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Control options for the nonnative common mullein are of increasing interest to land managers in the west. Common mullein is a prolific seed producer, with a single plant able to produce well over 100,000 seeds. We found that mechanical control of common mullein before mature seed capsules developed along the raceme significantly reduced viable seed production. Seeds from immature capsules had very low viability (early reproductive stage 5 0.08%, 95% CI = 0.06%, 0.67%; mid reproductive stage = 1.52%, 95% CI = 0.49%, 3.11%). This information allows managers to time their management efforts so that they can reduce the amount of plant material that must be disposed of in order to control the spread of common mullein seeds.
Nomenclature: Common mullein, Verbascum thapsus L.
Key words: physical control, mechanical control, management, seed viability.
Common mullein (Verbascum thapsus L.), a native of Eurasia, is a weedy herbaceous plant that was naturalized in North America by the 18th century (Gross and Werner 1978). It is semelparous and follows a biennial lifecycle throughout most of its range with some annual forms found further south, and triennials occurring in the northern range limits (Rienartz 1984a). In midtemperate latitudes where it is biennial, seedlings germinate in the spring and develop into rosettes. In the second growing season, an inflorescence forms that can reach up to 2 m (6ft) in height (Gross 1980, Reinartz 1984a, Whitson et al. 1996).
Each flower along the inflorescence develops into a twohalved capsule that can house as many as 500 small seeds (H.Wilbur and R. Hufbauer, unpublished data, 2009). An indeterminate flowering pattern results in plants simultaneously having mature seed capsules with a brown and brittle casing, immature seed capsules with a green and pliable casing, and open flowers. More mature seed capsules are generally found lower on the plant, as the inflorescence continues to extend upwards and produce flowers throughout the growing season (Gross and Warner 1978, Reinartz 1984a, Whitson et al. 1996). Seed limitation is likely rare for common mullein, as a single healthy plant may produce 136,000 (Gross and Werner 1978) to 223,000 (Stevens 1932) seeds in its lifetime. Furthermore, seeds can remain viable in the seed bank for at least 100 years (Kivilaan and Bandurski 1981).
Individuals are most abundant...