Great, or common, mullein (Verbascum thapsus) is a flowering perennial known as woolly mullein, so-called because of its furry leaves. The plant produces copious seeds and is a prolific spreader. It was introduced to the U.S. from Europe in the mid-18th century, possibly because of its purported medicinal qualities.
In its first year of growth, the mullein grows low to the ground, producing rosettes of bluish-gray-green leaves that have a velvety, felt-like surface. Beginning with the second year, the mullein produces leafy spikes or stalks of yellow flowers; each flower has five petals. A few flowers bloom at one time on each stalk from June to August. The leaves, larger at the base of the plant, alternate along the flowering stalks. The bottom leaves grow up to 4 inches wide and 12 inches long. As the leaves progress up the plant they get smaller and narrower.
Mullein likes the sun. It grows in meadows, neglected pastures, vacant lots, dry waste areas and alongside highways and railroads. Mullein prefers dry sandy soils, but will thrive elsewhere; it needs a growing season of at least 140 days and annual rainfall from 3 to 6 inches.
The mullein develops a tap root in its first summer of growth. The next spring, the mature plants produce seeds during the summer then die. The flowers at the base of the stalk bloom first, followed by those higher up. Longer stalks produce more seeds and can continue to flower into early fall. Seeds that drop and stay near the surface are more likely to generate. A single mullein plant is said to produce from 100,000 to 180,00 seeds that may remain capable of germinating for more than 100 years.
Mullein is an invasive plant that can push its way into fields and gardens, crowding out indigenous and cultivated plants. The U.S. Department of Agriculture has determined that the European curculionid weevil (Gymnaetron tetrum) matures in the mullein seed capsules and can devour up to half of the seeds. The mullein moth, Cucullia verbasci, is also considered a relatively safe biological control agent. Herbicides containing the active ingredient triclopyr, which is selective to broadleafed plants, are used to eradicate mullein. Herbicides containing the active ingredient glyphosate can eradicate mullein, but they kill all plants, including desirable ones.
Mullein contains coumarin, glycosides, saponins and mucilage that alternative health proponents assert have medicinal properties. Mullein has been used since ancient times to treat breathing ailments and problems with the skin and throat. Teas made from the leaves have been used to treat bronchitis, dry cough and hemorrhoids. Preparations made from its flower oil have been used to treat boils, carbuncles warts and other skin conditions. The usefulness of mullein to treat these and other ailments for which it is widely touted are based on anecdotal evidence. There have been no clinical studies of the medicinal usefulness of mullein or any of its components.