The forewings are generally dark brown or grey with few distinguishing marks apart from a small white tornal spot which may not be present on the frequent melanic forms. The wingspan is 18–24 mm. Two broods are produced each year with the adults flying in May and June and again in August. Moths of the spring brood are usually darker in colour than the later specimens.
The larva is bright yellow-brown with brown and greenish markings, most strikingly a variety of large, brown-green spots along the back. It has numerous, small white warts all over the body.
The caterpillars of the three races have different food plants:
Waring, Paul; Townsend, Martin & Lewington, Richard (2003) Field Guide to the Moths of Great Britain and Ireland. British Wildlife Publishing, Hook, UK. ISBN0-9531399-1-3.