I'm not sure why you wrote this: "I don't think it's good practice to rely on the Catalogue of Life for names of non-naturally ocurring hybrids." Non-naturally occurring hybrids are never listed in our database.
Hybrid taxa (the names with an "x" in them) are naturally occurring hybrids. The CoL does include the names of naturally occurring hybrids, but only if they are accepted botanical names. In contrast to some other databases, including ours, the CoL doesn't include the x, but if you scroll down on this page for Cistus nigrescens, you'll see the words "hybrid taxon." That's how we know the "x" should be included in the name for our database.
http://www.catalogueoflife.org...
The name Helleborus x nigercors should not appear in our database because it is not an accepted name. It was supposed to be H. niger x H. argutifolius, but H. argutifolius is now merely a synonym, so the hybrid taxon name is no longer valid. I'm not sure what happened to Viburnum x bodnantense. It's V. farreri x V. grandiflorum, both of which are accepted names, but it isn't recognized as a hybrid taxon by the CoL, so perhaps it wasn't a naturally occurring hybrid.
You say: "...horticulturally the polyanthus primroses are generally referred to as Primula x polyantha. In the absence of a better taxon, I suggest they be listed here under that name."
Our database is based on the botanical names accepted by taxonomists. The name may be used horticulturally, but that isn't enough to warrant its inclusion. I will remove the P. elatior subsp. elatior from the cultivar entries because they now would be classified as hybrids of P. veris (or P. elatior) x P. acaulis, and there is no botanical name for that hybrid.
