During the 1st year, the Great Mullein plant consists of a rosette of basal leaves about 2.5 cm to 5 cm across. During the 2nd year, it becomes up to 2.5 m tall and is usually unbranched. Occasionally, one or two side stems may develop in the upper half of the plant. These stems are covered with downy white hairs. The leaves are up to 30 cm long and 10 cm across, becoming progressively smaller and more narrow as they ascend the central stem. They are smooth and covered with fine downy hairs.
The central stem terminates in a dense spike of flowers up to 5 cm long. Each flower is about 4 cm across and pale yellow in color. The blooming period usually occurs during the summer and lasts about 1½ months. Only a few flowers are in bloom at the same time.
Each flower is replaced by a seed capsule with 2 cells, each cell containing numerous little seeds. The rectangular-oblong seeds have fine wavy ridges and tiny pits across the surface.
While the foliage withers away, the central stalk and its seed capsules turn brown and persist through the winter. The seeds are small enough to be carried aloft by the gusts of wind that shake the central stalk. This plant spreads by reseeding itself.
The root system consists of a stout taproot that runs deep into the ground.
Bumblebees are the most important pollinators of the great mullein, where they seek nectar and pollen. The seeds of Great Mullein are too small to be of much interest to birds, while the hairy foliage is not eaten by deer or elk, making it a great plant for gardeners who deal with browsing deer.