Ulmus 'Morton'

Ulmus 'Morton' (selling name Accolade) is an elm cultivar cloned from a putative intraspecific hybrid planted at the Morton Arboretum in 1924, which itself originated as seed collected from a tree at the Arnold Arboretum in Massachusetts. Although this tree was originally identified as Ulmus crassifolia, it is now believed to have been a hybrid of the Japanese elm (Ulmus davidiana var. japonica) and Wilson's elm (formerly Ulmus wilsoniana, but also sunk as U. davidiana var. japonica).[1] Accolade has proven to be the most successful cultivar tested in the US National Elm Trial, averaging a survival rate of 92.5% overall.[2]

Ulmus 'Morton'
Accolade, Morton Arboretum, Illinois
GenusUlmus
Hybrid parentageU. davidiana var. japonica × U. davidiana var. japonica
Cultivar'Morton' = Accolade
OriginUS

Description

The parent tree at the Morton Arboretum is noted for the resemblance of its habit to the American elm Ulmus americana, its upright-arching branches creating the familiar vase-shape, although the tree does not grow as large as the iconic native elm, reaching scarcely 20 m at maturity.[3] Its glossy, deep green leaves are also markedly smaller, rarely exceeding 8 cm in length. The tree is commercially propagated by grafting onto an Ulmus pumila rootstock.

Accolade has a propensity to produce co-dominant stems and major branches with bark inclusions, demanding corrective pruning on at least an annual basis where planted in towns.[4]

  • Accolade foliage, midsummer
    Accolade foliage, midsummer
  • American Elm (left) and Accolade leaf comparison
    American Elm (left) and Accolade leaf comparison
  • Accolade bark
    Accolade bark

Pests and diseases

In its 90+ years, the original tree at Morton has survived three epidemics of Dutch elm disease there unscathed.[5] However, two trees included in trials at the University of Minnesota were found to be afflicted by the disease in 2004, although one appears to have recovered completely.[6] The cultivar has also proved to be highly resistant to elm yellows and the elm leaf beetle Xanthogaleruca luteola [7] , but not to the Japanese beetle, which can cause extensive foliar damage.[8]

Cultivation

Accolade is reputed to grow well in almost all soils save those excessively wet, and is notably drought-tolerant and cold hardy. In artificial freezing tests at the Morton Arboretum[9] the LT50 (temp. at which 50% of tissues die) was found to be −35.5 °C. The tree grows vigorously at first, gaining as much as one metre per annum, but slows to approximately half that rate with maturity. Thus, a typical 20-year-old tree could be expected to have reached 14 m in height with a crown about 5 m in width.[10] Trees grown by Tollgate Education, Michigan State University, at Novi were the most vigorous and robust of the 37 different cultivars and species under assessment, described as looking like 'men among boys'.[11] Accolade is being evaluated in the National Elm Trial[12] coordinated by Colorado State University. Accolade was introduced to Europe in 2006 and is now in commerce in the Netherlands.

Hybrid cultivars

Accolade was crossed with the hybrid cultivar 'Morton Plainsman' = Vanguard. A selection of the resultant seedlings was marketed under the name 'Charisma', later changed to 'Morton Glossy' = Triumph.

Synonymy
  • Thornhill Elm: Anon.

Accessions
North America
Europe

Nurseries
North America
Europe
  • Batouwe Boomkwekerijen B.V., Dodewaard, Netherlands. Potted whips.
  • Boomkwekerij Gebr. Van den Berk B.V., Sint-Oedenrode, Netherlands.

References
  1. Fu, L.; Xin, Y.; Whittemore, A. (2002). "Ulmaceae, in Wu, Z. & Raven, P. (eds)" (PDF). Flora of China. Science Press, Beijing, and Missouri Botanical Garden Press, St. Louis, USA. 5. (Ulmaceae through Basellaceae)
  2. Griffin, J.; et al. (2017). "Ten-Year Performance of the United States National Elm Trial" (PDF). Arboriculture & Urban Forestry. International Society of Arboriculture, Atlanta, US. 43(3):107–120.
  3. "Accolade Elm - Ulmus x 'Accolade'". Cirrusimage.com. Retrieved 20 December 2018.
  4. Iles, J. (2009). Forget your fear of the elms. Western University website
  5. Ware, G. Morton Arb. Quarterly, 28(1): 1–5, 1992.
  6. Giblin, C. P. & Gillman, J. H. (2006). Elms for the Twin Cities: A Guide for Selection and Maintenance. University of Minnesota.
  7. McPherson, G. et al. (2008). National elm trial: Initial report from Northern California. Western Arborist, Fall 2009, 32–36.
  8. 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.
  9. Shirazi, A. M. & Ware, G. H. (2004). Evaluation of New Elms from China for Cold Hardiness in Northern Latitudes. International Symposium on Asian Plant Diversity & Systematics 2004, Sakura, Japan.
  10. Guries, R. P. & Smalley, E. B., (1986), Proc. Third Nat. Urban Forestry Conf., 214–218, 1986, Orlando, Florida.
  11. "Thinking of ash alternatives? Don't forget about elms". MSU Extension. Retrieved 20 December 2018.
  12. "National Elm Trial". 26 February 2008. Archived from the original on 26 February 2008. Retrieved 20 December 2018.
  13. "The Dawes Arboretum - Newark, Ohio". 25 October 2007. Archived from the original on 25 October 2007. Retrieved 20 December 2018.
  14. "Accolade". Archived from the original on 24 March 2012. Retrieved 20 December 2018.
  15. "Photograph of autumn colouring of young 'Accolade' elms before Suzzallo Library: Arthur Lee Jacobson, 'Plant of the Month, 2008'". Arthurleej.com. Retrieved 20 December 2018.
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