Having read Stearn again concerning E. grandiflorum f. koreanum from Japan and E. koreanum from Korea, I’m not much the wiser, but we haven’t bought any without grandiflorum being on the label. The issue as to whether plants from Korea have specific status may still be under debate. Also having looked on line, I see also plants referred to as subsp. koreanum. I think the picture below is of a plant bought from Beth Chatto’s nursery at Elmstead Market, Essex.
General information on Epimedium grandiflorum
Distribution – Japan (Kyushu, Shikoku, Honshu and Hokiado). Northern Korea and southern Manchuria.
Description – A medium sized fairly deciduous species with widespread distribution through Japan etc. growing in deciduous woodland. It was introduced to Great Britain in the 1830s. With its wide geographical range through the various Japanese islands etc., a good deal of variation has occurred. Flower colour of the plants found in the wild varies widely: yellow, white, pink, lilac and reddish to purple. Add to this the large number of plants attributed to E grandiflorum that have arisen in cultivation and there may be hundreds of cultivars already named.
Epimedium grandiflorum f. koreanum