Gunnera

When asked to behold our native Gunnera adorning riverine and marshy areas, many overseas visitors must wonder quite what they are supposed to say. Compared with their extravagant cousins, our Gunnera species are distinctly subtle (and diminutive) characters.

Even within New Zealand, gardeners and landscape architects are far more accustomed to the giant leaves of two South American brutes (Gunnera tinctoria and G. x cryptica) that have traditionally been planted adjoining ponds and waterways in countless gardens and parks around the world.

Although they don’t command attention to the same extent, our native Gunnera species are fine garden plants with often striking qualities, such as the intense bronze foliage exhibited by Gunnera hamiltonii (pictured below), G. prorepens and certain forms of G. monoica.

Members of Gunnera have a highly unusual symbiotic relationship with a cyanobacterium (from the genus Nostoc) that lives within the stems of Gunnera species. In this particular mode of nitrogen fixation, the cyanobacterium converts atmospheric nitrogen (which is unavailable to plants) into a form that the plant can access (ammonia in this case).

This is different to the way in which legumes (members of the bean family, like kōwhai and our native brooms) and actinorhizal plants (including Coriaria spp. and matagouri) fix nitrogen, and Gunnera is practically the only flowering plant known to form such a relationship with a cyanobacterium.

Water is an unsentimental editor of natural environments. Rushing floodwaters regularly strip the surface of many stream margins bare, whilst permanently damp banks create conditions that most flowering plants struggle with. As a result, habitats like these are often dominated by cryptogams – spore-producing organisms such as mosses, liverworts and algae.

As noted previously in this plant profile, Gunnera found a workaround for this a long time ago by becoming mates with a cryptogam (a cyanobacterium). The fact that this symbiotic relationship takes place in pockets within the plant’s stems (as opposed to the roots¹) is well suited to an environment where what little substrate is available to roots could be scoured by the next deluge.

This all helps with understanding Gunnera monoica‘s success at colonising ‘freshly bared surfaces’ in or adjacent to damp environments, as noted within Johnson & Brooke’s ‘Wetland Plants of New Zealand’². It is also interesting from a visual standpoint, due to the kind of morphological similarity that arises through convergent evolution – which in this case has resulted in Gunnera monoica exhibiting parallels with some of the cryptogams that inhabit the same places.

One can easily infer the kinds of conditions that G. monoica requires within cultivation, with some degree of summer moisture being the key to ensuring its tenure within plantings. This can be achieved through micro-topography (the creation of shallow depressions) for moisture retention, in addition to occasional supplementary watering in summer. In warmer, northern areas, a position in partial shade also assists with maintaining sufficient moisture throughout all seasons.

G. monoica is the most northerly of our native species, with records extending to the northern Hokianga. Within the Auckland region, it is comparatively rare, and several of the images here show the most significant remnant population of it – growing in the flood zone of a forest stream on the North Shore (where its leaves take on a particularly bronze hue).