Australia

 

AUSTRALIA, THE WORLD'S LARGEST ISLAND, has a land area of just over 2.9 million square miles, two‑fifths of which lies north of the Tropic of Capricorn. It has been isolated for at least 50 million years and 85 % of its unique flora is endemic. Although perhaps best known for its deserts, Australia has a wide diversity of climate and landscape which gives rise to a broad range of vegetation types, particularly in the coastal areas. In the north and north‑east the coastal areas have a tropical climate with distinct wet and dry seasons, the summer (November to March) being the wettest time of the year. These coastal areas support rain forest and mangrove swamps with woodland and savannah inland. The south‑west and south‑eastern areas of Australia have a temperate to subtropical climate with rainfall throughout the year. Their natural vegetation is mixed woodland and Eucalyptus scrub known as I mallee'. An alpine flora grows at higher altitudes in the mountains in the south‑east. Much of the vast area of central Australia is dry throughout the year, and rather few plants grow there in the stunted Acacia scrub.

 

There are estimated to be about 110 genera and over 700 species of orchids native to Australia. Forty per cent of the species are epiphytic or lithophytic and grow in the tropical northern and north‑eastern regions. Only five epiphytic species occur as far south as Victoria. The 60% of Australian orchids which grow terrestrially are found mainly in the south‑west and south‑east of the country and are nearly all endemic. The Kew orchid collection has many representatives both of terrestrial and epiphytic orchids from Australia.

 

Temperate Terrestrials

 

The Australian temperate terrestrials deserve to be much better known and more widely cultivated in Britain. Like the European terrestrials they grow in a symbiotic relationship with a mycorrhizal fungus which obtains organic matter from the compost and provides the orchid with nutrients.

 

The annual growth cycle at Kew starts at the end of October, when the first green shoots of Pterostylis plants appear. The early flowering species such as P. angusta and P. nutans quickly put up flower spikes. Gradually the leaves of Diuris species start to show

through, D. palustris being the first to flower, at the end of January. By this time the first Caladenia leaves are growing. After flowering, by about mid‑May, the plants die down and the tubers remain dormant through the summer when repotting is done.

 

Dormant tubers are removed from the old pots, but a third of the old compost is retained and incorporated in the new compost. This ensures an adequate supply of mycorrhizal fungus for the orchid to make vigorous growth. Some species increase readily in cultivation by forming up to five new tubers on stolons which can be divided when repotting. Many of the Pterostylis species can be propagated in this way, whereas most of the caladenias produce just one new tuber to replace the old one and so can only be increased from seeds. Tubers are repotted 3 cm below the surface of the compost with about 2 cm space between each one and then watered in. Normally another watering is not needed until the first shoots are growing. Watering is controlled according to the known habitat of the orchid in the wild. Diuris palustris, for example, grows in marshy areas and so is watered freely, while Thelymitra antennifera, which grows in dry areas, is kept just moist but watered infrequently.

 

The genus Pterostylis contains about 100 species, of which 15 are currently grown at Kew. The green hoods, as they are commonly known, are among the easiest of the Australian temperate terrestrials to maintain in cultivation. The genus is characterised by the hood which is formed by the fusion of the dorsal sepal and lateral petals of the flower. The hood is usually green, streaked with white, red or brown in different species. Like many orchids, Pterostylis flowers have evolved a complex method of achieving pollination. The lip of the flower is hinged, and when an insect, normally a small gnat, lands upon it, the lip springs backwards against the column trapping the insect inside the flower. The insect climbs up the column, gripping the conveniently placed hairs and squeezes out of the gap at the top. In doing so it brushes against the pollinia, which adhere to the insect as it takes flight. Landing on the lip of another flower, the insect is again trapped when the lip springs back. The pollinia which it has carried from the previous flower stick to the stigmatic surface as the insect climbs the column. As it gains its freedom again, it flies off with a fresh set of pollinia from that flower.

 

The readiness of the spring in the lip of a Pterostylis flower can easily be tested by touching it with a pencil. Usually the lip flies back straight away.

 

The largest species of Pterostylis is P. baptistii, the king green hood, which grows up to 40 cm tall with a hood up to 6 cm long. In Australia it grows in large colonies in the dense scrub surrounding streams and marshes. It has a translucent flower with green veins and brown markings at the tip of the hood. The hood is held erect. In P. nutans, the nodding green hood, the flower is bent over facing the ground. Most Pterostylis species bear a single flower at the top of the flowering shoot, but a few are multiflowered. Pterostylis vittata is one of this group and has up to eight green‑veined flowers on each stem. It grows naturally in the light scrub close to the southern coast of the country.

 

The genus Diuris, commonly called donkey orchids or double tails, is almost completely confined to Australia, with just one species found elsewhere ‑ Diuris fryana is endemic to Timor. Diuris carinata, D. palustris and D. emarginata all grow in damp areas while most other species occur in grassland or open woodland. It is thought that some Diuris species have evolved flower shapes and colours which mimic those of leguminous shrubs in order to attract insects to pollinate them.

 

There are about 50 species of Corybas, worldwide, ranging from the Himalayan foothills, through Malaysia to New Zealand, and 20 species are found in Australia. Corybas species grow in damp sheltered mossy areas under low bushes or ferns. They have a reputation for not flowering well in cultivation; and the partly formed flowers will abort if they are in a cold draught. To prevent this happening, Corybas plants are always grown in a sheltered spot in the greenhouse, under the shade of leaves of other plants. If kept in too much light their leaves become yellowish, and if in too much shade they stand rather unnaturally high above the surface of the compost. About 75% shade is most suitable. When conditions are right their fascinating flowers are ample reward for the extra trouble needed to cultivate them.

 

Tropical Terrestrials

 

Several terrestrial orchid genera originate from the northern tropical areas of Australia. The plants grow rapidly through the hot, wet summer and rest during the slightly cooler winter. The largest species in the Kew collection is Phaius tankervilleae. Copious watering and foliar feeding at fortnightly intervals result in its majestic leaves growing up to 100 cm. This species grows naturally along the margins of swamps, with its roots in the water for part of the year, and has flower spikes up to 2 m tall. The flowers are white on the outside and gingery brown within except for the lip which is usually pale on the outside and rose‑pink within. This species flowered first in England in Yorkshire in 1778, and was introduced to Kew in the same year by Sir Joseph Banks, the President of the Royal Society at the time. Phaius australis var. bernaysii, which has yellow and white flowers, is also grown at Kew.

 

The related genus Calanthe has 120 species worldwide, but only the widespread Calanthe triplicata is found in Australia. This species forms fine clumps of deep green, ribbed leaves about 60 cm tall through which the spikes of delicate white flowers protrude. Malaxis latifolia has equally handsome foliage and produces a raceme of tiny pale green flowers which turn purple after a few days.

 

The compost used for the tropical terrestrial orchids from Australia imitates the humus‑rich layer over the rain forest soils in which the wild plants grow. A mixture composed of leaf mould, peat and loam, lightened with coarse sand, is used at Kew. Repotting is done annually so that the roots never have to suffer stagnant compost. All these plants are grown at Kew in a glasshouse in shady conditions with a winter night minimum temperature of 14º C (61ºF).

 

Epiphytes

 

The Australian epiphytes, of which there are over 200 species, grow most commonly in the north‑east of Australia and along the east coast, with only five species occurring as far south as Victoria. They grow naturally on trees and rocks and have evolved root systems which adhere strongly to the substratum. Those grown at Kew are mainly from the genera Cymbidium, Bulbophyllum, Dendrobium and Sarcochilus.

 

The genus Cymbidium contains about 44 species worldwide. There are three species in Australia, each of which has a different habitat. Cymbidium madidum grows readily in cultivation in standard cymbidium compost, soon becoming a large plant in a 30 cm pot. It grows in nature in moist open forest or rain forest and needs a winter night minimum temperature of 16ºC (61ºF) in cultivation and quite shady conditions. Its stout leaves grow up to 35 cm long and are a dark glossy green. The flowers grow on pendulous spikes and are fleshy and long‑lasting, varying in colour from yellowy brown to green. Cymbidium canaliculatum grows in the drier inland woodland as well as coastal forests. It grows in the clefts of living or dead gum trees, where its roots penetrate deeply into the heartwood finding both moisture and protection from extremes of temperature. The leaves are few, thick, fleshy and gray‑green, the flowers variable, ranging from brown and red through purple to green. This species needs the maximum available amount of light in cultivation and is hung close to the glass in the warmest greenhouse with a winter night temperature of 17ºC (64ºF). Plants are grown in hollow logs made of cork oak bark, which seem to suit them better than plastic pots.

 

Cymbidium suave has soft, grass‑like foliage and a long slender stem. It is unusual in the genus in not forming pseudobulbs. The flowers are greenish in colour and sweetly scented. Like C. canaliculatum, C. suave is grown in hollow logs rather than pots but it is kept in a cooler glasshouse.

 

Twenty‑seven species of Bulbophyllum grow in Australia, 14 of which are grown at Kew. All are grown on slabs of cork oak bark and are kept quite shaded in a greenhouse with a winter night minimum temperature of 15ºC (60ºF). Over the years, these plants form mats of small, green pseudobulbs joined by short rhizomes. In Australia, bulbophyllums grow mainly in the rain forests close to the coast. Bulbophyllum baileys is the largest species with solitary flowers up to 4 cm across, white or yellow with purple spots.

 

The genus Dendrobium has over 70 species in Australia in a wide range of habitats. More than half this total is cultivated at Kew in three different glasshouses. Dendrobium bigibbum, the Cooktown orchid, is the floral emblem of Queensland. Its pseudobulbs grow up to 45 cm tall with a tuft of leaves at the top of each and arching sprays of mauve flowers in the autumn. The variety D. bigibbum ssp. bigibbum var. superbum has larger flowers than normal while the var. compactum has small flowers in cultivation. Dendrobium discolor forms dense thickets of enormous stems up to 3 m tall in the wild and is scarcely less vigorous in cultivation so it needs a high greenhouse. Its glossy new growth is most attractive and its arching racemes can be up to 30 cm long. The petals and sepals are brown and yellow and usually twisting and curled, adding to the interest of the plant. Both these species grow well in the brightly lit and warmest glasshouse at Kew.

 

The pendulous species, Dendrobium tetragonum and D. teretifolium, are grown in a shadier, cooler greenhouse with a winter night minimum temperature of 17ºC (60ºF). They are mounted on slabs of cork oak bark so that the flowers are presented in their natural posture. Dendrobium teretifolium, known sometimes as the bridal veil orchid, cascades like a chandelier and is covered with small white flowers each winter. Dendrobium tetragonum has distinctive square pseudobulbs with a tuft of dark green leaves at the end of each one. The flowers are yellow and brown or green and white and of spidery appearance. The tiny D. toressae grows under the same conditions. Its leaves are similar in size and shape to grains of rice and its yellow flowers are just as small.

 

A cooler glasshouse, with a winter night minimum temperature of 14ºC (57ºF), is used for growing the very spectacular D. speciosum. This variable species grows on rock outcrops in full sun and also on tree stumps in shade. Some clones flower better than others in cultivation. The best are absolutely magnificent with long spikes of densely packed, creamy‑coloured flowers growing from each stubby pseudobulb. Dendrobium speciosum grows well in pots of medium grade epiphyte compost. Dendrobium kingianum is another lithophyte which grows on cliff faces where its roots become entangled in mats of moss and ferns. Its flowers are usually pink, although a range from white through to mauve has been found. Dendrobium speciosum .  and D. kingianum form a natural hybrid, Dendrobium X delicatum, which has characteristics intermediate between its parents. It grows vigorously in cultivation and has pink or white flowers. Although rare in the wild, it is quite commonly cultivated, often erroneously labelled D. kingianum 'album'.

 

Seven species of the genus Sarcochilus are grown at Kew. They are all kept in a glasshouse with a winter night minimum temperature of 13ºC (55º F) but the individual species differ in the amount of shade they prefer. Sarcochilus ceciliae is grown in an open position in shallow pans of coarse compost where it forms compact tufts of foliage and dainty racemes of pink flowers. It needs to be grown with the crown of each plant standing proud of the surface of the compost so that it does not rot. Sarcochilus falcatus and S. olivaceus are both pendulous in habit and are grown mounted on to slabs of cork oak bark. They form large root systems which are misted over daily with water. Sarcochilus falcatus, the orange blossom orchid, has pure white flowers with the lip of each striped with yellow and orange, while S. olivaceus has olive‑green flowers with red markings. Sarcochilus hartmannii and S. fitzgeraldii are erect in habit and make compact plants. Both have white flowers with red markings at the bases of the petals. The flowers of Sarcochilus fitzgeraldii have a larger lip and the plants prefer a shaded position in the glasshouse. The natural habitat of this species is the dark, mossy ravines of the Dividing Range in New South Wales. Sarcochilus hartmannii is found in sunnier positions on rock faces and so does better in a more open position on the bench. Both species grow well either mounted on cork oak bark or in pots of medium grade epiphyte compost. Sarcochilus fitzgeraldii is now endangered in the wild and plants at Kew are deliberately pollinated so that many more seedlings can be raised and distributed around the world in the hope of reducing the need to remove more wild plants from their natural habitat. The artificial hybrid between these two species, Sarcochilus Fitzhart, grows vigorously, and is more common in cultivation than either parent.

 

Australia's most graceful orchid, Phalaenopsis amabilis var. papuana occurs sporadically along the Mossman and Daintree Rivers in dense forest. Each plant has several pendulous leaves and is secured to the host tree by its long flat roots. Five to ten glistening white flowers are borne on each arching inflorescence. This is the only species of Phalaenopsis which occurs in Australia. At Kew it is grown with other species of Phalaenopsis in a shady and very humid position in the warmest glasshouse with a winter night minimum temperature of 18ºC (64ºF). Its flowering period is one of the highlights of the year.