Hedychium gardnerianum
(kahili ginger)

Native to the Eastern Himalayas, H. gardnerianum has been widely introduced as an ornamental in different regions of the world. It continues to be available as an ornamental and is therefore likely to spread further.H. gardnerianum is an ecologically versatile plant with rapid vegetative growth, the dense rhizome system of the plant prevents regeneration of other species. This very aggressive, shade-tolerant plant is able to form dense thickets on undisturbed sites in the understorey of open and closed-canopy native rain forests and managed forests, as well as in open areas (forest margins, ravines or path sides). Its high seed production and efficient dispersal by rats and birds, allows it to persist in invaded areas for a long time and establish new points of invasion. As H. gardnerianum is able to invade native forests, ecological, conservation and cultural values are threatened and it is a major threat to native biodiversity.

H. gardnerianum is native to the Eastern Himalayas: India, Nepal, Sikkim, Khasia Hills (Hooker, 1897; Mitra, 1958), now widely introduced and naturalized in various tropical countries and probably present in areas neighbouring its native range. One of New Zealand's worst weeds of rainforests, a major weed of the Azores Islands of the mid-north Atlantic Ocean and also of Hawaii. It has been observed as widespread in Ecuador (Tye A, Charles Darwin Research Station, Galapagos, Ecuador, personal communication, 2003) though there is no specific literature record. First reported from Kruger National Park in 1999 (Foxcroft et al., 2008), it is a declared weed, naturalized and cultivated in South Africa.

Widely planted in Australia for its showy flowers, it is a weed of bushland, roadsides and riverbanks. In its very early stages of spread in Queensland, it has been highlighted as worthy of pre-emptive eradication (Blood, 2001).

It is probably far more widespread in tropical regions than indicated in the distribution table.

Further spread is highly probable, owing to the risks of both accidental movement as a seed contaminant of crop seed and other agricultural produce, and deliberate introduction as an ornamental. This is encouraged by availability from commercial nurseries by mail-order catalogues and websites, for example, in France, La Réunion and the Netherlands. H. gardnerianum is one of the top 100 worst invasive species of the world (PIER, 2000). There are no regulations in La Réunion, but H. gardnerianum is prohibited as a noxious weed in New Zealand and it is a proscribed species to be intercepted at entry points to New Zealand and Hawaii.

In South Africa, it is a declared weed of forests, plantations, riverbanks and moist shaded sites (Category 1 plants): “These are prohibited plants that will no longer be tolerated, neither in rural nor urban areas, except with the written permission of the executive officer or in an approved biocontrol reserve. These plants may no longer be planted or propagated, and all trade in their seeds, cuttings or other propagative material is prohibited. They may not be transported or be allowed to disperse”.

H. gardnerianum is native to the foothills of the Eastern Himalayas, in cool mountains areas at an altitude of 1200-2500 m, with a sub-tropical climate, cool winters and mild summers. It is present in forests and woodlands as an understorey species. It has been introduced to gardens as an ornamental, and escaped from these foci, especially along roadsides and stream edges, and is also a weed of natural forests, ravine sides, rail and roadsides, also wetlands and wastelands.
 

Genetics

A spontaneous naturalizing hybrid (H. coronarium x H. gardnerianum= H. sadlerianum) is thought to have been found in Medeiros, Haleakala National Park Maui, Hawaii which favours the simultaneous seed production of H. gardnerianum (P Bily, The Nature Conservancy, USA, personal communication, 2008). Other interspecific hybridization may be possible and a natural hybrid of “Tara” (form of H. coccineum) and H. gardneriaum is postulated as the two species coexist in Nepal (PIER, 2007).

Physiology and Phenology

The flowering period occurs from December to March in La Réunion and after fructification in April to August, the above-ground parts of the plant senesce and die. A new stem will grow from an axillary bud present in the rhizome. In its native range, H. gardnerianum flowers and fruits from July to December.

Reproductive Biology

H. gardnerianum spreads primarily by vegetative regeneration of rhizomes: it can reproduce from any underground parts of the plant, even from the smallest root fragments and disperse as parts detach from the main plant. H. gardnerianum produces conspicuous and fleshy red seeds which are carried long distances by frugivorous birds and rats, creating many new infestation sites, making H. gardnerianum a far greater threat to native plant ecosystems and exotic forests. In the Azores, a study of seed production and vegetative growth on São Miguel revealed that the number of seeds per spike ranged from 300 to 500 and concurred with the findings of Byrne (1992), which ranged from 20 to 600 depending on light conditions. H. gardnerianum seeds do not have dormancy (Cordeiro, 2001) and only remain viable in the soil for a short period, relying on high annual seed production as a mechanism for efficient dispersal. Each new corm usually originates new leaves and eventually a spike although many act as storage organs and lack leaves. Generally, the pseudostem, leaves and spike collapse in winter leaving a scar at the top of the corm. Vegetative growth is characterized by the development of new corms at the rhizome ends, creating a dense cover and crowding the soil, which is how it impairs regeneration of native species.

The scent of the flowers and the conspicuous petals and petaloid staminodes indicate insect pollination, while the long exerted stamen and style suggest that hovering insects in particular may be involved. Wing scales of Lepidoptera have been found adhering to stigmas of some Hedychium spp. in their native habitats and it is likely that long tongued butterflies in search of nectar produced by the prominent gland at the base of the style are likely to brush the stigma followed by the anther-cells with their wings and carry out cross pollination.

Environmental Requirements

H. gardnerianum prefers moist, wet and well-drained soil, but it is ecologically very versatile, thriving in bright light or dense shade, good or poor drainage, high or low fertility, thick humus or very little soil, even sprouting in tree forks above the ground and on tree trunks, like epiphytes in the native rainforest in La Réunion. In La Réunion, H. gardnerianum is present in wetlands with annual rainfall of 2000-5000 mm, and with an annual average temperature range between 11°C and 17°C (Cadet, 1977). It generally occurs at altitudes of 800-2000 m in the native wet forests and disturbed areas. In Hawaii, populations are found on all islands between sea level and 1700 m (Smith, 1985).

 

Winks et al. (2007) reported the results of a survey for the fungi, bacteria and invertebrate fauna associated with H. gardnerianum carried out in New Zealand in order to identify and assess their potential as biological control agents. However, no specialist invertebrates or pathogens were found and most of the foliar damage caused was minimal and associated with generalist herbivores. In Hawaii, the soil-borne plant pathogenic bacterium, Ralstonia solanacearum, was isolated from a number of ginger species including H. gardnerianum. It caused a significant bacterial wilt disease and is spread naturally via soil water and root-root transmission, as well as artificially through wounds and agricultural practices. The bacterial survey in New Zealand did not detect any isolates of R. solanacearum, or any other plant pathogenic bacterium of wild ginger.

A number of fungi have been recorded from Hedychium spp. and a search of herbarium and fungal databases lists the following on H. gardnerianum: Microthyriella azorica, Gliomastix luzulae and Phomopsioides natalinae from the Azores, Stachybotrys subsimplex from Canada, Antennularia sp. from Jamaica, and a Mycosphaerella sp. from its native range in India (CABI, 2007; USDA-ARS, 2008). Periconia minutissima and Stachybotrys subsimplex from New Zealand; Pythium sp. and Rhizoctonia (root rot) from Hawaii. Note that mycobiota associated with the plant in its introduced range would either represent host shifts or would have been introduced on the plant as they were imported.

Environmental

• Amenity
• Landscape improvement

Fuels

• Fuelwood

General

• Botanical garden/zoo
• Capital accumulation
• Ritual uses

Materials

• Fibre
• Oils

Medicinal, pharmaceutical

• Source of medicine/pharmaceutical
• Traditional/folklore

Ornamental

• Cut flower
• Propagation material
• Seed trade

Four Hedychium species are very common and can be distinguished by the colour of their flowers. H. coccineum has red or reddish orange flowers, and has more slender upright leaves. H. coronarium has white flowers which are sweetly fragrant. H. flavescens leaves are slightly narrower than those of H. gardnerianum. The flower head, 10-15 cm long, is also much smaller. Its fragrant flowers are creamy-white to pale yellow with yellow stamens. Flowers of H. flavescens do not produce seeds but it spreads relatively quickly by vegetative regeneration of rhizomes. H. flavescens is a major invader of native forests in New Zealand (Auckland Regional Council, 1999) and is also invasive in La Réunion: it is present in wet areas such as ravine sides, roadsides, native forest margins and disturbed forests (Radjassegarane, 1999). In Hawaii, H. flavescens and H. coronarium are weedy although these are usually confined to forest edges instead of invading the understorey.

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Control

Mechanical control

Hand pulling can be effective on young seedlings. Small populations may be dug out and all underground parts must be removed. In La Réunion, mechanical cutting is frequently used to clear forest understorey, forests margins and ravines or roadsides. This method provides no long-term control but slows down its propagation and spread. Cutting before flowering (December to April, in La Réunion) is more effective in reducing seed production. H. gardnerianum inflorescences can be cut and dropped on the ground prior to the seeds being formed: this removal will not kill the plant but it will stop it seeding. The National Biological Invasions research programme INVABIO in La Réunion was launched in 2000 by the French Ministry of Environment, and as part of this, a 3-year research project began in 2002 with the aim of studying the impacts of mechanical control measures on native biodiversity and dynamics of native communities. The results clearly showed that the removal of an invasive alien species causes modifications to the biodiversity and a quick return to the initial situation. Contrary to the results expected, the negative impacts on the ecosystem were numerous. From an economic point of view, the intervention was both very expensive (the removal of 70 T / ha of H. gardnerianum was estimated at 24,062 euros) and constrained (specialised workers, steep slopes, etc.). By contrast, non-intervention caused few disturbances. In the case of the very low-invaded native ecosystems, the recommendation for non-intervention and the least disturbance as possible is made when the ecosystem is little invaded, or invasion is recent, or the invasive alien species occupies a small area, then early detection and rapid eradication are of interest (preventive action) (Lavergne, 2005).

Chemical control

A number of herbicides have been investigated and used in the past 20 years for control of H. gardnerianum (Harris et al., 1996). Currently, the most effective herbicide reported for H. gardnerianum control is metsulfuron, sprayed on the leaves, stems and root system. The effects are noticeable after 3 months and the weed will die and its rhizomes will rot after 12 to 15 months (Auckland Regional Council, 1999). Glyphosate and amitrole are also used in New Zealand (Timmins and Mackenzie, 1995). Probably the most effective and cost-efficient control method is stump treatment with herbicide. The stems are cut close to the ground, just above the pink-coloured swelling at the stem base, and concentrated herbicide is applied to the freshly cut stump; glyphosate is the common herbicide for this in La Réunion. Due to the widespread distribution of H. gardnerianum, chemical control is more cost effective than manual or mechanical control. In Hawaii, unpublished research found that metsulfuron at 0.04lb/acre, imazapyr at 0.7lb/acre, and amitrol at 0.7lb/acre applied to visible rhizomes after mechanical clearing of top growth was very effective. A commercial mixture of 2,4-D + triclopyr was ineffective but 20% triclopyr ester product applied in straight stream to the stems and surface rhizomes provided 100% kill on Maui (Motooka et al., 2003).

Biological control

Chemical and mechanical control of wild ginger are labour intensive, expensive, time consuming and often ineffective, and environmental concerns such as soil leaching, ground water contamination often limit the usefulness of herbicides in some areas. Biological control is now considered the only practical approach for the long-term management of large H. gardnerianum infestations in native forests (Harris et al., 1996). 

Ralstonia solanacearum, a naturally occurring bacterium in Hawaii, was tested for its efficacy as a bioherbicide in Hawaiian forests. The ability of this bacterium to cause severe disease in H. gardnerianum, together with its apparent lack of virulence in other ginger species seemed to indicate high potential as a biological control agent (Anderson and Gardner, 1999). Plants were inoculated with an aqueous suspension of the H. gardnerianum-infecting strain of the bacterium by stem injection or root wounding and all inoculated plants developed irreversible chlorosis and severe wilting 3-4 weeks following inoculation. Systemic infection also caused death and decay of rhizomes. Subsequently, further experiments were carried out on species of ginger to evaluate their susceptibility to R. solanacearum (Rs) race 4 (ginger strains) by several methods of inoculation, including tests to simulate natural infection. In contrast to the previous reports that Rs strains from kahili ginger (H. gardenarium), were non-pathogenic on ornamental gingers, the kahili ginger strain wilted both ornamental and edible ginger (Zingiber officinale) species within 21 days. The ability of Rs race 4 to infect many ginger species without wounding and to survive for long periods indicates that high risks will be incurred if the kahili ginger strain is inadvertently introduced from the forest reserves into ginger production areas. Conflicts of interest can often limit which plant species can be targeted for biocontrol however and ornamentals, such as ginger (Hedychium spp.), cannot be targeted for biocontrol in Hawaii because they are seen as being of high value to the Hawaiian tourist industry, despite their status as serious invaders of Hawaiian forest (Gardner et al.,1995).

In 2008, a scoping study to investigate the potential for classical biological control of the invasive Hedychium species was funded by a consortium of sponsors from Hawaii and New Zealand. Surveys in the native range (Eastern Himalayan foothills of India) identified a large guild of natural enemies exerting considerable pressure on H. gardnerianum  populations. The biological control project led by CABI scientists in collaboration with Government organisations in India is ongoing and research is now focused on prioritizing natural enemies for host specificity testing in the UK (D. Djeddour, CABI, pers. com., 2011).

01/08/2008 Updated by:

Djamila Djeddour, CAB Europe - UK, Bakeham Lane, Egham, Surrey TW20 9TY, UK