Perennials or subshrubs, 50-150 cm. Stems mostly sparsely puberulent (hairs upcurved or apically bent). Leaves opposite; petioles 0-5 mm; blades lanceolate to triangular-lanceolate, (2-)3-6 × 0.8-2 cm, bases rounded, margins shallowly serrate to subentire, apices acute (not acuminate), abaxial faces eglandular or minutely gland-dotted, adaxial sparsely strigose to hispidulous. Involucres (3-)3.5-4 mm. Phyllaries: outermost ovate-elliptic to obovate to narrowly oblong-lanceolate, puberulent, margins herbaceous, usually weakly ciliate. Corollas white, (2-)2.4-2.8 mm, lobes glandular. Cypselae 1.6-1.8 mm, sparsely hispidulous.

Flowering Sep-Nov(-Dec). Shaded rocks along streams, crevices, oak woodlands; 800-1500 m; Ariz., N.Mex.; Mexico.

Koanophyllon palmeri is native primarily to western Mexico (Sinaloa, Chihuahua, and Sonora to Durango and Jalisco; R. McVaugh 1984; B. L. Turner 1997). Similar plants (known as Eupatorium palmeri var. tonsum B. L. Robinson) occur in near-coastal localities from Michoacan to Colima, Jalisco, Nayarit, and Sinaloa, Mexico.