Backyard Buddies
Orchids

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Orchids

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Many of Australia's 1200 species of orchids are epiphytic, meaning they grow up high using trees or rocks for support. Spring is the time for many of our beautiful epiphytic orchids to burst into flower.

One of the best known Australian epiphytic orchids is the New South Wales Rock Orchid, Dendrobium speciosum, which grows in frost free gardens from Gippsland in Victoria to northern Queensland.

This Rock Orchid has creamy sprays of fragrant blossoms, thick, leathery leaves and swollen, bulbous roots.

If you see this orchid, watch out for insect pollinators which alight on a perfect landing pad - a modified third petal - provided by the orchid in the centre of each flower.

If you catch the scent of oranges drifting down from a rainforest tree in your garden, you're not imagining things. It's Sarcochilus falcatus, another often cultivated native epiphytic orchid, commonly called Orange Blossom Orchid. Once common but with numbers depleted by over-collecting in the past, this orchid can often be seen growing on Acacia melanoxylon and Pittosporum undulatum on Australia's east coast.

Cymbidium, another epiphytic orchid, prefers to attach itself to the decayed hollows of trees, rather than tree branches.

Spring and summer is the time of year when many of Tasmania's native orchid species flower, though there are some species which flower only in autumn or winter.

Tasmania has about 210 species of orchids, with many listed as threatened, or with very limited distribution. Dry sclerophyll forests, heathland and native grasslands contain the greatest diversity of orchid species. Orchids do occur in rainforest and button grass moorland, but there are fewer species.

Orchids are often hard to detect or identify until flowering and may not necessarily flower each year. Leaves may appear many months before the flowers, but often these are hard to detect amongst leaf litter and vegetation. Flowering times can extend over months for some species, to only weeks or a few days for others.

Orchids use a wide range of methods to achieve pollination. Various techniques attract insects to the flowers which require cross-pollinating. These include colour, scent, mimicking other flowers or rewards such as nectar.

But one of the most cunning methods is the release of sex attractants to lure and trick male insects into thinking the flower is a female mate. The labellum, which is a large modified front petal, is the part of the flower that attracts insects.

Male thynnine wasps pollinate Large duck orchids Caleana major when they attempt to mate with the labellum. The large hairy labellum of beard orchids Calochilus spp. which have glossy eye-like glands at the base provide a similar appearance to the shape of female scoliid wasps, so beard orchids use visual display as well as scent which mimics the female wasp sex pheromone to achieve pollination.

During September, the native orchid season is in full bloom in Adelaide.

Adelaide has many types of orchids including species that are rare or threatened in Australia. The ongoing survival of many of these orchids is directly linked to pollinators.

Most plants provide their pollinators with nectar or pollen in return for pollination, however many orchid species trick their pollinators without offering any reward.

This might include pretending to offer a food reward by being bright or smelling sweet. Some orchids are so deceptive they imitate both the look and smell of a female insect to attract a male pollinator.

Some of Adelaide's Spider and Leek orchids make use of this method of pollination. Because the orchid may be mimicking a specific type of insect, for example a thynnine wasp, the conservation of the pollinator becomes essential to the orchid's survival.

One of Adelaide's rarest orchids, the Leafy Greenhood Pterostylis cucullata, is thought to be pollinated by fungus gnats Mycetophilids. Fungus gnats are small flies that look similar to mosquitoes. They transfer pollen between the flowers, causing them to close up. It is unclear what attracts fungus gnats to greenhood orchids; however fungus gnats live in cool, damp areas also favoured by the Leafy Greenhood.

Did you know?

Many terrestrial orchids team up with a root fungus to form a beneficial relationship. The fungus provides carbohydrates to help the orchid seeds germinate and feed the plant. These fungi can be difficult to establish and maintain in the garden but form naturally where wild orchids grow.

Tip

The tannins which naturally occur in eucalyptus bark are poisonous to the symbiotic fungi that orchids rely on. Orchids also avoid growing on trees which shed their bark seasonally, such as Brushbox. Orchids love to live in many rainforest species such as Casuarina and Melaleuca and some mangroves.

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Photo: OEH