Ulmus × intermedia Elowsky is a natural hybrid elm occurring across Nebraska and several other Midwestern states, derived from the crossing of Ulmus rubra and Ulmus pumila. As Red Elm U. rubra is far less fertile, and highly susceptible to Dutch elm disease (:DED), it could eventually be hybridized out of existence by U. × intermedia. Schlautman, B., Zalapa, J., & Brunet, J. (2012). Genotypic and phenotypic characterization of Ulmus pumila (Siberian elm), Ulmus rubra (Red elm) and their hybrids. Proc. Nebraska Acad. Sci. 2012. 106–107. The hybrid was first reported from the wild in the Chicago region in 1950 and was provisionally named U. × notha Wilhelm & Ware in 1994.
The horticulture industry made a number of artificial crosses of the two species, such as 'Coolshade' and 'Fremont', in an attempt to create ornamental trees resistant to DED.
Description Ulmus × intermedia is significantly different from both parental species, except in biometrics such as leaf length to width ratios, number of teeth, petiole length, and pollen size. The leaves are 4.5–12 × 2.5–6 cm, petiole 0.3 × 1.2 cm, ovate to lanceolate, apex acuminate to acute, base oblique. The samarae are 11.5–21.0 × 10.0–20.0 mm, cream to white."Herbarium specimen 0030674MOR ". Morton Arboretum Herbarium. Sheet labelled U. × notha Wilhelm & Ware, Chicago region (1993); Ulmus pumila x Ulmus rubra, Richland Center, Wisconsin; Missouri Botanical Garden leaves specimen (2015); Ulmus pumila x Ulmus rubra, Richland Center, Wisconsin; Missouri Botanical Garden flowers specimen (2015)
Atypically, fertility of the hybrid seed is high, occasionally in excess of 90%,Kaul, R. B. (1995). Natural and fertile hybrids of the native red elm, Ulmus rubra, with the introduced Siberian elm, U. pumila, are now producing hybrid swarms in Nebraska. Proc. Nebraska Acad. Sci. 105: 28–29. while Collins found in artificial hybridization no reduction of fertility in the F1 or F2 generations.Collins, P. E. (1967). Hybridization studies in the genus Ulmus. Ph.D dissertation, Univ. of Minnesota, Minneapolis.
Pests and diseases Morton Arboretum report the hybrid susceptible to DED and elm yellows (Elm phloem necrosis).
Cultivation Specimens of unnamed Ulmus × intermedia ( = Ulmus × notha) stand (2015) in Morton Arboretum, Illinois, received as Ulmus pumila from North Platte Agricultural Experiment Station, University of Nebraska.Photographs of Ulmus × intermedia, Morton Arboretum, 'Ulmus Series', acorn.mortonarb.orgU. × notha, Morton Arboretum, vplants.org
Cultivars
'Hamburg' may also belong to the Ulmus × intermedia group.U. pumila x U. rubra 'Hamburg', in Handbuch der Ulmengewächse, www.ulmen-handbuch.de
References