Estimating its size is difficult because its greatest diversity if in Asia, many parts of which are poorly known botanically speaking. An additional complication is a consequence of the differing species interpretations that have been adopted. North American species are thought to have low barriers to interspecific hybridization. Most species are tetraploids, but there are many hexaploids and possible some octoploids. All native North American species examined are tetraploids in which the second genome is the H genome.
Many Asian species are also tetraploids, but with the Y genome as the additional genome.
Other genomes known to occur in Elymus are the P and E genomes. The synonyms listed are those that seem to be in recent or current use and are provided to help in interpretation of the treatment reflected by the first column. Genomic information is provided ONLY if it has been obtained via investigation of the species involved. Reports that do not list voucher specimens made at the time of the genomic investigation are asterisked. They should be repeated, with vouchers being deposited in a herbarium that regularly makes loans available.