I first ran across Rock Harlequin, Corydalis sempervirens, while climbing around a rocky slope when I noticed the pink and yellow flowers poking up out of a crack in the rocks. It seemed so odd that such delicate flowers would be in such a seemingly harsh habitat. In fact these plants do very well in harsh conditions like exposed stone walls and shallow, dry soil conditions, but it does not like too much heat. Other common names for this plant are Pale Corydalis and Tall Corydalis. I like the Rock Harlequin name, since for me it pop out of the rocks like some kind of joker.
Its native range is all across Canada and south from Minnesota over to Georgia (in the higher elevations). C. sempervirens is found in throughout Massachusetts, excepting Barnstable County and the Islands. Its native habitat is in dry or rocky woods and often on sites after a disturbance.
It seems as if it can deal with poor soil better than it can handle competition with other plants on a richer site. This was borne out in my garden when I put a few plants on the edge of a mass of Yellow Corydalis , C. leutea, in cool moist soil. The Yellow Corydalis completely engulfed the Rock Harlequin within 2 weeks, never again to be seen. I had much better luck on a dry site on the edge of the driveway where not much else would grow.
Corydalis sempervirens is not so much an annual as it is a biennial or short lived perennial, producing a large amount of seed and moving around the garden. Like an annual it will flower in its first year from seed. The highly dissected gray-green leaves occur all along the upright, branched stem. The ¾ inch long pink and yellow tubular flowers are borne in panicles at the ends of the foot-long branches. Blooming time begins as early as May, for year old plants, and continues sporadically though September.