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. ^[1] 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. ^[2] The hybrid was first reported from the wild in the Chicago region in 1950 and was provisionally named U. × notha Wilhelm & Ware in 1994. ^[3]

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. ^[1]

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. ^{[1] [4]}

Atypically, fertility of the hybrid seed is high, occasionally in excess of 90%, ^[5] while Collins found in artificial hybridization no reduction of fertility in the F1 or F2 generations. ^[6]

Pests and diseases

Morton Arboretum report the hybrid susceptible to DED and Elm Yellows (Elm phloem necrosis). ^[7]

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. ^{[8] [9] [7]}

Cultivars

'Hamburg' may also belong to the Ulmus × intermedia group. ^[10]

References