Ulmus 'Frontier' is an American hybrid cultivar , a United States National Arboretum introduction (NA 55393) derived from a crossing of the European field elm Ulmus minor (female parent) with the Chinese elm Ulmus parvifolia in 1971. Released in 1990, the tree is a rare example of the hybridization of spring- and autumn-flowering elms. Tested in the US National Elm Trial coordinated by Colorado State University, 'Frontier' averaged a survival rate of 74% after 10 years.

Description 'Frontier' develops a vase or pyramidal shape, with glossy green foliage turning, unusually for elms, to burgundy in autumn.Photographs of burgundy colouring of 'Frontier' elm leaves in autumn: Arthur Lee Jacobson, 'Plant of the Month, 2008', photos 3 to 6 The twigs are pubescent.Jacobson, Arthur Lee, 'Plant of the Month, 2008': arthurleej.com/p-o-m-Oct08.html. Slow growing,McPherson, G. et al. (2008). National elm trial: Initial report from Northern California. Western Arborist, Fall 2009, 32–36. the ultimate height of the tree has yet to be determined, but should be > 15 m. The tree is autumn-floweringJacobson, Arthur Lee, 'Plant of the Month, 2008' arthurleej.com/p-o-m-Oct08.html but rarely does so, and has not produced seed.Santamour, J., Frank, S. & Bentz, S. (1995). Updated checklist of elm (Ulmus) cultivars for use in North America. Journal of Arboriculture, 21:3 (May 1995), 121–131. International Society of Arboriculture, Champaign, Illinois, US. Townsend, A. M., Schreiber, L. R., Masters, W. O. and Bentz, S. E. HortScience, 26: 80–81, 1991.

Pests and diseases 'Frontier' has a good resistance to Dutch elm disease, rated 4 out of 5,Heybroek, H. M., Goudzwaard, L, Kaljee, H. (2009). Iep of olm, karakterboom van de Lage Landen (:Elm, a tree with character of the Low Countries). KNNV, Uitgeverij. but tolerance of elm yellows from grafts in the United States was found to be poor.Sinclair, W. A., Townsend, A. M., Griffiths, H. M., & Whitlow, T. H. (2000). Responses of six Eurasian Ulmus cultivars to a North American elm yellows phytoplasma. Plant disease, Vol. 84, No.12, 1266–1270. American Phytopathological Society, St. Paul, MN However, no mortalities are known to have occurred from the latter disease in the field, the cultivar not known to be vulnerable to infection through natural means. The tree can be heavily to severely damaged by the elm leaf beetle Xanthogaleruca luteola, although it fared better than most of the cultivars assessed at U C Davis, suffering little more than 10% foliar damage. Although susceptible to attack by Japanese beetle, it is far less seriously affected than most hybrid cultivars available in the United States.Brady, C., Condra, J., & Potter, D. (2008) Resistance of Landscape-suitable Elm (Ulmus spp.) Cultivars to Japanese Beetle, Leaf Miners, and Gall Makers. 2008 Research Report, Nursery & Landscape Program, 15–16. University of Kentucky.

Cultivation In trials in eastern Arizona , 'Frontier' and another American hybrid, 'Regal', were found to have the highest tolerance of the hot and arid climate, notably exhibiting minimal leaf scorch. However, 'Frontier' is known to have sustained winter damage where planted in the Great Plains . This failing was repeated in the elm trials conducted by the University of Minnesota, although the tree often recovered the following year. It was also criticized for its form and integrity, considered "unsuitable" for urban forestry.Giblin, C. P. & Gillman, J. H. (2006). Elms for the Twin Cities: A Guide for Selection and Maintenance. University of Minnesota. 'Frontier' fared better in 10-year trials at Atherton, California, to evaluate replacements for Californian elms lost to disease: "Strong structure, rapid growth rate, attractive leaf color in spring and fall, and relatively low pruning requirement suggest that Frontier has promise...", although the tree again proved only moderately tolerant of elm leaf beetles.

'Frontier' has had a limited introduction to Europe, where it is largely restricted to arboreta and elm collections. In the Netherlands, however, dendrologist Martin Tijdgat reported (2020) that 'Frontier' "has become a popular tree, partly thanks to the efforts of Hans Kaljee", tree consultant for the city of Amsterdam.M. Tijdgat, 'Ulmus – Gebruikswaarde- en sortimentsonderzoek in de praktijk', Dendroflora 56, 2020. p.68; edepot.wur.nl 'Fontier' also featured in trials in New Zealand during the 1990s at the Hortresearch station, Palmerston North.

Accessions North America Bartlett Tree Experts, US. Acc. nos. 2001–097/8/9 Brooklyn Botanic Garden , US. Acc. no. 20040606. Chicago Botanic Garden, US. No details available. Dawes Arboretum, US. Newark, US. 3 trees. No acc. details available. Holden Arboretum, US. Acc. no. 95–140 Morton Arboretum, US. Acc. nos. 1284–2004, 433–2005, 270–2008 Parker Arboretum, US. No acc. details. Scott Arboretum, US. Acc. no. 91–242 Smith College, US. Acc. no. 19804, 36005 University of Idaho Arboretum, US. Acc. no. 1995010 U S National Arboretum, Washington, D.C., US. Acc. no. 68984 Europe Grange Farm Arboretum, Lincolnshire, UK. Acc. no. 504 Great Fontley Butterfly Conservation Elm Trials plantation, UK. One small sapling planted 2021

Nurseries North America Backyard Trees, Park Hill, Oklahoma, US. Carlton Plants, LLC , Dayton, Oregon, US. Charles J. Fiore , Prairie View, Illinois, US. ForestFarm , Williams, Oregon, US. Herd Farm Nursery, Belvedere, Tennessee, US. J. Frank Schmidt & Son Co. , Boring, Oregon, US. Johnson's Nursery , Menomonee Falls, Wisconsin, US. Jost Greenhouses, Missouri, US. North American Plants , Lafayette, Oregon, US. Pea Ridge Forest , Hermann, Missouri, US. Sester Farms , Gresham, Oregon, US. Sun Valley Garden Centre , Eden Prairie, Minnesota, US. Europe Golden Hill Plants , Marden, UK. Pan-Global Plants , Frampton-on-Severn, Gloucestershire, UK. Van Den Berk (UK) Ltd., , London, UK

References

External links http://www.extension.iastate.edu/Publications/SUL4.pdf Summary, inc. photographs, of elm cultivars resistant to Dutch elm disease available in the United States. https://web.archive.org/web/20030413074605/http://fletcher.ces.state.nc.us/programs/nursery/metria/metria11/warren/elm.htm Warren, K., J. Frank Schmidt & Son Co. (2002). The Status of Elms in the Nursery Industry in 2000.