Late June to late August. Common and well distributed throughout Britain. The largest of the British emeralds.

Easily identified by its large size and the scalloped edge to the wings. The green wing colour does not fade as in most other emeralds. White cross-lines are also scalloped with lines of white dots below.

The adults rest with wings spread and raised at an angle similar to a butterfly. They fly at night and are attracted to light but also occasionally fly high in the tree canopy on warm, sunny days.  The caterpillars can be found from July to the following June and are first brown becoming green with brown lumps after overwintering.