Microtis atrata, commonly known as the swamp mignonette orchid or yellow onion orchid and sometimes as Microtidium atratum, is a species of orchid endemic to southern Australia. It has a single thin leaf and up to forty or more yellowish-green flowers. The flowers are the smallest of any Australian ground-dwelling orchid. The orchid usually grows in large colonies after fire and although small, are easily seen because of their very large numbers in their blackened surroundings.
Microtis atrata is a terrestrial, perennial, deciduous, herb with an underground tuber and a single erect, smooth, more or less solid leaf, NaNsigfig=1NaNsigfig=1 long and about 3sigfig=1NaNsigfig=1 wide. Between two and forty or more yellowish-green flowers are densely crowded along a flowering stem NaNsigfig=1NaNsigfig=1 long, reaching to a height of NaNsigfig=1NaNsigfig=1. The plants are sometimes up to 200sigfig=1NaNsigfig=1 tall if growing in water. At about 2sigfig=1NaNsigfig=1 long and wide, the flowers are the smallest of any terrestrial Australian orchid, and as they age, they turn black. The dorsal sepal is egg-shaped to round, about 1sigfig=1NaNsigfig=1 long and wide and forms a hood over the column. The lateral sepals are oblong, less than 1sigfig=1NaNsigfig=1 long and spread apart from each other. The petals are egg-shaped and concave and the labellum is oblong to egg-shaped. Flowering occurs from September to December and is stimulated by fire the previous summer.[1] [2] [3] [4]
Microtis atrata was first formally described in 1840 by John Lindley and the description was published in A Sketch of the Vegetation of the Swan River Colony.[5] [6] The specific epithet (atrata) is a Latin word meaning "dressed in black",[7] referring to the colour of the dried flowers.[8]
Some authorities give this species the name Microtidium atratum but this name is not widely accepted.
The swamp mignonette orchid occurs in the south-west of Western Australia, the south-east of South Australia, in southern Victoria and northern Tasmania. It grows in swampy places and in winter-wet depressions, sometimes in standing water and can form colonies of enormous numbers of plants after summer fire.[9]
Microtis atrata is classified as "not threatened" in Western Australia by the Western Australian Government Department of Parks and Wildlife, but is regarded as "endangered" in South Australia[10] and as "rare" in Tasmania.