The hybrid elm cultivar Ulmus 'Hamburg' was originally raised by the Plumfield Nurseries, Fremont, Nebraska, circa 1932, after its discovery by Mr. Lloyd Moffet in a bed of Siberian Elm Ulmus pumila seedlings from Tekamah.^[1] It was later marketed by Interstate Nurseries, Hamburg, Iowa, from 1948, as 'Interstate's New Hamburg Hybrid Elm'. Green stated that it was originally said be a hybrid of Ulmus pumila (female parent) and Ulmus americana,^[2] but the Hamburg Nurseries of Iowa made no such claim for it in their catalogues from 1948 onwards.^[3][4][5] It is now considered more likely that Ulmus rubra was the male parent, as it was also known as 'Hybrid Chinese Elm',^[2] and therefore probably synonymous with Plumfield Nurseries' 'Hybrid elm' of the same date, a known crossing of U. pumila and U. rubra,^[6][7] – and so, perhaps, also synonymous with Ulmus × intermedia 'Fremont', an elm of the same parentage found a little later in Plumfield Nurseries.

An U. pumila × U. rubra parentage would, by present classification, make the cultivar Ulmus × intermedia 'Hamburg' .

A similar erroneously conjectured crossing with U. americana occurred at Plumfield Nurseries in the 1930s, with their so-called "Wheatley hybrid elm" (apparently not the European 'Wheatley'), briefly marketed in 1934: "Our variety, an attractive round-headed rapid growing tree, with medium sized glossy leaves; originally imported from Holland and hybridized with American white elm, Ulmus americana ".^[8]