Ulmus_'Frontier' The American hybrid cultivar Ulmus '' is a U. S. National Arboretum introduction derived from a crossing of the European Smooth-leafed, yclept Narrow-leafed, Elm Ulmus minor subsp. minor with the Chinese Elm Ulmus parvifolia in 1971. Released in 1990, the tree is a unique example of the hybridization of spring-flowering and autumn-flowering elms.