Giant Rhubarb
Introduction
Introduction Overview
Hamilton's Gunnera, scientifically known as Gunnera hamiltonii , is a remarkable miniature perennial herb that creates one of New Zealand's most distinctive and charming groundcover displays, forming dense spreading mats of tiny, perfectly rounded leaves just 5-15mm across that create an intricate carpet effect in moist, shaded environments. This diminutive cousin of the giant South American gunners grows barely 2-5cm high but spreads extensively through creeping rhizomes, producing thousands of minute heart-shaped to circular leaves with deeply crenated edges that turn beautiful bronze-red tones in autumn and winter. Endemic to New Zealand and found throughout both main islands in damp, shaded areas including stream banks, forest margins, and mossy ground under native bush, this tiny plant produces inconspicuous flower spikes followed by bright red berries just 2-3mm long that add jewel-like color to the forest floor. The plant 's extraordinary ability to form dense, weed-suppressing mats in challenging shaded and moist conditions, combined with its year-round interest and ease of propagation, makes it an exceptional choice for native groundcover plantings, shaded garden areas, and naturalistic woodland gardens where its delicate beauty and ecological authenticity create an enchanting miniature landscape that showcases the remarkable diversity of New Zealand's native flora. ground cover
Plant Description
Botanical Features
Gunnera hamiltonii, commonly known as Giant Rhubarb or Hamilton's Gunnera, is a small, creeping, dioecious herbaceous plant endemic to the South Island and Stewart Island of New Zealand. It forms compact, tight, overlapping rosettes of stiff, succulent leaves that lie flattened to the ground. The leaves are typically 2-7 cm long, deltoid, rhomboid, or broadly ovate, with finely toothed, crenulate margins. Their color ranges from slate grey, dark grey-green, reddish grey, or brown/green, and they have stout, prominently winged petioles. It spreads via fleshy, succulent, creeping stolons (underground stems) that can be up to 4 meters long. The plant produces inconspicuous flowers, with male and female flowers on separate plants, followed by red, fleshy, pendulous drupes (berries) in autumn. It typically grows in damp, sandy sites, often in sand dune hollows near the sea, in the transition zone between unstable dunes and stabilized shrubland, thriving in consistently moist soil and preferring damp, shaded environments.
Quick Facts
Key Features
| Scientific Name | Gunnera |
|---|---|
| Height | 5-10 cm (2-4 in) |
| Spread | 30-60 cm (12-24 in) |
| Water Needs | High |
| Light | Full sun to part shade |
| Frost Tolerance | Moderate |
| Salt Tolerance | Low |
| Growth Rate | Moderate |
| Lifespan | Several years |
Climate Best Suited to
Gunnera hamiltonii exhibits remarkable adaptation to New Zealand's challenging southern coastal climate conditions, thriving in the cool, humid maritime environment characteristic of Stewart Island and Southland's exposed coastal areas where temperatures remain consistently moderate year-round. This critically endangered species demonstrates exceptional tolerance for the region's high rainfall patterns, with some areas receiving over 2,000mm annually, combined with frequent fog and mist that maintain the consistently high atmospheric humidity essential for its survival. The species shows optimal growth in temperature ranges between 8-15°C, reflecting its adaptation to the cool temperate oceanic climate of its natural range, though it can tolerate occasional temperature drops to near-freezing during winter months. Native to areas experiencing minimal temperature variation throughout the year, Gunnera hamiltonii benefits from the thermal buffering effect of the Southern Ocean, which moderates both summer heat and winter cold extremes that could stress this delicate coastal specialist. The species requires the specific combination of high humidity, moderate temperatures, and protection from desiccating winds that characterizes the sheltered dune slack environments where it naturally occurs, making cultivation challenging outside of these specialized climatic conditions. Its extremely limited natural distribution reflects not only habitat specificity but also precise climatic requirements that have restricted its successful establishment to only the most suitable microenvironments within New Zealand's southern maritime climate zone.
Regional Suitability
| City | Climate Suitability |
|---|---|
| Whangārei | Ideal |
| Auckland | Ideal |
| Hamilton | Ideal |
| Tauranga | Ideal |
| Rotorua | Ideal |
| Gisborne | Ideal |
| New Plymouth | Ideal |
| Napier | Ideal |
| Whanganui | Ideal |
| Palmerston North | Ideal |
| Wellington | Ideal |
| Nelson | Ideal |
| Christchurch | Ideal |
| Dunedin | Ideal |
| Invercargill | Ideal |
Natural Habitat
Natural Habitat Overview
Gunnera hamiltonii occupies one of New Zealand's most specialized and restricted natural habitats, occurring exclusively in the unique coastal dune slack ecosystems of Stewart Island, Southland's south coast, and historically on the West Coast, where it colonizes the permanently moist sand and organic matter deposits that accumulate in sheltered depressions between stabilized sand dunes. These highly specific microhabitats are characterized by consistent year-round moisture from both groundwater seepage and frequent coastal fog, creating the precise conditions of high humidity and stable moisture availability that this critically endangered species requires for survival. The natural habitat consists of fine to medium sand mixed with decomposing organic matter from surrounding dune vegetation, providing both the excellent drainage and consistent moisture retention that allows the species' shallow root system to access water without experiencing waterlogging stress. In these coastal dune systems, Gunnera hamiltonii typically grows in association with other specialized dune slack species including various sedges, native grasses, and mosses that form distinct plant communities adapted to the challenging conditions of salt spray, strong winds, and fluctuating moisture levels. The species shows particular preference for sites that receive protection from direct salt spray and desiccating winds while maintaining adequate light levels, often establishing in areas where slightly taller vegetation provides partial shelter without creating excessive shade. The extreme rarity of suitable habitat, combined with the species' highly specific environmental requirements and limited dispersal ability, has resulted in Gunnera hamiltonii being restricted to only five known natural populations, making it one of New Zealand's most habitat-specialized and geographically restricted native plants.
Plant Conservation
Gunnera hamiltonii is classified as "Threatened - Nationally Critical" under the New Zealand Threatened Classification System. This designation is accompanied by the qualifiers "Conservation Dependent" (CD), "Range Restricted" (RR), and "Recruitment Failure" (RF).
This creeping herbaceous plant is endemic to the South Island and Stewart Island of New Zealand, where it is found in damp sand within coastal dune slacks and swales. It is considered one of New Zealand's rarest plants, with only five suspected natural habitats remaining.
The primary threats to Gunnera hamiltonii stem from its extremely limited distribution and small population size. A significant challenge is the difficulty of natural fertilization, as male and female plants are often isolated, hindering sexual reproduction and seedling recruitment in the wild. Habitat modification has already led to the extinction of one mainland population, and others face threats from weed invasion and coastal erosion.
Soil
Constantly moist , rich in organic matter.
Light
Partial to full shade.
Water
High humidity , avoid drought.
Successful cultivation of Gunnera depends on understanding and meeting its specific growing requirements. These requirements have evolved over thousands of years to suit New Zealand's unique environmental conditions.
By replicating these natural conditions in your garden, you can create an environment where the plant will thrive and contribute to a healthy, sustainable ecosystem that supports local wildlife and biodiversity.
Planting Guide
When to Plant
The best time to plant Gunnera is during spring or autumn when soil temperatures are moderate and rainfall is reliable.
How to Plant
Dig a hole twice the width of the root ball and slightly deeper. Place Gunnera in the hole, backfill with soil, and water thoroughly. Mulch around the base to retain moisture.
Ecological Role
Wildlife Interactions
Giant Rhubarb provides essential ground cover in wetlands and damp grasslands, helping to stabilize soil and retain moisture. Its dense mats suppress weeds and create microhabitats for small invertebrates and amphibians. The plant âs red berries are a food source for native birds, aiding in seed dispersal. By thriving in waterlogged conditions, Giant Rhubarb supports wetland biodiversity and contributes to the ecological health of riparian zones and restoration projects.
Uses & Significance
Traditional Uses
- Used in traditional Mori medicine for various ailments.
Gunnera hamiltonii, also known as Giant Rhubarb, holds cultural significance for Mori, who traditionally used it for medicinal purposes.
Landscaping Uses
Landscaping Uses Overview
Giant Rhubarb is an excellent ground cover for damp, shaded areas, bog gardens, pond edges, and wetland restoration projects. Its dense , mat-forming habit helps suppress weeds and stabilize soil, making it ideal for erosion control on stream banks and in rain gardens. The small , rounded leaves and red berries add texture and seasonal interest to woodland gardens and shaded borders. Giant Rhubarb pairs well with other moisture-loving natives such as Mazus radicans , Carex secta , and Blechnum ferns , creating lush, low-maintenance plantings. It can also be used as a living mulch under larger shrubs and trees. Avoid planting in dry or exposed sites, as it requires consistent moisture and protection from hot sun. In the right conditions, Giant Rhubarb will form a resilient, attractive carpet that supports biodiversity and enhances the naturalistic look of native gardens.
Seasonal Care Calendar
Spring
- New growth begins; keep soil moist.
- Apply organic mulch to retain moisture.
Summer
- Monitor for pests such as slugs and snails.
- Water regularly during dry spells.
Autumn
- Berries ripen ; remove dead leaves.
- Divide clumps if needed.
Winter
- Dormant period ; minimal care required.
- Protect from severe frost if possible.
When to Prune and How Much
When to Prune and How Much Overview
Minimal pruning needed. Remove dead or damaged leaves as required.
Regular pruning of Gunnera helps maintain its health and desired shape. Remove any dead, diseased, or crossing branches to improve air circulation and reduce the risk of pest and disease problems.
For most native plants, light pruning throughout the growing season is better than heavy pruning at once. This approach encourages healthy new growth while maintaining the plant's natural form and character.
How to Grow Giant Rhubarb
Giant Rhubarb, also known as Hamilton's Gunnera, is a remarkable miniature perennial herb that forms dense, spreading mats of tiny, rounded leaves. It is a unique and charming groundcover for moist, shaded environments, creating an intricate carpet effect. While it is a relatively low-growing plant, its resilience and ability to spread make it an excellent choice for native groundcover plantings, shaded garden areas, and naturalistic woodland gardens. Understanding its propagation methods is key to successfully growing this delightful species.
From Seed
Propagating Giant Rhubarb from seed is a viable method, though germination can be slow and erratic. Collect the small, red berries when they are ripe, typically in autumn. Clean the seeds thoroughly to remove any fleshy pulp. Sow the seeds in a well-draining seed-raising mix, lightly covering them. Maintain consistent moisture in the seed tray and keep it in a cool, shaded location. Germination can take several weeks to months, so patience is key. Once seedlings have developed a few true leaves, they can be potted into individual containers and grown in a sheltered environment before planting out. Note that plants grown from seed may show some variation from the parent plant.
From Division
Division of established clumps is the easiest and most reliable method for propagating Giant Rhubarb. This is best done in spring when new growth is emerging. Carefully dig up sections of the creeping rhizomes, ensuring each piece has several growing points and intact roots. Replant the divisions immediately into well-prepared soil that is rich in organic matter and consistently moist. Water thoroughly after planting and keep consistently moist until established. This method is excellent for quickly expanding your stock of plants and for rejuvenating older, less vigorous clumps.
Pests and Diseases
Common Pests
Gunnera is generally resistant to most pests due to its native adaptations. However, it may occasionally be affected by common garden pests such as aphids or scale insects.
Disease Prevention
To prevent diseases, ensure good air circulation around Gunnera and avoid overwatering. Remove any diseased plant material promptly to prevent spread.
Cultural Significance
Gunnera hamiltonii, an extremely rare plant endemic to New Zealand, primarily found on the coasts of Southland and Stewart Island, does not appear to have significant cultural significance in terms of traditional uses or folklore, unlike some other Gunnera species.
Ecological and Conservation Significance:
- Rarity and Conservation: It is classified as "Threatened - Nationally Critical" in the New Zealand Threatened Classification System, making it one of the rarest plants in the country. This rarity gives it considerable scientific and conservation significance.
- Discovery and Naming: The plant was named after W.S. Hamilton from Southland, who first discovered it in 1884 or 1895. Its discovery and subsequent rediscovery by various individuals highlight its importance to botanists and naturalists in New Zealand.
- Ecological Importance: It plays a role in its specific coastal habitat of damp sand in dune slacks and swales. The genus Gunnera is also notable for its unique symbiotic relationship with a cyanobacterium that allows it to fix nitrogen, a rare trait among flowering plants.
- Cultivation for Survival: Due to its limited natural distribution and challenges with sexual reproduction in the wild (male and female plants are often isolated), efforts have been made to cultivate Gunnera hamiltonii from cuttings and seeds to ensure its survival. This cultivation in botanical gardens and for transplanting back into the wild underscores its value in biodiversity conservation.
While Gunnera hamiltonii itself doesn't have widely documented traditional cultural uses, other species within the Gunnera genus, such as Gunnera perpensa in southern Africa, are used in traditional medicine, for food, and even in making beer. However, these uses are not attributed to Gunnera hamiltonii.
Bonus Tip
A Symbiotic Superstar
Gunnera hamiltonii has a secret weapon that allows it to thrive in its nutrient-poor, sandy habitat. It has a symbiotic relationship with a nitrogen-fixing cyanobacterium called Nostoc. The cyanobacteria live in special glands within the plant's stems and convert atmospheric nitrogen into a form that the plant can use. This is a rare and remarkable adaptation for a flowering plant, and it is one of the keys to the survival of this critically endangered species.
