Coefficient of Conservatism:
Coefficient of Wetness:
This is one species surely native in Michigan, where it favors moist sites: wet prairies, fens, sedge meadows, tamarack swamps; river banks and floodplain forests; borders of upland forests and wet depressions; marshes, and ditches.
Helianthus giganteus is an immensely variable species, The leaves are rarely opposite or whorled, and may be densely gland-dotted beneath. The roots may be thickened or tuberous.
This species hybridizes with H. grosseserratus, producing H. ×luxurians E. Watson; it is said to differ from H. giganteus in having petioles 2–3 cm long and stems green, yellow, or light red (rather than dark red or purple), although one or both of these characters is lacking in many of the specimens examined. It differs from H. grosseserratus in having stems pubescent (rather than glabrous) and phyllaries usually pubescent and with marginal cilia (not glabrous or with short scattered hairs). The hybrid with H. divaricatus is mentioned under that species.