|Hybrid parentage||U. pumila × U. rubra|
The hybrid cultivar 'Willis' originated as a selection made by a Mr Minnick of Kansas simply as a hybrid. Almost certainly derived from a crossing of the Siberian Elm Ulmus pumila and the Red Elm Ulmus rubra, it was originally believed that the American parent was the American Elm Ulmus americana, not the Red Elm.
'Willis' was described as having a smooth grey bark, large leaves similar to U. americana, and capable of very rapid growth.
Pests and diseases
First marketed by the Willis Nursery Co., Ottawa, Kansas, circa 1959, it is not known whether the cultivar remains in cultivation in the USA, nor whether it was ever introduced to Europe or Australasia.
- Green, P. S. (1964). Registration of cultivar names in Ulmus. Arnoldia, Vol. 24. Arnold Arboretum, Harvard University. 
- Santamour, F. S., & Bentz, S. E. Updated checklist of elm (Ulmus) cultivars for use in North America. Journal of Arboriculture, 21(3): May, 1995