Phyllaries 3-5 mm, apices not cuspidate. Flowering Jul-Oct(-Nov). Moist forests, cove forests; 10-800 m; N.B., N.S., Ont., Que., Sask.; Ala., Ark., Conn., Del., Fla., Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Mich., Minn., Miss., Mo., Nebr., N.H., N.J., N.Y., N.C., N.Dak., Ohio, Okla., Pa., R.I., S.C., S.Dak., Tenn., Tex., Vt., W.Va., Wis. Plants with narrow leaves, generally in the southwest part of the range of Ageratina altissima, have been recognized as var. angustata and were so mapped by A. F. Clewell and J. W. Wooten (1971), who indicated that all var. angustata occurs west of the Mississippi River and that this taxon was completely congruent in distribution with var. altissima. The present treatment confirms the westward tendency toward size reduction and observes that narrow-leaved plants occur widely through the southeast United States (including Arkansas, Kentucky, Louisiana, North Carolina, Oklahoma, Tennessee, and Texas). The transition is gradual and the region of intergradation is wide. In Texas, where the leaves mostly are narrow, plants with broad, cordate leaves are scattered through the range.
From Flora of Indiana (1940) by Charles C. Deam
Frequent to common in most of the dry and moist woods of the state. It is more common in beech and sugar maple and black and white oak woods. This plant is poisonous to grazing animals and if it is eaten in a sufficient quantity it proves fatal. A symptom of having eaten too much of this weed is a trembling of the animal and because of this characteristic, the disease has been called "trembles." The plant is frequently eaten by sheep and by cattle when the pasturage becomes scarce, and many of those animals are killed in Indiana each year by this weed. When it is eaten by milch cows, the poisonous principle (a barium salt) is communicated to the milk; such milk, when consumed by people, has the same effect as the plant has upon stock. The pioneers called it "milk sickness," and many of them died from drinking too much of the affected milk. A pioneer informed me that a family of four in my own county died from this cause. Indiana specimens show some variation in leaf form. All of my specimens are generally densely short-pubescent in the inflorescence and on the upper half of the stem, and in a few plants the stem is villous. (See Rhodora 10: 87. 1908.) The leaves of all of my specimens are abruptly cuneate at the petiole except in my Lake and Warren County specimens in which they are slightly cordate at the base.