This is a much newer variety of E x rubrum which we bought from Edrum Nursery, in East Scotland. It was bred by Darrell Probst in America using E. alpinum with E. sempervirens, rather than E. grandiflorum. This may be slightly more vigorous than the original E x rubrum. The young leaves have an attravtive red edge which emphasises the heart shape of the leaf, which is why I imagine it received the name 'Sweetheart'.
Epimedium x rubrum 'Sweetheart'