The tree has a broader crown than its siblings', whilst the generally obovate leaves, < 11 cm long by 7 cm wide, are less acuminate at the apex. Like 'Lobel', the tree flushes markedly later than most other elms, and is rarely in full leaf before mid-May.
Pests and diseases
Whilst the resistance of 'Clusius' to Dutch elm disease proved marginally greater than that of its Dutch predecessors, rated 4 out of 5, it has been eclipsed by later developments such as Lutèce. Consequently, sales in the Netherlands declined from over 8,000 in 1979 to 600 in 2004.
While 'Clusius' remains for sale in Europe, its planting where the disease remains prevalent cannot be recommended. 'Clusius' featured in New Zealand government trials during the 1990s at the Hortresearch station, Palmerston North, but is not known to have been introduced to North America.
^Heybroek, H. M., Goudzwaard, L, Kaljee, H. (2009). Iep of olm, karakterboom van de Lage Landen (:Elm, a tree with character of the Low Countries). KNNV, Uitgeverij. ISBN978-90-5011-281-9
^Hiemstra, J.A.; et al. (2007). Belang en toekomst van de iep in Nederland [Importance and future of the elm in the Netherlands]. Wageningen, Netherlands: Praktijkonderzoek Plant & Omgeving B.V. Retrieved 26 October 2017.
^Burdekin, D.A.; Rushforth, K.D. (November 1996). Revised by J.F. Webber. "Elms resistant to Dutch elm disease"(PDF). Arboriculture Research Note. Alice Holt Lodge, Farnham: Arboricultural Advisory & Information Service. 2/96: 1–9. ISSN1362-5128. Retrieved 26 October 2017.