Invasive of the Week: Common Mullein

By Katie Stannard

Common mullein, Verbascum thapsus, is known by its upright appearance, fuzzy leaves, and yellow flowers. From the Latin “mollis,” meaning soft, even nicknames sound benign: velvet leaf, flannel plant, big taper, cowboy toilet paper. But it’s location, location, location combined with how (seeds) and where (open sites) that elevate it from common weed to invasive plant in 20 states, including Wisconsin, Ohio, Illinois, and all western states. A biennial forb (a herbaceous flowering plant that’s not a grass) native to Eurasia and Africa, common mullein was introduced in the 1700s, and noted in Michigan by 1839.

Preferring sunny, dry soils though highly adaptable, it requires open areas like meadows and forest openings for growth. It can form dense, persistent stands quickly as it grows faster than native plants, then endures due to copious, long-lived seeds. Pastures, industrial areas, and sites with poor to average soil disturbed by logging, fire, and storms are also ideal: the more soil disturbance, the more seeds emerge and germinate. While an ephemeral plant in some locations, on others, once established, it’s very difficult to eliminate.

Mullein roots.

Native Americans utilized it for ceremonial and other purposes, as an aid in teething, rheumatism, cuts, and pain. It’s also used for a variety of traditional herbal and medicinal purposes for coughs and other respiratory ailments. Some sites indicate use by early settlers as a fish bait or poison. Additionally, methanol derived from mullein has been utilized as mosquito larvae insecticides.

Mullein seeds.

Mullein flowers.

First year growth is a rosette of flannelly, oblong, whitish green decurrent leaves (leaves that partially wrap around stems and grow down or along the stems) around 4-12 inches long and 1-5 inches wide, covered in woolly hairs, all anchored by thick, deep taproots. While the hairs can cause a skin reaction in humans, they prevent evaporative water loss, and also disperse the effect of wind on the plant. Second year growth is an unbranched long stem which bolts upward, 5-10 feet, with leaves tapering in size toward the top of the stalk. Tiny yellow 5-petaled flowers lead to ovoid fruit capsules. Some research notes a relationship between stalk length and blooming–if taller, it may bloom into October. Seeds generally don’t fall far from the parent plant–around 3 feet on average, and common mullein does not reproduce vegetatively. 

The problem with mullein is this: it’s an incredibly prolific seed producer, whose seeds persist in the seed bank for decades: a parent plant produces 100,000-180,000 seeds. (One seed study noted 232,000 seeds from a single plant!)  However, seeds germinate in water and open soil–and can lie dormant in the soil for decades–more than 100 years!!!–before germinating. One study in Denmark involved successfully germinating common mullein seeds of an archaeological soil sample dated to 1300 A.D.

Ironically, one research article referred to a 1921 study with seeds from familiar territory: “In the laboratory, newly harvested common mullein seeds collected from the University of Michigan’s Botanical Gardens germinated at over 90% in the light and about 2% in the dark. Older seeds were also light sensitive. When seeds were in soil or sand, germination in dark conditions was better, 24% to 34%.” 

Control methods depend on the circumstances of the affected site. One positive: mullein seeds require bare ground for germination, so sowing a series of native plants and grasses that emerge early in the season can help to reduce mullein plants from growing. Pulling is ideal, especially before the flowers set seed, as the tap root is more easily removed than tap roots of many other invasive plants. Cut, remove, dispose of all obvious flowering parts; avoid disturbing the soil as that’s a certain welcome mat for other mullein plants to grow. Biological controls have been tested in Europe for extensive infestations and only where warranted, (as they’re not like animals that can be herded back to the barn when they’ve done their work!): the mullein moth (Cucullia verbasci) feeds on all plant parts, and European curculinoid weevil (Gymnaetron tetrum) larvae grow in seed capsules and reduce seed production around 50%. Chemical use is utilized in sites where hand-pulling is dangerous, difficult or impossible (steep slopes, inaccessible areas where soil disturbance would actually lead to greater germination, etc.).

Sources:

common mullein: Verbascum thapsus (Scrophulariales: Scrophulariaceae)

Plant Conservation Alliance®s Alien Plant Working Group Common Mullein Verbascum thapsus L. Figwort family (Scrophulariaceae)

Native American Ethnobotany Database

Herb to Know: Mullein (Verbascum thapsus)

Photo credits: Bugwood.org

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