The classification of British elms has ranged from one variable species, to two, three, five or eight, often with named hybrids and varieties. Oliver Rackham (Woodlands, 2006) remarks 'There are arguably more kinds of elm in England than of all other native trees together' and elsewhere regarded Ulmus as 'the most critical genus in the British flora'. Sell & Murrell (Flora of Great Britain & Ireland, 2018) proposed a classification with 62 species and no hybrids or varieties. I have spent the summers of 2018-21 trying that classification out. I have seen 61 of the 62 taxa in the field, plus half a dozen not included in the Flora. I have produced a more practicable and better-illustrated key to elms: it can be downloaded free from https://www.wildlifebcn.org/sites/default/files/2021-07/Complete key to native and naturalised elms.pdf I hope more recorders will try out the new classification, and give me feedback on the key, so we can learn more about their distribution and ecology.
Amateur botanist for four decades, professional ecologist/entomologist/conservationist, now chief exec of Wildlife Trust for Beds, Cambs and Northants, and obsessed with elms.