Hamilton’s Gunnera

Botanical name: Gunnera hamiltonii
Synonyms: None
Common name(s): Hamilton’s gunnera

About:
An exceptionally rare, creeping coastal herb and one of New Zealand’s most threatened native plants. Gunnera hamiltonii forms remarkably compact mats of tightly overlapping, grey-green to reddish-grey leaves connected by long fleshy stolons hidden just beneath the sand. Unlike the huge ornamental species commonly associated with the genus Gunnera, this species is tiny and ground-hugging. Male and female flowers occur on separate plants, and the isolation of the sexes within most surviving populations severely limits natural reproduction. It is highly distinctive among New Zealand Gunnera for its angular, finely toothed leaves and stout, conspicuously winged leaf stalks.

Natural habitat:
A highly specialised coastal species restricted to damp, consolidated sand in dune slacks and swales. It occupies sparsely vegetated hollows within coastal dune systems where the substrate remains consistently moist. These environments combine exposed coastal conditions with reliable ground moisture and relatively little competition from taller vegetation.

Growing environment:
Frost, Coastal, Wind, Damp & Sand tolerant. Prefers cool, consistently moist conditions and damp sandy or similarly free-draining substrates. Although naturally occurring in open coastal habitats, cultivated plants have been reported to perform particularly well in damp, lightly shaded sites free from competing weeds.

Endemic distribution:
Endemic to southern New Zealand. Restricted to coastal dune systems near Invercargill in Southland and along the western side of Stewart Island/Rakiura. Only a handful of natural populations are known.

Height:
Approximately 5–10 cm tall, with the foliage generally held very close to the ground.

Width:
Forms spreading mats potentially extending several metres, with individual underground stolons capable of reaching approximately 4 m long.

Flowering:
August – December
Produces very small, inconspicuous flowers on short, fleshy upright spikes arising amongst the leaves. Plants are dioecious, with male and female flowers occurring on separate individuals. Male spikes may carry 10–40 or more flowers with conspicuous yellow anthers.

Fruiting:
March – July
Female plants produce upright fruiting spikes approximately 10–35 mm long bearing widely spaced, fleshy, pendulous drupes. Mature fruits are small and may develop yellowish, reddish or red-brown colouring.

Uses:
• Conservation collections
• Threatened-plant collections
• Specialist native ground cover
• Coastal dune garden displays
• Botanical and educational collections
• Ecological restoration within its natural range
• Preservation of threatened native plant diversity

Appearance:
Growth habit:
A creeping, stoloniferous perennial forming extremely compact, low mats. Thick, succulent stolons run just beneath the substrate and may reach approximately 4 m long. Short internodes result in dense groups of tightly packed rosettes.

Leaves:
Leaves are arranged in compact, overlapping rosettes. The fleshy, leathery blades are approximately 20–30 × 10–25 mm and range from triangular and rhomboid to broadly ovate. Colour varies from slate-grey and dark grey-green to reddish-grey. The margins are finely crenulate with small, blunt teeth. Petioles are stout, 20–50 mm long and prominently winged, broadening toward their clasping bases. This combination of angular greyish leaves and strongly winged petioles readily distinguishes the species from other New Zealand gunneras.

Flowers:
Flowers are extremely small and carried on stout, fleshy spikes amongst the foliage. Male flowers are largely reduced but possess one or two stamens with yellow anthers. Female flowers are even more reduced, lacking or having only rudimentary petals and sepals, but possessing two conspicuous slender styles.

Fruit:
Small fleshy drupes are produced along an exposed upright fruiting spike. They are approximately 3 mm long, club-shaped and pendulous. The enclosed endocarp is broadly obovate and dull yellow to pale reddish-brown.

Ecology:
Gunnera hamiltonii is a highly specialised component of southern coastal dune-slack vegetation. Its long underground stolons allow established clones to spread horizontally through damp sand while keeping most vegetative growth close to the substrate.

Like other members of Gunnera, it has the remarkable ability to form a symbiotic relationship with nitrogen-fixing cyanobacteria of the genus Nostoc. This unusual association provides biologically available nitrogen and is especially advantageous in nutrient-poor environments such as coastal sand.

The species’ dioecious reproductive system has become a major conservation issue. Male and female plants are separated in most surviving populations, preventing or severely restricting natural pollination, fruit production and seedling recruitment. At Doughboy Bay on Stewart Island/Rakiura both sexes occur in the same general area, although they are not necessarily intermixed.

Fleshy fruits are adapted for dispersal through consumption by animals, with ants also considered a possible dispersal agent.

Associated habitats:
Coastal dune slacks, dune swales, damp consolidated sand, sparsely vegetated coastal turf and sheltered depressions within established dune systems. Associated vegetation is generally low-growing and adapted to persistent moisture, exposure and nutrient-poor sandy substrates.

How to grow:
Despite its extreme rarity in nature, Gunnera hamiltonii can be relatively straightforward to maintain under appropriate cultivated conditions. It requires consistently damp conditions and should never be allowed to dry severely.

A moist, free-draining substrate that approximates its natural sandy habitat is suitable. Cultivated plants have performed particularly well in damp, lightly shaded situations where competition from weeds is carefully controlled.

The species is remarkably cold hardy. Vegetative pieces have reportedly survived being accidentally frozen for several days and resumed growth after thawing.

Because Gunnera hamiltonii is a highly threatened species, cultivation should use plants obtained from legitimate conservation or cultivated sources. Wild populations should never be disturbed or collected.

Propagation:
Readily propagated vegetatively from rooted stolon sections and by division of established cultivated plants. Sections containing healthy roots and growing points can be separated and established in consistently damp substrate.

Fresh seed produced through controlled cultivation has also germinated readily. Because plants are dioecious, both male and female individuals are required for sexual reproduction. Maintaining genetically distinct male and female plants is particularly important in conservation cultivation.

Cultural use:
There are no well-documented traditional Māori uses specifically associated with Gunnera hamiltonii. The species is named for Southland naturalist William Stewart Hamilton, who was associated with its original discovery. Today its significance is primarily botanical, ecological and conservation-related.

Garden value:
A fascinating but highly specialised native ground cover, valued primarily as a conservation and botanical collection plant rather than as a conventional landscaping species. Its dense mosaics of slate-grey, grey-green and reddish foliage are highly distinctive, and its ability to spread through underground stolons can produce attractive tightly packed mats.

Its greatest value lies in conserving one of New Zealand’s rarest and most geographically restricted plants. Cultivated populations are especially important because natural populations are extremely few, some are vulnerable to coastal erosion, and separation of male and female plants has severely compromised natural recruitment.