Rhizanthella Gardneri: The Underground Orchid

Rhizanthella gardneri, often called the Western underground orchid, is one of the most remarkable plants in the world because it spends its entire life cycle underground, including flowering. It’s native to Western Australia, where it grows beneath the leaf litter and soil of the broom honey-myrtle and Acacia shrubland.

Unlike almost all other plants, Rhizanthella gardneri lacks chlorophyll, meaning it cannot photosynthesize. Instead of making its own food from sunlight, it depends entirely on a symbiotic relationship with fungi.

The orchid connects to a specific mycorrhizal fungus, which in turn, forms associations with the roots of nearby broom bushes (Melaleuca uncinata). Essentially, the orchid taps into the underground fungal network that links it to the photosynthetic Melaleuca plants. The orchid parasitically draws the nutrients and carbon it needs through the fungus, indirectly obtaining the energy that originates from the Melaleuca’s photosynthesis.

Credit: Jean and Fred Hort

Flowering of Rhizanthella gardneri begins in late May, early June when each plant produces up to 100 small, inward facing, cream to reddish coloured flowers, surrounded by 6 to 12 large, cream or pinkish-cream bracts. These bracts form a tulip-like head that curves over the flowers forming a small opening at the soil surface. A layer of leaf and bark litter covers this opening. The flower is usually 5–10 cm below the surface, and sometimes just visible through the soil or litter.

Because it never sees the light, pollination and seed dispersal are adapted to its subterranean existence. The flowers emit a strong, musty odour, likely to attract small soil-dwelling insects such as gnats or termites. The seeds are minute, typical of orchids, and are probably dispersed locally by ants or soil movement rather than wind.

Rhizanthella gardneri was discovered in 1928, by a Western Australian farmer named Jack Trott, who was digging in the sandy soil near Corrigin, about 230 km east of Perth, when he noticed something strange beneath the surface of a broom bush (Melaleuca uncinata). What he saw resembled a cluster of small pinkish buds, completely buried under the soil.

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Trott took a specimen to Richard Sanders Rogers, who quickly realized it was something quite new. He formally described the discovery the same year in the Journal of the Royal Society of Western Australia, and named it in honour of Charles Gardner, assistant botanist to the Western Australian Government at that time.

For decades, R. gardneri was known only from a handful of sites in the wheatbelt region of Western Australia. Its dependence on specific soil fungi and host plants made it extremely difficult to study or cultivate. The plant is found in two disjunct areas some 300 km apart—between Corrigin and Babakin and northwest of Munglinup.

The species is classified as "critically endangered" with only three of the known populations of Rhizanthella gardneri protected within nature reserves. Much of his habitat in central and southern Wheatbelt of Western Australia has been cleared for agriculture, or affected by drought, resulting in the loss of broombush habitat or a reduction in the level of bark and leaf litter necessary to protect the underground orchid and a reduction in the area suitable for translocation. The main threats to the species include lack of suitable habitat, degraded habitat, drought and rising soil salinity.

Credit: chookman.id.au