Microtis, commonly known as onion orchids or mignonette orchids is a genus of about 20 species of plants in the orchid family, Orchidaceae. Onion orchids are terrestrial herbs with a single leaf at the base of the plant. They are similar to orchids in the genus Prasophyllum in that they have an onion-like leaf. The flowers are small but often scented and attractive to their insect pollinators. They are widespread in Asia, Australia and some Pacific islands.

Quick Facts Onion orchids, Scientific classification ...

Onion orchids
Microtis parviflora growing on Black Mountain in the A.C.T.
Scientific classification
Kingdom:	Plantae
Clade:	Tracheophytes
Clade:	Angiosperms
Clade:	Monocots
Order:	Asparagales
Family:	Orchidaceae
Subfamily:	Orchidoideae
Tribe:	Diurideae
Subtribe:	Prasophyllinae
Genus:	Microtis R.Br., 1810
Synonyms[1]
Goadbyella R.S.Rogers Hydrorchis D.L.Jones & M.A.Clem Microtidium D.L.Jones & M.A.Clem

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Orchids in the genus Microtis are terrestrial, perennial, deciduous, sympodial herbs with a few inconspicuous, fine roots and an egg-shaped to almost spherical tuber. The tuber often produces two tubers on the end of long, root-like stolons. There is a single, linear, cylindrical, onion-like leaf at the base of the plant. The leaf resembles that of the closely related genus Prasophyllum except that is entirely green (usually red at the base in Prasophyllum) and exudes clear mucilage when damaged.^{[2][3][4][5][6][7]}

The inflorescence is a raceme with a few to many resupinate green flowers spirally arranged on a flowering stem. Each flower has a short stalk with a small bract near its base. The broad dorsal sepal is sharp-pointed, dished on the lower side and forms a horizontal hood over the column. The lateral sepals are similar to, but much narrower than the dorsal sepal. The petals are smaller than the sepals, thin and are spread below or under the dorsal sepal. As is usual in orchids, one petal is highly modified as the central labellum. The labellum is egg-shaped to oblong and hangs or is curved against the ovary. The sexual parts of the flower are fused to the column, which is shaped like half a cylinder. Flowering time depends on the climatic region where the species is found and the fruit that follows flowering is a non-fleshy, dehiscent capsule containing up to 500 seeds.^{[2][3][5][6][8]}

The genus Microtis was first formally described by Robert Brown in 1810 and the description was published in Prodromus Florae Novae Hollandiae.^[9] Brown described five species at the time (M. parviflora, M. rara, M. media, M. alba and M. pulchella) but did not nominate a type species.^[10] The name Microtis is derived from the ancient Greek words mikros (μικρός) meaning "small" and ous, genitive ōtos (οὖς, genitive ὠτός) meaning "ear",^[11] referring to the small, ear-like appendages on the column.^[3]

Onion orchids are widespread in temperate areas of Australia and in sub-tropical Queensland, occurring in all states but not the Northern Territory. Microtis parviflora and M. unifolia are the most widely distributed species and also occur on both main islands of New Zealand and as far south as Stewart and possibly Auckland Islands as well as New Caledonia, and Norfolk, Lord Howe and the Kermadec Islands in the western Pacific. These two species are also found in Malaysia, Indonesia, the Philippines, Taiwan, southern Japan, the Ryukyu Islands and possibly on the Chinese mainland. They usually grow in areas that are wet in winter or in areas of high rainfall.^[3]

The taxonomy of Microtis has been confused ever since Robert Brown first described the genus. He did not include Ophrys unifolia G.Forst. (1786)^[13] and Epipactis porrifolia Sw. (1800)^[14] and these names were used by later authors, although now recognised as Microtis media. The small size of the flowers and their tendency to change when dried as herbarium species led to further confusion.^[3]

Most Microtis are very easy to grow, and readily volunteer themselves in other pots. Onion orchids are often found in gardens around Melbourne from wind-born seed.^[4]