Kanuka and Manuka by Chris Ward

Environmental News Issue 22 Winter 2010

Chris Ward examines the ecological characteristics and many values of these common but under-appreciated New Zealand plants.

Kanuka and manuka are the stuff of controversy. Are they farmland weeds, or valuable resources? Is that hillside covered with scrub, or forest? Is it a precious example of native biodiversity and natural character, or simply a wasteland? For that matter, some layfolk will argue there’s no such thing as kanuka, only manuka.

Though frequently look-alikes, and often confused with each other, there are major differences between kanuka and manuka. (Details in box on page 4). Both trees can adapt their forms according to the growing conditions. Kanuka and manuka may look much the same, but often the differences are obvious, even from a distance. 

Until about 30 years ago, kanuka and manuka were both identified as closely related species in the genus Leptospermum. Then, fundamental differences, especially in the flowers and seed capsules, led Australian botanists to reclassify them in different genera. Kunzea ericoides is the ‘new’ scientific name for kanuka while Leptospermum scoparium remains the name for manuka. (Recent PhD research by Peter de Lange has recognised several new species of Kunzea previously included with kanuka – watch this space!)

We might feel aggrieved at the Aussies fiddling with the names of New Zealand species in this way, but kanuka and manuka are part of a large complex of similar species shared with Australia.

Could this be the origin of the widespread misunderstanding that kanuka and manuka aren’t New Zealand natives? How often have I heard something like ‘Oh, we’re only cutting scrub, we wouldn’t touch the native!’ – implying the ‘scrub’ (kanuka-manuka) isn’t native. Far from it; both were present well back in New Zealand’s geological past.

That word ‘scrub’ – it can be used with the emphasis of a four-letter word to suggest the ‘weed’ status of a vegetation, its illegitimacy and lack of value, indeed its negative value. For that reason, many conservation-focused people avoid using the word. But ‘scrub’ is also a straightforward technical term for closed-canopy woody vegetation dominated by stems less than 10 centimetres diameter, a purely descriptive term without any judgmental implications.

‘Shrubland’ is an alternative word sometimes used instead of ‘scrub’ by people concerned with its negative connotations. But technically, shrubland is another form of vegetation – essentially scrub with an open canopy, where shrub cover is less than 80%. On this basis, there is plenty of both scrub and shrubland dominated by kanuka and manuka in many parts of New Zealand and notably Great Barrier.

Much of the vegetation that is commonly referred to as scrub is, however, technically forest. Scrub becomes forest when the dominant stems forming the canopy are more than 10cms diameter at breast height. For kanuka this normally occurs when the stand is about 30 years old, at which time it is typically 8-12 metres tall. These are small trees, agreed, but kanuka will keep on growing to a large size if not felled or burnt.

Many of us will know a corner with some large ‘old man’ kanuka. My favourite is on the high marine terrace surface of Whetumatarau, a dramatic plateau immediately behind Te Araroa on the East Cape. Large kanuka trees are a component of a mixed forest there. I measured a single stemmed specimen to be 94 centimetres diameter, and estimated its height at 25 metres – a great forest tree laden with perching lilies, orchids and other epiphytes. By comparison with nearby kanuka, known to post-date 1857, I estimate these larger kanuka trees to be 300-400 years old.

So while some people see all kanuka vegetation as scrub, I cannot accept this. Kanuka and manuka are ‘seral’ or successional species, which dominate key parts in the series or succession of vegetation types which follow the colonizing of a new site. Here size really does matter. Kanuka grows en masse to form dense scrub; then, as the dominant stems grow and the others are suppressed and die, it matures to form a kanuka forest. This will generally diversify to a mixed forest and ultimately be replaced in a natural succession – if we wait long enough.

Hill country farmers are very familiar with the most basic ecological feature of kanuka and manuka – their ability to colonize the smallest of bare patches in sparse pasture. Those tiny airborne seeds get around, and the essentially unpalatable seedlings do well in full-light conditions. They may also colonize extensive bare sites after fires or on slips.

Kanuka grows well on soils of middling-to-good natural fertility and drainage. Manuka by contrast favours wetter soils and low-fertility leached soils. It is not so much an active preference for poor conditions; rather, that with competition between the two, kanuka fails in such circumstances. In contrast, manuka often establishes with kanuka on the average or better sites but is suppressed by the faster-growing kanuka and dies out within 10-20 years, after being overtopped.

The net result is that kanuka dominates in some areas, such as most of the Gisborne District. Manuka persistently dominates on wetland margins, and on some particularly hard, ‘bony’ or burnt sites. It also flourishes in areas with consistently high annual rainfall, and at higher altitudes.

The different lifespans of manuka and kanuka is the basis of another important distinction between them. Manuka is comparatively short-lived – generally to about 60 years. As a stand approaches this age, there is a progressive breakdown of the canopy as individual manuka die or fall. This allows seedlings or saplings of other species to come through. Now there is an early succession to forests dominated by broadleaved species such as rewarewa or kamahi. In some cases, where browsing by stock or wild animals is excessive, this natural succession may fail – then the manuka may be replaced by mingimingi and bracken, in patches, or a second generation of manuka establishes itself.

Kanuka by contrast is long-lived. Stands dating from the abandonment of land during the economic depression of the 1930s, or before, are widespread. Whether the plants beneath them are heavily browsed by animals or not is to some extent immaterial as far as survival of kanuka forest is concerned. The kanuka will still be there at the end of another century. Removing browsing animals from the understorey would, however, allow a diverse forest to establish and eventually succeed the kanuka.

Manuka and kanuka have other values, too. While some iwi leaders have declared that manuka (including kanuka) have no worth, others consider that its former use for prized tools and weapons represents a cultural value of high importance.

On the utilitarian front, perhaps the best known value, for kanuka especially, is as a source of firewood. Alas, in the absence of sustainable management systems, this use tends to be an opportunistic ‘mining’ of the resource.

The quality of kanuka timber also suits it to machining for tool handles, with far higher value-added potential. While there have been encouraging thoughts of harvesting kanuka for such purposes, using sustainable management practices on quite modest areas, no one has yet got that off the ground on a commercial basis.

There has been a tendency in the past to regard kanuka and manuka as significant only as a ‘nursery crop’ allowing a ‘real’ forest to develop, but there is far more that makes kanuka-manuka vegetation valuable. High density kanuka-manuka scrub/forest is very effective at holding the land in severe rainstorms – in maintaining slope stability on the steep hill slopes so prone to soil-slips when in pasture. Landcare Research has shown that the combination of canopy interception of the rain, and the strong interlocking roots, means kanuka stands 16-20 years old or more are as effective at erosion control as close-planted pines of eight or more years old. With kanuka there is the added opportunity for that stability to be maintained for centuries. Kanuka-manuka scrub/forest won’t stop an existing gully eroding out, but it will stop a gully initiating on a slope that in pasture would be vulnerable.

Both kanuka and manuka yield honey in large volumes. This is generally sold as manuka honey with a significant price premium over clover honey. Spectacularly effective antibiotic activity has been tested in some manuka honey (but not from kanuka).

The oil extracted by steam distillation from manuka leaves from the East Coast also has striking anti-bacterial and fungicidal properties. Manuka oil is being used in a variety of medicinal and cosmetic products, commercially produced and marketed from a number of sites including Barrier Gold’s operation at Port FitzRoy.

A remarkable recent discovery is that manuka oil varies dramatically in chemical composition and properties from district to district. This illustrates firstly that manuka goes a long way back into the geological past and has evolved locally, and secondly how much we have still to learn about New Zealand’s plants and biodiversity generally.

Kanuka and manuka-dominant vegetation also provide habitat for a remarkable variety of other plants and animals – they are major repositories of New Zealand’s indigenous biodiversity. Many native orchid species, for example, particularly favour older kanuka or manuka-dominant vegetation.

Measured by species numbers and ecological complexity the most important elements of biodiversity are the invertebrates – moths, beetles, millipedes, spiders, snails and the rest. Research work in the Gisborne district shows that the significance and diversity of the invertebrate fauna in 60-year-old kanuka forest is as great as that in primary forest. Extensive kanuka-manuka areas support large numbers of the forest birds including threatened species such as whitehead/popokatea and fernbird.

To some, kanuka-manuka areas are but signs of recent farming gone wrong; an essentially unnatural phenomenon of the past century. But there is a long history of forest modification, extending back to the first Maori occupation and long before, due to fires and other disturbances. So kanuka and manuka scrub forests are an important part of the natural character of most districts.

The character of kanuka and manuka vegetation ranges as a continuum, from nuisance weeds invading pasture at one end, to a treasure trove of indigenous biodiversity and natural character at the other. The challenge is deciding at what point in the continuum do the positive values become dominant.

There is no simple answer, and to a great extent it must depend on the specific context of an area. Landowners will commonly argue that their own views on the subject must be paramount. I would like to think it possible to stimulate increasing respect among landowners for kanuka and for manuka.

Adapted (with permission) from an article published in Forest and Bird by Christopher Ward.

Telling differences between kanuka and manuka

Kanuka and manuka are distinctly different species, though they can look very similar. Kanuka grows faster and bigger than manuka, but you can’t simply call it manuka for the small stuff and kanuka when it’s bigger! The following features help define the differences:

• Kanuka has narrow parallel-sided leaves several times longer than wide and notably soft to the touch

• Manuka leaves are more ovoid but sharp-pointed (‘lanceolate’) with the prickly apex giving the foliage a harsh feel.

• Kanuka foliage is generally a rather bright olive-green. Specific colour features of kanuka and manuka vary with the seasons, and regionally.

• Manuka is duller, generally darker (not so obvious when very young).

In overview manuka often has a grey-brown look, from a combination of the leaf colour and the branches/stems which typically have a covering of sooty mould (which thrives on the sugary excretion of an introduced scale insect). This mould is much more prevalent on manuka than kanuka.

• Kanuka bark is a light tawny brown. Narrow vertical strips of bark are characteristic of kanuka.

• Manuka bark is darker with a reddish tinge. It comes off in very thin flakes, wider and less regular than kanuka bark.

• Kanuka flowers are notably smaller, 4-5mm across, creamy white.

• Manuka flowers are 10-12 millimetres across and generally pure white.

• Kanuka flowers are carried in dense elongated clusters (or ‘cymes’) towards the end of the branchlets

• Manuka flowers are more evenly scattered over the plants as single flowers.

• Kanuka generally flowers once a year only, in midsummer.

• Manuka flowers strongly a little earlier than kanuka, and additionally in irregular bursts at other times.

• The kanuka seed capsule is less woody, only 2-3 millimetres across and generally disappears after a month or two. Generally kanuka does not carry seed capsules, except briefly in late summer.

• Manuka has a hard woody seed capsule 5-6 millimetres across which persists on the plant for a year or more after flowering. At any time of the year you will see seed capsules of various ages.

• Kanuka generally has faster growth rates and reaches a larger size so it is commonly seen as trees, 10-15 m tall and more, and 15-40 cm diameter.

• Manuka generally stops at about 6-8 m height and 7-10 cm diameter, or less on the poorest soils (e.g. 1–2 m on the Te Ahumata plateau).

The growth forms of kanuka and manuka are slightly different – the somewhat droopy branchlets of kanuka often contrast with more erect manuka – but there is much variation caused by the character of the site, the density of the stand, and tree age.