Late summer and autumn.
Flower-heads: growing singly, or clustered loosely in a corymb and composed of both ray and disk flowers; the rays three to five-cleft at the summit. Leaves: alternate; lanceolate; thick. Stem: one to six feet high; smooth; angled; branched.
The swamp sunflower, while greatly pleasing the eye by illuminating the low fields and swamps in the autumn, is on the high road to making itself a most disagreeable member of the floral world. The flowers of the older plants are very poisonous to animals. Usually their instinct prevents them from eating of them; but the plant is one of those insidious things for which a taste can be cultivated. Cows have been known to cultivate this fatal taste, when their milk and meat were made bitter. If the plant be eaten in great quantities the animal dies. In a dried and powdered form it causes violent sneezing, for which purpose it is well known in medicine. Once that it has established itself in a field it is most difficult to exterminate and adds one more to the trials of the poor farmer.
H. nudiflorum, purple-head sneezeweed, grows in the south and west. It blossoms from June until October. The name purple-head alludes to the disk flowers, as the rays are yellow with a brownish base.