Editing Ulmus americana (section)

==== Hybrids and hybrid cultivars ====
* [[Ulmus 'Rebella'|''Ulmus'' 'Rebella']] (''U. americana'' × ''U. parvifolia'')

Thousands of attempts to cross the American elm with the Siberian elm ''[[Ulmus pumila|U. pumila]]'' failed.<ref>{{FEIS |type=tree |genus=Ulmus |species=americana |last=Coladonato |first=Milo |date=1992 |access-date=December 14, 2014}}</ref> Attempts at the [[Arnold Arboretum]] using ten other American, European and Asiatic species also ended in failure, attributed to the differences in [[ploidy]] and operational [[dichogamy]],<ref name="Hans" /> although the ploidy factor has been discounted by other authorities.<ref name="Ager">{{cite journal|last1=Ager|first1= A. A.|last2=Guries|first2=R. P.|title= Barriers to Interspecific Hybridization in ''Ulmus americana''|journal=Euphytica|volume= 31|issue= 3|year=1982|pages=909–920|doi=10.1007/bf00039231|s2cid= 34278784}}</ref>

Success was eventually achieved with the autumn-flowering Chinese elm ''[[Ulmus parvifolia]]'' by the late Prof. [[Eugene Smalley]] towards the end of his career at the [[University of Wisconsin–Madison]] after he overcame the problem of keeping Chinese elm pollen alive until spring.<ref name="Heybroek, b">{{cite book|last1=Heybroek|first1= H. M.|last2=Goudzwaard|first2= L.|last3=Kaljee|first3= H.|year=2009|title=Iep of olm, karakterboom van de Lage Landen (Elm, A Tree With Character of the Low Countries|publisher=KNNV Uitgeverij|location=Zeist, Netherlands|language=nl |isbn=978-9-0501-1281-9}}</ref> Only one of the hybrid clones was commercially released, as [[Ulmus 'Rebella'|'Rebella']] in 2011 by the German nursery Eisele GmbH; the clone is not available in the United States.

Other artificial hybridizations with American elm are rare, and now regarded with suspicion. Two such alleged successes by the nursery trade were [[Ulmus 'Hamburg'|'Hamburg']], and [[Ulmus 'Kansas Hybrid'|'Kansas Hybrid']], both with Siberian elm ''[[Ulmus pumila]]''. However, given the repeated failure with the two species by research institutions, it is now believed that the "American elm" in question was more likely to have been the red elm, ''[[Ulmus rubra]]''.<ref>{{cite web|url=http://systematics.mortonarb.org/ulmus/elmCultivar.244.html|title=Ulmus 'Hamburg'|publisher=Morton Arboretum|access-date=December 27, 2014|archive-url=https://web.archive.org/web/20141227190218/http://systematics.mortonarb.org/ulmus/elmCultivar.244.html|archive-date=December 27, 2014|url-status=dead}}</ref>