Common mullein (Verbascum thapsus L.) is a self-seeding biennial which has become an invasive species in many natural areas. Although a plant that technically lives only two years in lifespan seems like a small or negligible threat to native species, the durability of mullein seeds and their adaptability to a wide variety of growing conditions, among other attributes, serve as a good illustration of how a non-native plant can crowd out diversity.
Mullein plants first appear as rosettes of silver-green, fuzzy leaves, approximately 3 inches high and 6 inches in circumference. Leaves are attractive, reminiscent of the lambs ears plant, offering a soft background color to brighter blooms. Very often, lifespan is established because these small, blending rosettes add visual charm without taking space from other plantings.
The plant emerging from a mullein location during its second year is visually an enormous cousin to the first. Plants keep the general rosette cluster of leaves, but they can be up to a foot in length, and the rosette can be a foot or more in height. Extending upward, a gradually blooming flower stalk can bring the full plant height to over 6 feet. Roots are not enormous, but plant size makes it a competitor for flower-bed nutrition, sun and water. Second-year mullein needs space--and takes it, often crowding or shading out anything close by.
Maintaining the Second Season
Ideally, you can spot second-year mullein early and pull it out by hand. Waiting till the flower stalk blooms is poor--and potentially painful--planning. Mullein's multi-budded flower stalk provides sustenance and shelter to hornets, wasps and bees, who will happily defend their floral summer house against all interlopers, including weed-pulling gardeners. Flying tenants leave as the time comes for mature seeds to leave the parent plant and disperse.
Other Mullein Hazards
In addition to crowding out other plants and oversupplying a garden with stinging insects, the mullein plant illustrates one more disadvantage of alien plants: alien pests. The mullein plant serves as summer host to mullein plant bugs, which damage fruit tree crops. Moving back and forth between woody tree twigs and mullein, bugs can create serious problems for commercial fruit growers. Their close association with the mullein plant suggests that removing this host may remove the problems for fruit trees as well.
Perpetuation after Two Years
Mullein seeds are numerous and durable, making it possible for new first-year plants to sprout in the same location or close to established second-year plants. Seeds tolerate a number of growing conditions, able to make the best of poor soil and little water, and the Plant Study Group notes that seeds remain viable for a number of years. Vigilance is required, therefore, to keep mullein-prone areas weeded thoroughly and to spot and dispose of new first-year plants. Without consistent watching, the two years of a mullein plant's natural lifespan transform into many years of annoying mullein.