Editing Ulmus americana (section)

=== Cultivars ===
{{Further|List of elm cultivars, hybrids and hybrid cultivars}}

Numerous [[cultivar]]s have been raised, originally for their aesthetic merit but more recently for their resistance to Dutch elm disease<ref name="Townsend">{{cite journal|last1=Townsend|first1=A. M.|last2=Bentz|first2=S. E.|last3=Douglass|first3=L. W.|url=http://www.elmpost.org/19elms.pdf|title=Evaluation of 19 American Elm Clones for Tolerance to Dutch Elm Disease|journal=Journal of Environmental Horticulture|date=March 2005|volume=23|pages=21–24|doi=10.24266/0738-2898-23.1.21|access-date=2006-11-07|archive-url=https://web.archive.org/web/20050511204924/http://www.elmpost.org/19elms.pdf|archive-date=2005-05-11|url-status=live|doi-access=free}}</ref> The total number of named cultivars is circa 45, at least 18 of which have probably been lost to cultivation as a consequence of DED or other factors:
{{#section:List of elm cultivars, hybrids and hybrid cultivars|Ulmus_americana_cultivars}} and others.<ref>{{cite web|url=https://www.mortonarb.org/trees-plants/tree-plant-descriptions/elm-cultivars|title=Elm cultivars|year=2021|location=[[Lisle, Illinois]]|publisher=[[Morton Arboretum]]|access-date=February 8, 2020|quote=Accolade® (Ulmus davidiana var. japonica 'Morton') .... Commendation™ (Ulmus ‘Morton Stalwart’) .... Danada Charm™ (Ulmus ‘Morton Red Tip’) .... Emerald Sunshine® (Ulmus davidiana var. japonica 'JFS-Bieberich') .... Frontier (Ulmus ‘Frontier’) .... Homestead (Ulmus ‘Homestead’): .... Jefferson (Ulmus americana 'Jefferson') .... New Harmony (Ulmus americana 'New Harmony') .... New Horizon (Ulmus ‘New Horizon’) .... Patriot (Ulmus 'Patriot') .... Pioneer (Ulmus 'Pioneer') .... Prairie Expedition® (Ulmus americana 'Lewis & Clark'): .... Princeton (Ulmus americana 'Princeton') ....  Prospector (Ulmus davidiana var. japonica 'Prospector') .... Regal (Ulmus ‘Regal’) .... Triumph™ (Ulmus ‘Morton Glossy’) .... Valley Forge (Ulmus americana 'Valley Forge') .... Vanguard™ (Ulmus ‘Morton Plainsman’)|archive-date=May 14, 2020|archive-url=https://web.archive.org/web/20200514021746/https://www.mortonarb.org/trees-plants/tree-plant-descriptions/elm-cultivars|url-status=live}}.</ref>

The disease-resistant selections made available to commerce to date include [[Ulmus americana 'Valley Forge'|'Valley Forge']], [[Ulmus americana 'New Harmony'|'New Harmony']], [[Ulmus americana 'Princeton'|'Princeton']], [[Ulmus americana 'Jefferson'|'Jefferson']], [[Ulmus americana 'Lewis & Clark'|'Lewis & Clark']], [[Ulmus americana 'Miller Park'|'Miller Park']], [[Ulmus americana 'St. Croix'|'St. Croix']], [[Ulmus americana 'Endurance'|'Endurance']], and a set of six different clones collectively known as [[Ulmus americana 'American Liberty'|'American Liberty']].<ref name="Costello">{{cite journal|last=Costello |first=L. R. |title=A 10-year Evaluation of the Performance of Four Elm Cultivars in California, U. S. |journal=Journal of Arboriculture |date=March 2004 |url=http://www.findarticles.com/p/articles/mi_qa4107/is_200403/ai_n9387070/print |archive-url=https://web.archive.org/web/20081006074656/http://findarticles.com/p/articles/mi_qa4107/is_200403/ai_n9387070/print |url-status=dead|archive-date=2008-10-06}}</ref> The [[United States National Arboretum]] released 'Valley Forge' and 'New Harmony' in late 1995, after screening tests performed in 1992&ndash;1993 showed both had unusually high levels of resistance to DED. 'Valley Forge' performed especially well in these tests.<ref>(1) {{cite web|first=Bruce|last=Carley|url=https://www.elmpost.org/|title=The Solution|work=Saving the American Elm|date=January 2, 2017|access-date=February 8, 2020|archive-date=January 25, 2021|archive-url=https://web.archive.org/web/20210125005127/http://www.elmpost.org/|url-status=live}}<br />(2) {{cite journal|first1=A.M.|last1=Townsend|first2=S.E.|last2=Bentz|first3=G.R.|last3=Johnson|year=1995|title=Variation in Response of Selected American Elm Clones to Ophiostoma ulmi|journal=Journal of Environmental Horticulture|volume=13|number=3|pages=126–128|doi=10.24266/0738-2898-13.3.126|oclc=8656790939|issn=0738-2898|lccn=83643944|doi-access=free}}</ref>

'Princeton' has been in occasional cultivation since the 1920s. 'Princeton' gained renewed attention after its performance in the 1992–1993 screening tests showed that it also had a high degree of disease resistance. A later test performed in 2002&ndash;2003 confirmed the disease resistance of 'Princeton', 'Valley Forge' and 'New Harmony', as well as that of 'Jefferson'.  Thus far, plantings of these four varieties generally appear to be successful.

In 2005, approximately 90 'Princeton' elms were planted along [[Pennsylvania Avenue (Washington, D.C.)|Pennsylvania Avenue]] in front of the [[White House]] in [[Washington, D.C.]] The trees, whose maintenance the [[National Park Service]] (NPS) manages, remain healthy and are thriving.<ref>(1) {{cite web|archive-url=https://web.archive.org/web/20190501223905/https://nativeforestnursery.com/trees/elm-princeton/|archive-date=2019-05-01|url=https://nativeforestnursery.com/trees/elm-princeton/|url-status=dead|title=Elm, Princeton - Ulmus americana 'Princeton'|year=2016|location=South [[Chatsworth, Georgia]]|publisher=Native Forest Nursery|access-date=2019-04-28}}<br />(2) {{cite journal|first1=Jessica R.|last1=Sanders|first2=James W.|last2=Woodworth Jr.|archive-url=https://web.archive.org/web/20190501223542/https://digitalcommons.lmu.edu/cgi/viewcontent.cgi?referer=https%3A%2F%2Fwww.google.com%2F&httpsredir=1&article=1141&context=cate|archive-date=May 1, 2019|url=https://digitalcommons.lmu.edu/cgi/viewcontent.cgi?referer=https://www.google.com/&httpsredir=1&article=1141&context=cate|title=Proactive, Not Reactive: Evolving Elm Management in the Nation's Capital|journal=Cities and the Environment|volume=6|number=1, article 8|date=2013-11-25|access-date=2019-04-28|via=Digital Commons @ LMU and LLS|url-status=live}}<br />(3) {{cite book |last=Sherald |first=James L |url=http://www.nps.gov/nationalmallplan/Documents/Studies/ElmsoftheMonuCore_HistandMgmtPlan_122009.pdf |title=Elms for the Monumental Core: History and Management Plan |id=Natural Resource Report NPS/NCR/NRR--2009/001|page=39|place=Washington, D.C.|publisher=Center for Urban Ecology, National Capital Region, [[National Park Service]]|date=December 2009|access-date=2010-10-14 |archive-url=https://web.archive.org/web/20101129074707/http://www.nps.gov/nationalmallplan/Documents/Studies/ElmsoftheMonuCore_HistandMgmtPlan_122009.pdf |archive-date=2010-11-29|url-status=live}}</ref> However, it has  been noted that ''U. americana'' cultivars are not recommended for more than singular plantings as they have unresolved DED and elm yellows concerns.<ref name="Zetterstrom">{{cite journal|first=Tom|last=Zetterstrom|editor1-last=Pinchot|editor1-first=Cornelia C.|editor2-last=Knight|editor2-first=Kathleen S.|editor3-last=Haugen|editor3-first=Linda M.|editor4-last=Flower|editor4-first=Charles E.|editor5-last=Slavicek|editor5-first=James M.|year=2017|url=https://www.fs.fed.us/nrs/pubs/gtr/gtr-nrs-p-174papers/13zetterstrom-gtr-p-174.pdf|title=Report From the Street|journal=Proceedings of the American Elm Restoration Workshop 2016; 2016 October 25–27; Lewis Center, OH. Newtown Square, PA|pages=119–121|oclc=1231892730|doi=10.2737/NRS-GTR-P-174|publisher=[[United States Department of Agriculture]], [[United States Forest Service]], Northern Research Station|id=Gen. Tech. Rep. NRS-P-174|access-date=February 7, 2021|quote=Because of unresolved DED and elm yellows concerns, American elm cultivars are not recommended for more than singular plantings, according to Elm Watch and Bruce Fraedrich of Bartlett Tree Research Lab (Charlotte, NC). .... Monoculture plantings, such as alongside Pennsylvania Avenue in Washington, D.C., have disproportionate vulnerabilities to disease, and because of a poor understanding of pruning goals, uniformity of streetscape design may likely become disrupted over time along that Inauguration Day parade route due to expected failures of major structural leaders.|archive-date=May 1, 2019|archive-url=https://web.archive.org/web/20190501223229/https://www.fs.fed.us/nrs/pubs/gtr/gtr-nrs-p-174papers/13zetterstrom-gtr-p-174.pdf|url-status=live|doi-access=free}}</ref>

It has also been noted that monoculture plantings of ''U. americana'' cultivars, such as those along Pennsylvania Avenue, have disproportionate vulnerabilities to disease.<ref name="Zetterstrom" /> Further, long-term studies of 'Princeton' in Europe and the United States have suggested that the cultivar's resistance to DED may be limited (see [[Ulmus americana 'Princeton'#Pests and diseases|Pests and diseases of 'Princeton']]).

The [[National Elm Trial]] evaluated 19 elm cultivars commercially available in the United States in scientific plantings throughout the nation to assess and compare the strengths and weaknesses of each. The trial, which started in 2005, lasted for ten years. Based on the  trial's final ratings, the preferred cultivars of ''U. americana'' are 'New Harmony' and 'Princeton'.<ref>(1) {{cite web|url=http://bspm.agsci.colostate.edu/national-elm-trial/|title=National Elm Trial|work=Bioagricultural Sciences & Pest Management|location=[[Fort Collins, Colorado]]|year=2018|publisher=[[Colorado State University College of Agricultural Sciences]]: [[Colorado State University College of Agricultural Sciences#Academic Departments|Department of Agricultural Biology]]|access-date=February 8, 2021|archive-date=March 30, 2018|archive-url=https://web.archive.org/web/20180330152322/http://bspm.agsci.colostate.edu/national-elm-trial/|url-status=live}}<br />(2) {{cite journal|first1=Jason J.|last1=Griffin|first2=William R.|last2=Jacobi, E.|first3=Gregory|last3=McPherson|first4=Clifford S.|last4=Sadof|first5=James R.|last5=McKenna|first6=Mark L.|last6=Gleason|first7=Nicole Ward|last7=Gauthier|first8=Daniel A.|last8=Potter|first9=David R.|last9=Smitley|first10=Gerard C.|last10=Adams|first11=Ann Brooks|last11=Gould|first12=Christian R.|last12=Cash|first13=James A.|last13=Walla|first14=Mark C.|last14=Starrett|first15=Gary|last15=Chastagner|first16=Jeff L.|last16=Sibley|first17=Vera A.|last17=Krischik|first18=Adam F.|last18=Newby|display-authors=4|url=https://webdoc.agsci.colostate.edu/bspm/NationalElmTrial/AUF2017.pdf|year=2017|title=Ten-Year Performance of the United States National Elm Trial|journal=Arboriculture & Urban Forestry|volume=43|number=3|pages=107–120|doi=10.17660/ActaHortic.2018.1191.5|oclc=7347020445|issn=0567-7572|publisher=[[International Society of Arboriculture]]|access-date=February 7, 2021|quote=Based on the ratings, the preferred cultivars of American elm were ‘New Harmony’ and ‘Princeton’, and the preferred cultivars of Asian elm were The [[Morton Arboretum]] introductions and ‘New Horizon’.}}</ref>

'Jefferson' was released to wholesale nurseries in 2004 and is becoming increasingly available for planting. However, 'Jefferson' has not been widely tested beyond Washington, D.C. The National Elm Trial provided no data on ‘Jefferson’ because an error in tree identification had occurred earlier in the nursery trade.<ref>{{cite journal|first=Tom|last=Zetterstrom|editor1-last=Pinchot|editor1-first=Cornelia C.|editor2-last=Knight|editor2-first=Kathleen S.|editor3-last=Haugen|editor3-first=Linda M.|editor4-last=Flower|editor4-first=Charles E.|editor5-last=Slavicek|editor5-first=James M.|year=2017|url=https://www.fs.fed.us/nrs/pubs/gtr/gtr-nrs-p-174papers/13zetterstrom-gtr-p-174.pdf|title=Report From the Street|journal=Proceedings of the American Elm Restoration Workshop 2016; 2016 October 25–27; Lewis Center, OH. Newtown Square, PA|pages=119–121|oclc=1231892730|doi=10.2737/NRS-GTR-P-174|publisher=[[United States Department of Agriculture]], [[United States Forest Service]], Northern Research Station|id=Gen. Tech. Rep. NRS-P-174|access-date=February 7, 2021|quote=Consumers need to also be aware that a nursery trade mix-up a dozen years ago still plays out in the marketplace, and ‘Princeton’ elms continue to be sold as ‘Jefferson’ unknowingly by reputable nurseries from New York to Minnesota. .... National Elm Trial (NET) results were inconclusive and provided no data on ‘Jefferson’ due to the cultivar mix-up.|doi-access=free}}</ref> The error may still be causing nurseries to sell 'Princeton' elms that are mislabeled as 'Jefferson', although one can distinguish between the two cultivars as the trees mature.<ref name="Zetterstrom" /><ref>{{cite web|url=https://trees.umn.edu/elmid|title=Jefferson American Elm vs. Princeton American Elm|work=Disease-Resistant Elm Identification Guide|publisher=[[University of Minnesota]]: Urban Forestry Outreach, Research & Extension: The UFore Nursery & Lab|access-date=February 7, 2021|archive-date=February 7, 2021|archive-url=https://web.archive.org/web/20210207235201/https://trees.umn.edu/elmid|url-status=live}}</ref>

In 2007, the 'Elm Recovery Project'<ref>{{cite web|url=http://www.uoguelph.ca/arboretum/SpProjects/Elm_Recover1.htm|title=Elm Recovery Project|publisher=[[University of Guelph]]|archive-url=https://web.archive.org/web/20071202083136/http://www.uoguelph.ca/arboretum/SpProjects/Elm_Recover1.htm|archive-date=December 2, 2007|url-status=dead}}</ref> from the [[University of Guelph]] in Ontario, Canada, reported that cuttings from healthy surviving old elms surveyed across Ontario had been grown to produce a bank of resistant trees, isolated for selective breeding of highly resistant cultivars.<ref>{{cite web|title=Scientists Breed Supertrees to Beat Dutch Elm|url=https://www.pressreader.com/canada/ottawa-citizen/20070911/281509336805186|publisher=Canada.com|date=September 11, 2007|via=[[PressReader]]|access-date=December 21, 2014|archive-url=https://web.archive.org/web/20210402161220/https://www.pressreader.com/canada/ottawa-citizen/20070911/281509336805186|archive-date=April 2, 2021|url-status=live}}</ref>

In 1993, Mariam B. Sticklen and James L. Sherald reported the results of NPS-funded experiments conducted at [[Michigan State University]] in [[East Lansing, Michigan|East Lansing]] that were designed to apply [[genetic engineering techniques]] to the development of DED-resistant strains of American elm trees.<ref>{{cite book |last1=Sticklen |first1=Mariam B. |url=https://books.google.com/books?id=avvxBwAAQBAJ&pg=PA171 |title=Chapter 13: Strategies for the Production of Disease-Resistant Elms |last2=Sherald |first2=James L. |publisher=Springer-Verlag |year=1993 |isbn=9781461568728 |location=New York |pages=171–183 |lccn=93017484 |oclc=851736058 |access-date=22 November 2019 |via=[[Google Books]]}}</ref> In 2007, AE Newhouse and F Schrodt of the [[State University of New York College of Environmental Science and Forestry]] in [[Syracuse, New York|Syracuse]] reported that young [[Transgene|transgenic]] American elm trees had shown reduced DED symptoms and normal [[mycorrhiza]]l colonization.<ref>{{cite journal|last1=Newhouse|first1=AE|last2=Schrodt|first2=F|last3=Liang|first3=H|last4=Maynard|first4=CA|last5=Powell|first5=WA|title=Transgenic American elm shows reduced Dutch elm disease symptoms and normal mycorrhizal colonization|journal=Plant Cell Rep.|year=2007|volume=26|number=7|pages=977–987|pmid=17310333|doi=10.1007/s00299-007-0313-z|s2cid=21780088}}</ref>

==== 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>