Carmichaelia

Some of New Zealand’s most extraordinary plants belong to this genus of shrubs and small trees. Some species exhibit beautiful flowering displays (amongst the finest in our flora), whilst another attraction of the native brooms is the range of highly unusual (normally leafless) growth forms that they assume. Carmichaelia spp. are members of the pea family, to which the well-known native species, kowhai (Sophora spp.) and kakabeak (Clianthus spp.) also belong. Members of this family are also referred to as legumes; a term that is commonly used by agriculturists and horticulturists to describe them.

Legumes are one of a limited range of plants with the capacity to ‘fix’ nitrogen into soils (from the air), through the actions of various species of soil bacteria, known as rhizobia, which live in nodules on the roots of legumes. This special ability has been utilised for a long time by agriculturists, for increasing soil fertility by periodically planting legumes on cultivated ground.

Within nature, it plays a role in helping such species to occupy impoverished or open ground, as legumes gain access to the nitrogen required for plant growth from their symbiotic relationship with the rhizobial bacteria. It is therefore unsurprising that many of our native brooms (Carmichaelia spp.) specialise in growing within open or disturbed habitats.

The many species of Carmichaelia are a good example of adaptive radiation; a term that refers to the development of a wide range of species from common ancestors, as they adapt to different environments. There is a great proliferation of Carmichaelia spp. in the eastern South Island, due to the isolation created by the dramatic geology, and the prevalence of dry, open habitats. Many of these eastern South Island species have very limited natural ranges, and a considerable number of them are threatened within the wild.

In addition to their often specialised habitat preferences, the rarity of many species is also due to the fragility of the kinds of habitats that they occupy. Disturbed or open ground is often highly vulnerable to invasion by weeds or pests, erosion, development (whether for agriculture or other commerce) and fire.

The most sought-after of the native brooms are the spectacular-flowering species from Marlborough and eastern Canterbury, including C. muritai, C. stevensonii, C. glabrescens, C. torulosa and C. carmichaeliae. These range in flower colour from pale lavender through to intense pink. All of them, with the exception of Carmichaelia glabrescens, are serioiusly threatened in the wild.

Northern areas are better suited to the cultivation of two species that naturally occur in the north, C. australis and C. williamsii (pictured, above right). The latter is a rare coastal species, with comparatively large, cream-yellow flowers, and a marked weeping habit. The former is a polymorphic species (one that occurs in many forms), which is well worth a place in the garden for its often graceful weeping form, and dainty flowers. One form of Carmichaelia australis that shows particularly good flowering and form was sold as Carmichaelia cunninghamii (notably by Oratia Native Plant Nursery), by which name it was officially classified previously.

This highly unusual shrub bears very apt common names, as it can appear like a pile of sticks (albeit an elegant pile of sticks) from a distance, but has the look of coral when viewed close up. Of the many New Zealand broom species, none may be easily confused with coral broom. Its extraordinary appearance is related to the harshness of the eastern South Island habitats in which it is naturally found. Reduced leaves (or indeed a complete absence of leaves) and brownish pigmentation are favourable adaptations for plants that are exposed to high winds, drought and extreme insolation.

Both the main subspecies and the closely-related C. crassicaulis subsp. racemosum (slender coral broom) are threatened in the wild (the latter more acutely), mainly from browsing by mammals. Slender coral broom, which is found in different localities than where C. crassicaulis subsp. crassicaulis occurs, differs primarily in its more spreading habit and delicate branches.

Coral broom will grow up to 2m high, but is often seen in a relatively stunted state in nature, due to the attentions of livestock or other animals. It is not a species for humid areas of New Zealand, and should be provided with free-draining conditions when planted within gardens.

Although eccentric in appearance, this is one shrub that should be considered more by gardeners and landscapers in the eastern South Island for its sculptural qualities. Its branching structure is reminiscent of the dynamic abstract paintings of Russian Suprematists and Constructivists, such as Aleksandr Rodchenko and Kazimir Malevich.

Although interesting for its compact form and attractive flowers, this critically endangered broom is also fascinating for the spectacular landforms on which it grows. Carmichaelia hollowayi is naturally confined to limestone outcrops on the south side of the Waitaki River, such as the impressive escarpment on which photos in this plant profile were taken.

The main distinguishing features of C. hollowayi are its suckering habit and thick branches (which have rounded tops). Its flowers appear from January until early autumn in the wild, although we have had it in full flower in December, within Auckland (as might be expected in a warmer climate). Thus far, Carmichaelia hollowayi appears to be fairly amenable to cultivation, despite the very specific conditions of its natural habitat.

As with other native brooms, C. hollowayi should be given a well-drained, open spot within the garden. A good amount of air movement is also beneficial, as many brooms resent excessive humidity. In the case of C. hollowayi, it should be noted that the substrates upon which it naturally grows are relatively fertile – therefore, the regular addition of fertiliser is a good idea within the garden. Originating from the dry eastern flank of the South Island, it is better suited to cultivation in cold, dry southern parts of New Zealand (although it has taken surprisingly well to cultivation at Oratia Native Plant Nursery in Auckland).

The limestone bluffs on which I have viewed this species are also home to another nationally endangered species, Lepidium sisymbrioides – a bronze-leaved herb that occurs in a range of dry, fertile habitats within Canterbury and Otago. The unusual colour and feathery appearance of L. sisymbrioides’ foliage render this a very attractive species, whose appeal is enhanced when it throws up its comparatively large heads of white flowers.

C. hollowayi‘s species name commemorates Rev. J. Holloway, who lectured in botany at Otago University, and carried out a significant amount of work on Lycopodium and other fern-allies. Holloway, who was a contemporary of Leonard Cockayne, botanised extensively in the Waitaki Valley – where he discovered the shrub that would bear his name.

Carmichaelia hollowayi suffers from a similar problem to other South Island brooms – a lack of young plants coming through to eventually replace existing populations. This is due to a number of factors, the most significant of which is likely to be the effect of browsing animals, such as rabbits. With just c. 250 mature plants left in the wild, the potential exists for horticulture to play a positive role in safeguarding the future of this species.

Kirk’s broom is a weirdo amongst weirdos; being the only climbing member of this idiosyncratic genus of trees and shrubs. It is most frequently found as a member of the grey scrub communities that are a feature of landscapes of the eastern South Island. It was in one of these habitats, on a river terrace in the vicinity of Lake Benmore, that I had the chance to view this rare species within the wild.

Although unusual in appearance, this is an attractive species that is worthy of a place in gardens. The brownish stems form a dynamic, interlacing network, whilst the abundantly-produced pinkish-white flowers make a beautiful display in early summer. As is so often the case with plants of a subtle nature, C. kirkii benefits from being planted in association with species with which it contrasts. In the Tekapo garden where we have planted a considerable number of this species, this is established by its combination with Hebe subalpina – a bright green counterpoint to C. kirkii‘s whippy stems.

Considering that this plant is seldom cultivated (as well as the fact that it is a threatened species¹) I had mistakenly assumed it to be a challenging plant to grow within the garden. On the contrary, it has proven to be a particularly robust character within the Mackenzie Basin’s challenging conditions – both in the open and under the shade of shrubs. In the space of three years, it attained a height of 1.2m, winding its way through the structure of many shrubs in the montane garden that we designed at Lake Tekapo. Even more surprisingly, it did not suffer noticeably from browsing by rabbits – one of its most pressing problems in the wild, due to its palatability.

Within the garden, C. kirkii may be planted to scramble its way through supporting shrubs (as it does in nature), form densely interlacing bushes, or even trail over a wall (against which its dynamic growth form would stand out).

Carmichaelia kirkii is unusual amongst New Zealand’s predominantly leafless brooms, for bearing a great number of leaves during spring and summer, when growing in a moist situation. It was named by Sir J.D. Hooker, in honour of the great early botanist, Thomas Kirk (who was one of the earliest people to collect the plant). However, the species was first discovered (prior to 1870) by J.B. Armstrong, on the site where Christchurch City now stands. Its natural range extends from Marlborough to Otago.

“Right, here.” ………

“I’m pretty sure it’s left up here … Yep, definitely, left” ………

“Nooow, was it left or right at this fork … I think it was right”.

And so it unfolded, that the higher we ascended into the Seaward Kaikouras, the less Winston trusted his memories from a hunting trip several years prior. As it turned out, there was nothing wrong with his memory, although the relief was writ large upon his face when we arrived at what must be one of the largest remaining populations of the nationally endangered Carmichaelia stevensonii.

In Winston’s defence, the walk did involve many forks in the stream, as well as climbing around and up several rushing cascades in the DoC hunting block at George Stream where we were looking for this spectacular weeping species of tree broom. Furthermore, Winston’s hunting trip took place prior to the 2013 earthquake that shook this region so violently, adding another layer of uncertainty (and caution) to our scramble up George Stream.

Weeping tree broom needs little introduction to many gardeners and landscape architects, as it has become relatively commonly available within nurseries – on account of both its remarkable growth form and attractive lilac flowers. Seeing Carmichaelia stevensonii in the wild, on the other hand, is a privilege that far fewer people get to experience, due to the fact that most significant populations are in locations as inaccessible as this one.

Although I had grand visions of seeing this species amidst clear skies, I was not disappointed with the misty conditions into which we ascended. Similar to experiencing a rainforest within conditions to which its inhabitants have adapted, observing Carmichaelia stevensonii shrouded in the kind of mist that regularly settles upon the foothills of the Seaward Kaikouras afforded us some insight into possible reasons for its distinctive weeping form – in which its weeping stems offer obvious benefits with respect to shedding water.

In this part of the country, sea fog frequently adds to the sum total of atmospheric moisture (which is already elevated in upland areas in terms of rainfall); especially on mountain ranges, which capture cloud and fog as they roll over the land. The substrate in which Carmichaelia stevensonii grows (‘ground’ would be a generous description in many locations) frequently consists of deep layers of rocky colluvium (as pictured below) – material that has accumulated at the base of slopes due to any one of several environmental factors (including rockfalls associated with seismic activity).

This helps to explain one aspect that I have noted regarding its cultivation, wherein trees often become unstable in more conventional soil profiles. It is reasonable to assume that a species that customarily grows through deep, porous, rocky substrates (such as colluvium or riverbeds) is more at home sending its roots down a long way into the ground, thereby anchoring it. A firm stance would certainly be of assistance in a place where ferocious winds are funnelled through montane gorges and valleys.

As noted previously, C. stevensonii has become popular in cultivation over the last few decades – especially in southern regions, to which it is better suited. Its lilac flowers appear in mid-summer (centred on the start of December), and whilst it can produce a great profusion of blooms, the degree of flowering varies considerably from year to year.

Within plantings, it prefers a well-drained position (with some additional moisture providing during periods of drought), and the main challenge that I have encountered with growing this species is keeping it upright – a situation that is only exacerbated by overzealous staking. To avoid this pitfall, it is advisable to establish this species from younger plants (which almost invariably develop better root systems), and it would be interesting to undertake trials with layers of sharp rock around the rootball (to further encourage a more solid root system).

Carmichaelia stevensonii was recognised as distinct (in 1904) by a farmer in the Clarence River area, George Stevenson, whilst looking over the large sheep run on which he had recently taken up a lease². Despite not being a botanist, Stevenson evidently had a keen eye for natural history (like many farmers), noting differences not just relating to C. stevensonii‘s overall growth form, but also its seed morphology.

Several years later (in 1911), it was officially named as the sole member of a new genus, Chordospartium³, by T. F. Cheeseman, an important early botanical figure within New Zealand and then curator of the Auckland Museum (who had received material from Stevenson). At the time, Cheeseman expressed the opinion that a case might be made in future for sinking Chordospartium back into Carmichaelia, a change that transpired nearly 80 years later.