I’ve been looking for this for quite a while, and emailed almost every nursery I could find out in the UK, and I’m starting to write out letters too. This year I’ve obtained Tigris and Nigel Hawthorne, not necessarily to breed with, but more for the novelty value. Right now I’m looking out for Euphrates (anyone has this?) But the first email I sent was to Harkness, since it was their creation… Pity that they have discarded such a treasure like Xerxes… Here’s what they’ve written to me for anyone who’s intrested in the Harkness Persica hybrids.:
You have done very well to find these roses in the USA but unfortunately we do not know of anywhere that you can obtain Xerxes. We had the original plant in our greenhouse for some years but eventually discarded it. We are still working with h. persica as a breeding line so there may some more varieties to add to this collection in future years.
All best regards