Brachmia inornatella is a rather dull, greyish brown moth with three black dots on each front wing. When resting two black dots on each wing are quite conspicuous. Together with the same spots on the other wing they make out a kind of square. The third dot isn't visible in this position. There are also some very tiny black dots along the edge of the front wing. The wingspan is 11 to 15mm.
Little is known about the caterpillar of Brachmia inornatella, except for the fact it is monophagous and lives inside the stem of reet (Phragmites). Apparently it overwinters inside the stem as well.
Brachmia inornatella is a species considered to be rare in most parts of Europe. It is however not well known. It comes to light sometimes, but nobody knows whether the species is really attracted to light, or only some individuals are. The species appears over much of Europe, but always seems to be very local. It is on the wing in June and July. In Britain only known from the fens and marshes of East Anglia and south-eastern England. A scarce and local species in Holland and a rare and local species in Belgium.