August’s plant of the month is a bit of a pandora’s box - kānuka are well-known native trees, but they are made up of more than one species. In fact, a genetic study published in 2014 has shown that there are ten different species native to Aotearoa. Here in Kaipātiki you can find two species growing naturally; Kunzea linearis (rawiri mānuka) and Kunzea robusta (rawirinui).
Kānuka can easily be confused with mānuka and we are often asked how to tell them apart. When you are face to face with a plant this is actually quite straight forward. The real trick is the saying “mean mānuka, kind kānuka” - if you grip the leaves of mānuka they are stiff and spiky and painful to hold tight, kānuka leaves, on the other hand, are soft and don’t hurt to grab. Mānuka are smaller trees (growing to maximum height of 5m), with seed capsules that remain on the plant year-round and larger flowers (1cm or more across) that grow individually down plant stems and may be on the plant at any time of the year. Kānuka are much taller, wispier trees, with a short flowering season and small flowers (around 5mm across) that cluster together at branch tips.
Mānuka are smaller trees (growing to maximum height of 5m), with seed capsules that remain on the plant year-round and larger flowers (1cm or more across) that grow individually down plant stems and may be on the plant at any time of the year. Kānuka are much taller, wispier trees, with a short flowering season and small flowers (around 5mm across) that cluster together at branch tips.
L: Kānuka and mānuka flowering together, larger mānuka flowers are on the right . R: Kanuka seed capsules
But what are these two local species of kānuka and where are you likely to see them? Rawiri mānuka is a fairly uncommon species that grows in coastal shrublands and cliff faces (and coastal podzols - a soil type created by kauri forests). The name of this species might be a little confusing, but it comes from a local dialect of te reo Māori appropriate to this species’ range, where ‘mānuka’ is used as the name for kānuka. The southern limit of rawiri mānuka is the Kaipātiki coastline around the Waitematā harbour. You can see a stand of the trees amongst the pо̄hutukawa on the sea cliffs as you begin driving up the Harbour Bridge towards the city. There is some discussion amongst botanists and ethnobotanists about whether Kaipātiki is a natural part of rawiri mānuka’s range, as most of the populations in this area are found near pā sites and it is possible that the seeds were spread through thatching that may have been brought from further afield. Rawiri mānuka is a smaller species than rawirinui, growing to a maximum of 12m (compared to rawririnui’s 30m). Rawiri mānuka flower in early summer and are threatened by hybridisation with rawirinui where they grow in close proximity to one another.
Rawirinui is one of the most common kānuka species in Aotearoa and by far the most commonly seen in Kaipātiki. It grows in both coastal and inland habitats and often makes up the canopy in successional forest (bush that is gradually restoring through natural stages). This kānuka grows well on poor soils, including clay and is drought-tolerant. You can spot this species in many, if not most, of the reserves around Kaipātiki. Rawirinui are tall trees - the biggest of all the kānuka species, usually with a single trunk that opens up to a broad flattened canopy. They have very hard wood which was reputedly used to make the timber seats for Auckland’s historic trams. Rawirinui flower from late spring to early summer and can hybridise with other species of kānuka, including rawiri mānuka where they grow close to one another. This is a good reason to identify your local kānuka species and try not to introduce any new species that may disrupt local genetics.
Kānuka - of all species, hold a very important role in the ecosystems in which they grow. They can be thought of as healers - helping transition an area from bare ground to a flourishing forest. Kānuka, unlike many of our native trees, like to grow in full sun. They can also grow on poor soil, including damaged, compacted soils. This means that they are capable colonisers of areas that have lost their tree cover - through a slip, fire, or more deliberate tree removal. If you look closely you will notice that most kānuka seem to grow in groups that are all the same height, indicating that they are around the same age and colonised an area together. Their canopy cover is airy and lets in enough dappled light to allow seedlings of more sensitive forest trees to grow. They also have a fantastic microbiome that enables lots of healthy fungi and other micro-organisms in the soil to thrive, which then benefits surrounding plants as well. A study from Victoria University has even shown that kānuka compounds have a negative effect on kauri dieback spores!
Kānuka don’t just nourish and shelter other forest plants, they also play an important role for many of our native birds, insects and reptiles. Their loose, flaky bark provides lots of shelter to spiders and other invertebrates and you will often see tui rummaging about kānuka trunks hunting for them. Kākā also fossick through kanuka bark looking for insects and break apart dead branches to get at grubs inside. You might see some large kānuka with huge flattened diamond-shaped scars. These are from puriri moth caterpillars, which can use kānuka as one of their host trees. Once the caterpillars emerge their holes become shelter to tree wētā and spiders. Up in the canopy, kānuka leaves are a firm favourite for our native stick insects as well as some of our native lizards. When they flower, kānuka flowers provide a wealth of nectar and pollen for our small native bees, as well as a range of flies and beetles.
You might think of kānuka as a common tree - especially rawirinui, but every species of kānuka is listed as threatened. Both rawiri mānuka and rawirinui are classed as nationally vulnerable and many species are listed as nationally critical. There are a number of reasons for this serious classification. The first is that because kānuka are thought of as common and are sometimes described as ‘scrub’, they aren’t considered precious or ecologically valuable in the same way that our larger forest trees are, leading to greater loss through clearance. Compounding this is the general understanding that there is only one species of kānuka, despite some of the ten different species having small populations, very few of which are on protected land. Rawiri mānuka is among species threatened by habitat loss - as a coastal species it is regularly cleared for development. Kānuka of all species make good firewood and trees are regularly felled and cleared to harvest timber for this purpose. All kānuka are also threatened by myrtle rust - a fungal plant disease spread by air-borne spores. Myrtle rust attacks new growth, buds, flowers and developing fruit, which can prevent plant reproduction. Repeated infections can damage tree canopies and eventually kill trees entirely. The full impact of this disease is not yet understood, but with the critical role kānuka play in our environment it cannot be underestimated.
L: Kānuka leaves. R: Young kānuka with ‘wispy’ new growth.
So we encourage you all to go out there and love your kānuka! Look closely at the flowers and leaves and see if you can tell what species you have growing (and what other species may be depending on it). Record trees on iNaturalist to help build a better understanding of dispersal and population sizes, as well as the growing impact of myrtle rust. Include locally sourced kānuka in your restoration plantings or pop one in your garden as a beautiful specimen tree. And hold off on clearing kānuka, or collecting it as firewood. We have many native species that were once thought of as so common they didn’t warrant a second glance, but are now rare or even extinct. Let's make sure our wonderful kānuka never suffer the same fate.