Common in woodland, parks and gardens throughout much of England, spreading into Scotland, a species of conservation concern under the UK Biodiversity Action Plan. In Hampshire and on the Isle of Wight Common and widespread. Although the flight-time histogram shows a smooth curve of emergence, in most years there are two peaks, one in July and another, usually smaller, in September. Wingspan 38-42 mm. The most likely confusion species is August Thorn E. quercinaria
, which has the antemedian fascia (the line nearest the base of the wing) acutely angled at the costa (almost straight in September Thorn). Larva feeds on Oak, Lime, Birch and Beech, over-wintering as an egg.