Ulmus × hollandica 'Pioneer'
|Ulmus × hollandica|
'Pioneer', Netherlands. Photo: Ronnie Nijboer
|Hybrid parentage||U. glabra × U. minor|
The elm hybrid cultivar Ulmus × hollandica 'Pioneer' is an American clone arising from the crossing of two European species, Wych Elm U. glabra and Smooth-leaved Elm U. minor subsp. minor. Raised by the USDA station at Delaware, Ohio, in 1971, 'Pioneer' was released to commerce in 1983.
'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, and casting a heavy shade. The leaves are deep green, and similar in shape to the Wych Elm, colouring yellow and red in the fall. 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, 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., and Japanese Beetle. Tolerance of Elm Yellows in the USA was also found to be poor. 
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. 'Pioneer' has had a very limited introduction to Europe, featuring in street tree trials in several Dutch cities in the late 1990s.
- Bartlett Tree Experts. Acc. nos. 85-0195, 85-0199
- Denver Botanic Gardens. No acc. details.
- Dominion Arboretum, Ottawa, Canada. No acc. details.
- Holden Arboretum. Acc. no. 90-58
- New York Botanical Garden. Acc. no. 955/97
- Smith College. Acc. no. 5603.
- University of Idaho Arboretum. One tree. Acc. no. 1990013
- U S National Arboretum , Washington, D.C., USA. Acc. no. 76233.
- Brighton & Hove NCCPG Collection: Tilbury Road, Hove (5 trees)
- Grange Farm Arboretum, Sutton St. James, Spalding, Lincolnshire, UK. Acc. No. 544
- Bailey Nurseries , St. Paul, Minnesota
- ForestFarm , Williams, Oregon
- J. Frank Schmidt & Son Co. , Boring, Oregon
- Johnson's Nursery , Menomonee Falls, Wisconsin
- Pea Ridge Forest , Hermann, Missouri
- Sun Valley Garden Centre , Eden Prairie, Minnesota
- 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
- 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.
- Photograph of autumn colouring of young 'Pioneer' elms, University of Washington campus: Arthur Lee Jacobson, 'Plant of the Month, 2008' 
- 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
- McPherson, G. et al. (2008). National elm trial: Initial report from Northern California. Western Arborist, Fall 2009, pp 32-36.
- 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.
- 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
- Giblin, C. P. & Gillman, J. H. (2006). Elms for the Twin Cities: A Guide for Selection and Maintenance. University of Minnesota.
- 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.