants, which then mature into large, weeping trees. Small-leaved kōwhai tend to be found in riparian forest near streams, rivers and wetlands throughout Aotearoa.

Flowering coastal kōwhai

The coastal and riparian forest habitats that kōwhai prefer indicates the strong connection these trees have with water. Even though kōwhai can be quite drought tolerant, water is their main seed dispersal mechanism. Kōwhai seeds have a very tough outer coating that is both waterproof and buoyant, enabling the seeds to float and tumble through water for years. As the seed surface is scraped against rocks and sand it gradually abrades and this triggers chemical changes that start the germination process. Anyone who has tried growing kōwhai at home will know that the recommended process mimics this natural assault: with sandpapering, clipping, scratching, soaking and even boiling seeds to start germination. Because kōwhai seeds are so tough and capable of long-term immersion in water, they are able to survive sea journeys and have been found washed up on beaches around New Zealand, including far-flung offshore islands.

As well as the eight native species of kōwhai, there are many other closely related species in other countries, including Chile and Australia. The family resemblances are very strong and it can be hard to tell these exotic kōwhai apart from native species. Unfortunately this confusion extends to nurseries who sometimes sell exotic species under the names of our native species. This is a problem because kōwhai readily hybridise. Planting species that are not locally native can result in hybrids that weaken genetic differences as well as the special features that have developed through the long process of localised adaptation. The different kōwhai species have evolved through their physical separation in different areas and when people move them to grow close to one another this process is reversed. Too much of this could even cause extinction through hybridisation. Another reason why eco-sourcing is so important!

Necklace pod, Sophora tomentosa, is an Australian relative of

kōwhai, with different proportions to our native species.)

While other kōwhai have made it back to our shores carried by people as exotic garden plants, it seems our native species may have made this journey in reverse millions of years ago. Botanists use clues to trace the evolution of species and the fact that there are eight species of kōwhai in New Zealand, compared to only one in each of the other countries which have related species, indicates that this group of plants originated here and have gradually spread abroad to evolve into new species in different countries. Kōwhai have other, more distantly related plants growing in New Zealand as well. They belong to the legume family, made most famous by peas and beans, but also including gorse, broom and kākābeak. This family of plants have distinctive flowers with a ‘keel’ petal and rounded seeds that usually grow in long pods. Kōwhai and kākābeak have flowers that are distinctly adapted to bird pollination with downward-hanging clusters of large, brightly coloured blooms.

Karakabeak flowers and buds Native broom flowers

If one thinks of kōwhai it is hard not to also think of tūī. Their flowers are so attractive to tūī that birds will often stake out favourite trees weeks before they flower, scaring off any other birds that might dare to come and perch. But tūī are not the only ones attracted to these wonderful trees. Kākā also come to feed on their nectar and pollinate them using their brush-tongues. Bees plunge into their flowers. Kererū come to feed on buds and leaves and can strip branches bare. Legumes are often high in proteins and so it is likely that the kererū are gaining nutrients from kōwhai that they can’t easily get from other plants, and this may help them gain condition for breeding.

Kākā and tūī on kōwhai

Young kōwhai may lose leaves to another native culprit; the kōwhai moth (Uresiphita polygonalis maorialis). The caterpillars of this moth feed on kōwhai leaves, as well as broom, lupin, gorse and clover. Unfortunately possums also enjoy munching on kōwhai leaves and flowers. All this attention can mean that kōwhai are victims of their own popularity and are eaten faster than they can grow. As such important food sources for so many species there are lots of reasons to plant more kōwhai groves - and keep up your trapping.

Kōwhai moth and caterpillar, images by Steve Kerr, Manaaki Whenua

Kōwhai are hardy and can cope with growing in poor and dry soils. They are able to ‘fix’ atmospheric nitrogen into a usable form using nodules on their roots, which gives them an advantage in nitrogen depleted areas. This also means that they help enrich the soil for surrounding plants. Kōwhai are also among the very few native plants to be deciduous. They lose their leaves for a brief period in late winter, just before flowering. This gives them their striking golden appearance when in bloom as the flowers usually develop before the new leaves. It also means that they are an ideal tree to plant in sites where you may not want to block out winter sun.

It would be a joy to see more of these beautiful trees gracing our coastline and stream margins and we highly recommend planting them - just take care that they are from a reputable eco-source nursery to avoid accidentally mixing species from other places into our local ecosystems. The long coastline and many stream-edges of Kaipātiki are excellent habitats for our two locally native species of kōwhai. If you feel like a cheerful spring walk, beautiful specimens can be seen at both Kahika and Witheford Reserves. Here’s to a golden spring!

Historic kōwhai tree in flower beside former Mumme family home at what is now Kahika Reserve, photo courtesy of Mumme family

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