|Hybrid parentage||U. minor subsp. minor × U. parvifolia|
The American hybrid cultivar Ulmus 'Frontier'  is a United States National Arboretum introduction (NA 55393) derived from a crossing of the European Smooth-leafed, or Narrow-leafed, Elm Ulmus minor subsp. minor (female parent) with the Chinese Elm Ulmus parvifolia in 1971. The USNA released it as "a selection that resulted from controlled pollination of U. carpinifolia Geldtisch with U. parvifolia Jacq. Released in 1990, the tree is a rare example of the hybridization of spring-flowering and autumn-flowering elms.
'Frontier' develops a vase or pyramidal shape, with glossy green foliage turning - unusually for elms - to burgundy in autumn. The twigs are pubescent. Slow growing, the ultimate height of the tree has yet to be determined, but should be > 15 m. The tree is autumn-flowering  but rarely flowers, and has not produced seed.
Pests and diseases
'Frontier' has a good resistance to Dutch elm disease, rated 4 out of 5, but tolerance of Elm Yellows in the USA was also found to be poor. No mortalities are known to have occurred from the latter disease, however, and the cultivar is 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 USA.
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. '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. The tree is currently being evaluated in the National Elm Trial  coordinated by Colorado State University.
'Frontier' has had a limited introduction to Europe, where it is largely restricted to arboreta and elm collections; it also featured in trials in New Zealand during the 1990s at the Hortresearch station, Palmerston North.
- North America
- Bartlett Tree Experts. Acc. nos. 2001-097/8/9
- Brooklyn Botanic Garden , New York. Acc. no. 20040606.
- Chicago Botanic Garden, Glencoe, Illinois. No details available.
- Dawes Arboretum , Newark, Ohio. 3 trees. No acc. details available.
- Holden Arboretum. Acc. no. 95-140
- Morton Arboretum. Acc. nos. 1284-2004, 433-2005, 270-2008
- Parker Arboretum, Parker, Colorado. No acc. details.
- Scott Arboretum. Acc. no. 91-242
- Smith College. Acc. no. 19804, 36005
- University of Idaho Arboretum. Acc. no. 1995010
- U S National Arboretum, Washington, D.C., USA. Acc. no. 68984
- Brighton & Hove City Council, UK, NCCPG Elm Collection .
- Grange Farm Arboretum, Sutton St James, Spalding, Lincolnshire, UK. Acc. No. 504
- North America
- Backyard Trees, Park Hill, Oklahoma.
- Carlton Plants, LLC , Dayton, Oregon
- Charles J. Fiore , Prairie View, Illinois
- ForestFarm , Williams, Oregon
- Herd Farm Nursery, Belvedere, Tennessee
- J. Frank Schmidt & Son Co. , Boring, Oregon
- Johnson's Nursery , Menomonee Falls, Wisconsin
- Jost Greenhouses, Missouri
- North American Plants , Lafayette, Oregon
- Pea Ridge Forest , Hermann, Missouri
- Sun Valley Garden Centre , Eden Prairie, Minnesota
- Sunshine Nursery , Clinton, Oklahoma
- Photographs of burgundy colouring of 'Frontier' elm leaves in autumn: Arthur Lee Jacobson, 'Plant of the Month, 2008', photos 3 to 6 
- Jacobson, Arthur Lee, 'Plant of the Month, 2008': arthurleej.com/p-o-m-Oct08.html.
- McPherson, G. et al. (2008). National elm trial: Initial report from Northern California. Western Arborist, Fall 2009, pp 32-36.
- Jacobson, Arthur Lee, 'Plant of the Month, 2008' arthurleej.com/p-o-m-Oct08.html
- 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, USA.
- Townsend, A. M., Schreiber, L. R., Masters, W. O. and Bentz, S. E. HortScience, 26: 80-81, 1991.
- 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. ISBN 9789050112819
- 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
- 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, pp 15, 16. University of Kentucky.
- Giblin, C. P. & Gillman, J. H. (2006). Elms for the Twin Cities: A Guide for Selection and Maintenance. University of Minnesota.
- Costello, L. R. (2004). A 10 -year evaluation of the performance of four elm cultivars in California, U. S. Journal of Arboriculture, March 2004. 
- Burdekin, D. A. & Rushforth, K. D. (Revised by Webber J. F. 1996). Elms resistant to Dutch elm disease. Arboricultural Research Note 2/96. Arboricultural Advisory and Information Service, Alice Holt, Farnham, UK.
- http://www.extension.iastate.edu/Publications/SUL4.pdf Summary, inc. photographs, of elm cultivars resistant to Dutch elm disease available in the USA.
- 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.