2. Helianthus debilis Nutt. (N-Am.) – A very rare and always ephemeral alien. First recorded as a wool alien in the Vesdre valley near Ensival in 1911. Probably last seen in 1954 in Brussel on wasteland near the former botanical garden, most likely as an escape from cultivation.
Plants in cultivation of Helianthus debilis belong to subsp. cucumerifolius (Torr. et A. Gray) Heiser (syn.: H. cucumerifolius Torr. et A. Gray, H. debilis var. cucumerifolius (Torr. et A. Gray) Torr.), characterised by erect stems and rather small, shallowly serrate leaves (25-90 mm). Helianthus debilis often looks like H. annuus and might have been overlooked. It usually has smaller heads on erect peduncles and the inflorescence is sometimes branched (1-3 heads).
Wain R.P. (1982) Genetic differentiation in the Florida subspecies of Helianthus debilis (Asteraceae). Amer. J. Bot. 69(10): 1573-1578.