Timing is Key for Managing Common Mullein Invasion
Common mullein (Verbascim thapsus) is an increasing concern to grassland managers as the aggressive forb spreads from old fields, disturbed areas, and rights-of-ways into healthy, native grasslands. This invasion has prompted state and county officials in Nebraska, Colorado, and Wyoming to list the weed as a state or county noxious weed.
Common mullein spreads solely by seeds that typically germinate in the fall and exist as a rosette, a circular leaf formation at ground level, for a full growing season without producing a stalk or seed. Leaves are light green, egg shaped, and densely covered in hair significantly obstructing herbicide contact and discouraging most grazers.
Stems are hairy and can grow from 1 to 7 feet tall without branching. Flowers are small, 1 inch in diameter, sulfur yellow, and sprawl out along the upper portion of the stem from June to July. A single flower stem is most common, although multiple can occur. The distinct brown, dried corn-stalk-looking stems can remain standing for several years. Each plant can produce over 100,000 seeds one millimeter in diameter or smaller that remain viable in the soil for decades.
Aggressive management while the population is low is the most effective and economical control of common mullein. Preventing seed production and spread are critical due to substantial seed production and long-term seed viability. Mowing before seed formation, digging plants just below the soil surface, or removing and burning seed heads can reduce seed production while infestations are small.
Chemical control may be necessary as infestations establish and spread. Timing is key when spraying common mullein. Spring applications in late April and early May provide the greatest control for first- and second-year plants prior to bolting. Additionally, fall herbicide application on rosettes can be effective, however, herbicide is not effective to mature stalks.
Common mullein’s woolly leaves significantly reduce herbicides effectiveness. To combat this, include 0.25% v/v non-ionic surfactant or 1% v/v crop oil concentrate with herbicide to reduce the solution’s surface tension, penetrating the hair layer and improving leaf adherence, uptake, and herbicide effectiveness.
The Nebraska Extension Circular EC 130, 2021 Guide for Weed, Disease, and Insect Management in Nebraska, which is available at your local Extension office, provides herbicide recommendations and estimated costs. Recommendations for controlling common mullein include Cimarron Plus® (0.5 oz/acre; $7/acre) or Escort®XP (0.5 oz/acre; $6/acre) for rosette application and Grazon P+D® (4 pints/acre; $18/acre) or Overdrive® (6 oz/acre; $19.50/acre) on rosettes or prior to stem elongation.
A 2020 University of Nebraska-Lincoln study found no difference whether using non-ionic surfactant or crop oil concentrate with Picloram (Tordon®), Aminopyralid (Milestone®), Aminocyclopyrachlor (Method®240SL), Metsulfuron Methyl (Escort®XP), or Chlorsulfuron (Telar®XP).
Researchers at Kansas State University reported the greatest control with a combination of aminopyralid (Milestone®) and 2,4-D followed by combinations of metsulfuron methyl (Cimarron Plus®, Escort®XP, Chaparral™) + dicamba + 2,4-D or picloram (RESTRICTED USE; Grazon P+D®, Graslan® L, Tordon®) + 2,4-D.
Always read, understand, and follow herbicide label directions. It is recommended to use a herbicide that includes the target species on the label and not just the application site. Spot spraying is recommended for range and pastures because herbicides to control common mullein will also negatively impact native broadleaf plants competing with undesirable plants.
Once common mullein reaches maturity, chemical control becomes far less effective, but seed heads can be cut and destroyed to reduce the seed bank. A lone plant from one migrant seed can rapidly become a colony of prolific seed producing plants, making early detection and management a top priority. Extensive, mature infestations will likely require multiple management practices to control the plant and reduce seed spread.
Biological control utilizing insects has also been successful at limiting the spread of common mullein. The mullein seed head weevil (Gymnetron tetrum) is a natural common mullein predator in Europe and has been approved for release in North America. The weevil larvae mature in the seed capsules and feed on the seeds, helping to substantially reduce seed production. Contact a county weed superintendent to learn the protocol for obtaining and releasing biological control agents (neweed.org).
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