Trochetiopsis ebenus (St Helena ebony)
From the two surviving wild St Helena ebony plants, Kew’s horticulturists and conservationists have taken part in propagation programmes with colleagues in St Helena to produce thousands of new plants from cuttings and seeds.
About this species
St Helena ebony is one of St Helena’s many unique plants facing extinction. Although it was believed to be extinct for over 100 years, two plants clung to life, confined to a cliff face near the Asses Ears where they were out of reach of grazing animals. Many thousands of St Helena ebony plants have been propagated from these two survivors, mainly from cuttings. Using seed collected from the propagated cuttings, staff in Kew’s Conservation Biotechnology Unit developed a novel method of growing strong healthy seedlings with good root systems that could withstand transfer from laboratory culture jars to glasshouse growing conditions.
Geography & Distribution
The last two remaining wild St Helena ebony plants (Image: Nick Johnson, RBG Kew)
The three species in the genus Trochetiopsis are unique to the remote island of St Helena in the South Atlantic, one of the UK Overseas Territories. St Helena ebony (T. ebenus) originally grew in the drier areas of the north and west of the island, at an altitude of 200 to 500 m. Redwood (T. erythroxylon) was found on the island’s wet uplands, and is extinct in the wild. The now extinct dwarf ebony (T. melanoxylon) occupied arid areas in the rain shadow to the north of the central highland ridge.
Although St Helena ebony was originally recorded as a small tree, reaching no more than 5 m in height, the only recorded wild plants, which were the parent plants for those now in cultivation, are short rounded bushes.
Characteristic star-shaped hairs form a soft bronze-coloured felt covering the lower surface of the leaves and the young branches. In the flowers, five creamy white petals surround the purple stamens (which produce the pollen) and the star-shaped stigma (the pollen receptor).
Threats & Conservation
St Helena ebony has been reintroduced to St Helena (Image: Nick Johnson, RBG Kew)
By the middle of the nineteenth century, the St Helena ebony was already considered to have vanished from the island, principally due to over-grazing by goats. Cutting down trees to utilise the very hard wood also contributed to its demise.
Over a hundred years later, two small plants were rediscovered, clinging to a cliff face. An islander managed to collect cuttings which were sent to Cambridge Botanic Garden, where they were rooted and grown on for propagation. Cuttings from these cultivated plants were distributed to Kew and other botanic gardens. Some propagated plants have been reintroduced to the wild at various sites on St Helena and others have been established in gardens there.
During the nineteenth century, islanders burnt the wood in lime kilns to make mortar for buildings and used it to make household furniture, utensils and ornaments.
The hard black wood, which is responsible for the tree’s common name, is so dense that it sinks in water.
Millennium Seed Bank: Seed storage
Kew's Millennium Seed Bank partnership aims to save plant life worldwide, focusing on plants under threat and those of most use in the future. Seeds are dried, packaged and stored at a sub-zero temperature in our seed bank vault.
Number of seed collections stored in the Millennium Seed Bank: 1
Germination testing: 100% germination. Seedcoat chipped with scalpel before germination.
St Helena ebony seedlings grown on Sorbarods (cellulose rods) produce vigorous plants (Image: Andrew McRobb, RBG Kew)
All the St Helena ebony plants currently in cultivation are derived from the two plants recently discovered on the island. This means that they are likely to have a relatively narrow genetic diversity compared to the original population and may be particularly susceptible to pests and diseases.
Horticulturists have found that cuttings root easily but this type of propagation produces new plants that are genetically identical to their parents. Crossing plants to produce seed, however, offers the potential for new combinations of genetic material and maximises variability within the species.
Although the plants set seed quite easily, possibly by self-pollination, the seeds have a very hard coat that must be removed or chipped before the embryo can germinate. Initially the extracted embryos were grown on agar gel enriched with nutrients, but their roots developed poorly and it was difficult to establish them on conventional compost. Kew’s Conservation Biotechnology Team developed a method of growing the seedlings in sterile culture, using cellulose rods (Sorbarods) or a mixture of paper pulp and vermiculite (Florialite) soaked with a nutrient solution. The seedlings grown this way develop good root systems and can be successfully transferred to compost for growing on.
As the number of St Helena ebonies in cultivation has increased, seeds have become more readily available, and horticulturists now germinate them under conventional glasshouse conditions.
Where to see St Helena ebony at Kew
St Helena ebony is part of the Island Floras display in the central section of the Temperate House, among other plants from St Helena and remote islands elsewhere in the world.
Preserved specimens of St Helena ebony are held in the Herbarium, one of the behind-the-scenes areas of Kew. The details, including some images, of two dried and one alcohol-preserved specimen can be seen in the on-line Herbarium Catalogue. The specimens themselves are made available by appointment to bona fide researchers from around the world.
St Helena and Kew
St Helena ebony cuttings are being propagated on the island for reintroduction (Image: Colin Clubbe, RBG Kew)
Kew’s links with St Helena go back to the early nineteenth century, when Sir Joseph Banks (the unofficial director of the Gardens at the time) recommended that a botanic garden should be set up on the island to act as a transfer station for plants that were being moved from one continent to another. One of the earliest directors of the St Helena garden was William Burchell, whose collection of dried plant specimens, together with his illustrations of the island’s landscapes and plants, is preserved at Kew.
Today, Kew staff are involved in a number of active projects on the island. One of these is the South Atlantic Invasive Species project (co-ordinated by the Royal Society for the Protection of Birds). Invasive plant species are a serious problem on the island, overwhelming the natural habitats of the native plants, many of which are now extinct or critically endangered there. Kew botanists and horticulturists are also supporting a project to propagate and reintroduce native plants, in sufficient numbers to compete with alien species. This includes helping to re-develop the St Helena Government nursery at the Agriculture and Natural Resources Department (ANRD) in Scotland and to help build local skills and experience in horticulture.
St Helena ebony is one of the UK Overseas Territories plants used in the Remembrance Sunday wreath that Kew staff make every year. The wreath is placed on the Cenotaph in Whitehall by the Foreign Secretary on behalf of the UK Overseas Territories.
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