The hybrid elm cultivar Ulmus × hollandica 'Pioneer' is an American clone arising from the crossing of two European species, Wych Elm U. glabra (female parent) and Field Elm U. minor. Raised by the USDA station at Delaware, Ohio, in 1971, 'Pioneer' was released to commerce in 1983.

Dutch dendrologist Martin Tijdgat, noting that the "branch fragility" of the cultivar could indicate the influence of Ulmus pumila, and that the leaf recalled 'Hoersholmiensis',Martin Tijdgat and Karlijn Raats, 'Genoeg te kiezen in iepenland', p.66, boomzorg.nl 'conjectured that 'Pioneer' might not be a straight cultivar of Ulmus × hollandica, but a complex hybrid with Heybroek's clone '215' ('Commelin' × (U. pumila 'Pinnato-ramosa' × U. minor 'Hoersholmiensis')), used in the breeding of 'Homestead', in its parentage.M. Tijdgat, 'Ulmus – Gebruikswaarde- en sortimentsonderzoek in de praktijk', Dendroflora 56, 2020. p.70; edepot.wur.nl

Description

'Pioneer' is a fast-growing tree distinguished by a dense, globular crown, which as it matures becomes more broad than tall, like its U. glabra parent,Arthur Lee Jacobson, 'Plant of the Month, 2008' (arthurleej.com/p-o-m-Oct08.html). Photographs of 'Pioneer' elm: www.ca.uky.edu and plants.bachmanslandscaping.com and casting a heavy shade. The leaves are deep green, and similar in shape to the Wych Elm,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. colouring yellow and red in the fall.Photograph of autumn colouring of young 'Pioneer' elms, University of Washington campus: Arthur Lee Jacobson, 'Plant of the Month, 2008' The perfect, apetalous wind-pollinated flowers appear in early March.

Pests and diseases The tree's 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. is somewhat less than more recent American hybrids, and for this reason the tree was omitted from the elm trials in eastern Arizona conducted by the Northern Arizona University. 'Pioneer' is also severely damaged by the elm leaf beetle Xanthogaleruca luteola, sustaining more foliar damage (50%) than any other cultivar in an assessment conducted as part of the National Elm Trial at U C Davis,McPherson, G. et al. (2008). National elm trial: Initial report from Northern California. Western Arborist, Fall 2009, pp 32-36. and Japanese beetle.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. Tolerance of elm yellows in the United States was also 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

The tree's foliage was adjudged "resistant" to black spot by the Plant Diagnostic Clinic of the University of Missouri .

Cultivation Considered "quite hardy in Saint Paul", and "definitely a good selection for the Twin Cities (St. Paul and Minnesota) urban forest" although very different in appearance to the American elm.Giblin, C. P. & Gillman, J. H. (2006). Elms for the Twin Cities: A Guide for Selection and Maintenance. University of Minnesota. 'Pioneer' has had a very limited introduction to Europe, featuring in street tree trials in several Dutch cities in the late 1990s. The tree has reputedly been planted in Preston Park, Brighton, and along Tisbury Road, Hove, but does not feature in the NCCPG National Elm Collection held there.

Accessions North America Bartlett Tree Experts, US. Acc. nos. 85–0195, 85–0199 Denver Botanic Gardens, US. No acc. details. Dawes Arboretum, Newark, Ohio, US. 1 tree (2022), accession no. D1986-0021.001Ulmus × hollandica 'Pioneer', The Dawes Arboretum, Arboretum Explorer (Newark, Ohio) dawesarb.arboretumexplorer.org Dominion Arboretum, Ottawa, Canada. No acc. details. Holden Arboretum, US. Acc. no. 90–58 New York Botanical Garden, US. Acc. no. 955/97 Smith College, US. Acc. no. 5603. University of Idaho Arboretum, US. One tree. Acc. no. 1990013 U S National Arboretum , Washington, D.C., US. Acc. no. 76233.

Europe Grange Farm Arboretum, Sutton St. James, Spalding, Lincolnshire, UK. Acc. No. 544

Nurseries North America Bailey Nurseries , St. Paul, Minnesota, US. ForestFarm , Williams, Oregon, US. J. Frank Schmidt & Son Co. , Boring, Oregon, US. Johnson's Nursery , Menomonee Falls, Wisconsin, US. Pea Ridge Forest , Hermann, Missouri, US. Sun Valley Garden Centre , Eden Prairie, Minnesota, US.

Europe Batouwe Boomkwekerij B.V., , Dodewaard, Netherlands. Noordplant , Glimmen, Netherlands. Van Den Berk (UK) Ltd., , London, UK Westerveld Boomkwekerij B.V. , Opheusden, Netherlands.

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.