The tubular flowers of castilleja tenuiflora
are rather larger and more prominent that those of most other castilleja species, and the exposed section is often longer than the lower portion, enclosed by the red calyx. Flowers are greenish at the upper end, yellow to pale orange towards the base. The calyx is unequally four lobed, and similar in appearance to the surrounding red bracts. Calyx, bracts and corolla are covered by short white hairs. The stigma is slightly exserted from the end of the corolla tube, while the four stamens remain largely hidden.
Stems are rigid and relatively tall, up to 3 feet, and much branched. Leaves are unlobed and have entire edges, often purplish. Both sides of the leaves have a covering of white hairs, the hairs longer on the undersurfaces. Stems are also hairy.