What are the best references for determining ploidy of potential parents?
Modern Roses is the best. Over the years information from scientific investigations have been included and compiled in Modern Roses. However, there are some references that never made it in as of yet, so you can find that the ploidy of roses here and there have been confirmed. Krussman in his ‘Complete Book of Roses’ has a nice listing of species and some cultivars as well, but most of them are in Modern Roses.