Reproducing only by seed.
Stems of second-year plants erect, 30-180cm high, with harshly spiny, irregularly lobed, leaf-like wings running lengthwise on all stems and branches except just below each flower head.
First-year plants forming a large, circular, nearly flat rosette, each leaf often 30cm long by 10cm wide, bright green to gray-green, margins deeply lobed, the lobes close together, twisted and wavy, with long, sharp spines pointing in all directions. The actual upper and lower surfaces of the leaf blade and its lobes (apart from the harshly spiny margins) are finely woolly-hairy and soft to the touch. The stem leaves of second-year plants similar to rosette leaves but gradually smaller and less lobed upwards, alternate (1 per node).
Flower heads single on slender, smooth, long, bare (not spiny-winged) stalks at ends of branches and from axils of upper leaves, each head large, 4-7.5cm across but occasionally smaller, with no ray florets but with many, large, bright purple disk florets, these surrounded by an involucre of many, overlapping, broad-based, greenish bracts with outward- or backward-pointing, long, sharp spiny tips. The heads at end of stems and branches usually bent to one side ("nodding"), those from leaf axils often nearly erect. The seeds are light brown, shiny, 4mm long, egg-shaped with a small knob at the tip and a pappus (parachute) of short, unbranched (non-plumose), light beige hairs. Flowers from June to October.
Nodding thistle is common throughout southern Ontario in pastures, waste places, roadsides and around buildings, especially on coarse-textured soils.
Flowering plants are distinguished by their large, showy, bright purple flower heads surrounded by an involucre of broad-based bracts narrowed to long, sharp, outward- or backward-pointing spiny tips and their heads nodding on long, non-winged stalks at ends of stems or upright in leaf axils (compare with Plumeless thistle); non-flowering plants and first-year rosettes are distinguished by their deeply lobed, bright green to gray-green leaves, the lobes much twisted, wavy and spiny margined, and both upper and lower surfaces (apart from the spiny margins) always finely woolly-hairy and soft to the touch, in contrast to the harshly, almost prickly-hairy surfaces of Bull thistle. Plants which seem to be intermediate between Nodding thistle and Plumeless thistle may be hybrids between these two species.
A. Basal rosette. B. Flower head on naked stalk.