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 = 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.
Management Implications: Common mullein is an increasing priority for control in natural areas of the western United States. This biennial species produces a large number of seeds that can lay dormant in the seed bank for at least 100 years. Managers detach reproductive flowering stalks in an effort to limit seed production but then must remove the reproductive portion from the field or risk adding seeds to the seed bank. At the request of land managers, the relationship between plant phenology and seed viability was studied to facilitate timing of the removal of reproductive stalks to minimize additions of common mullein seeds to the seed bank.
This study provides specific information about the timing of removal of the reproductive inflorescences of common mullein to best limit seed set. If management efforts are carried out before seedpods turn brown on the stalk, seeds will not develop and the risk of adding additional seed to the seed bank is diminished. However, if seedpods have begun to mature, land managers should take precautionary measures by removal of stalks from the field, followed by proper disposal.