Larval food plants include White Clover and Bird's Foot Trefoil. There is a large area of White clover in the paddock at Roseland House. The adults are common in sheltered corners of the paddock in late July, where they settle on the wild Thistles, then in early August favouring the Water Mint as the Thistles cease flowering.

In the south of England, Common Blues produce two broods per year. The adults flying May and June are from the second brood of eggs laid the previous autumn which have overwintered as larvae. These early summer adults lay the first of brood of eggs, which are on the wing as adults in July to September. In 2010 a peak number of adults on the wing was noticed in Great Yeldham in the second week of August with over half a dozen at a time flying over the Water Mint patch and landing to take nectar. Even so they were regularly outnumbered by Brown Argus sharing the same nectar plants, the Brown Argus making distinctly shorter, busier and lower flights.

To identify the males (with blue upper wing surfaces), it is necessary to see that the underside is brown-tinted and has black spots ringed with white, unlike Holly Blues whose under wing surfaces are clearer pale blue, and has small black spots without white rings.

Females have upper wing surfaces blue, merging into brown towards the margins, or just brown. If they are brown make sure that they have the extra spots on the undersides of the forewings to distinguish them from Brown Argus and that the pair of spots on near the leading edge of the underwings and away from the body are staggered and not one vertically arranged like a colon.