Common Mullein Control: Herbicide Choice and Application Timing

 

Common mullein (Verbascum thaspsus L.) has been a problematic invasive weed in Nebraska for a number of years. In in the past two years however, there has been a greater concern among ranchers and landowners in central and western Nebraska about controlling common mullein. Several articles have been previously posted on CropWatch regarding the biology and management of common mullein, and common mullein herbicide options are included in the annually updated Guide to Weed Management in Nebraska.

So why the recent increasing interest in common mullein? The past few winters have favored survival of weed species that overwinter, including winter annuals such as cheatgrass and marestail (horseweed), and common mullein a biennial. Common mullein seed germinates in the spring and spends the first year in a vegetative rosette. After surviving the winter in this vegetative form, common mullein bolts and moves into a reproductive growth habit to later produce seed. Mild, wet winters help ensure good germination the following year for new seedlings and good winter survival of plants entering the second year. The last few winters have benefited common mullein. Common mullein can especially thrive on sites that have considerable amounts of plant residue at, or near the soil surface.

 

 

Another reason for greater interest in common mullein is a frustration among some stakeholders who are not achieving effective control of common mullein. Often stakeholders have been disappointed in the level of control a particular herbicide product provides, and they want to know what might work better. So what product should you use to control common mullein?

 

Effective weed control in range, pasture, and CRP depends on selectivity and timing. Selectivity simply means the herbicide you choose to use will control the weed you wish to target. Timing refers to when an herbicide will be applied. In selecting an herbicide for common mullein control, there are a lot of options (Table 1).

 

When choosing a herbicide product for use in rangeland or CRP, picking a product with a mixture of more than one active ingredient (Table 1) (for example: Graslan®L, GrazonNext®HL, DuraCor™) is a safe bet as products with more than one active ingredient may control a larger number of broadleaf weeds and potentially offer a larger application window compared to a single active ingredient product. However, all the products listed in Table 1 do claim to control common mullein. When timing of application is specified on the various product labels, the recommendation is always before bolting.

 

Common mullein bolts in the early summer of its second year of growth. Applying an herbicide after bolting likely will control first year common mullein that is present but miss the second-year mullein that has already bolted. A fall application can control common mullein before it overwinters, but the herbicide likely will not persist in the soil into the following spring when new seedlings emerge. A spring application in April or May is generally the best time to control common mullein as you can control both the newly emerged weeds, and those which overwintered before they bolt.

 

 

So why have some stakeholders observed poor mullein control even when using registered products? Either the application was not well-timed or some labeled products do not provide good enough control. In response to stakeholders, the Panhandle Range Management and Integrated Weed Management Programs put out a trial in April of 2020 to test which herbicide products are providing the best control of common mullein. Herbicides were applied on April 21st, after spring germination of first year common mullein had started but before the second year mullein had bolted.

 

At the time of application, no common mullein plants were larger than 4” in height. Rather than use products with more than one active ingredient, the choice was made to only use standalone herbicide products (Table 2). Additionally, each herbicide was applied both with 0.25% v/v non-ionic surfactant (NIS), and as a separate treatment with 1% v/v crop oil concentrate (COC) added.

 

 

Common mullein control was assessed in early August, after first year plants had germinated and second year plants had bolted. In non-treated plots, the average density of common mullein was 15 plants per yd2. 2,4-D and dicamba are not labeled for common mullein control, but were included as they are both commonly included in labeled herbicide product mixtures. 2,4-D provided poor control, only reducing mullein density by 22% and 29%, with NIS and COC respectively.

 

Dicamba however reduced common mullein density by 93% and 100% with NIS and COC, respectively. All other products, which are labeled for common mullein control, resulted in 100% common mullein control. There was no significant difference in the levels of control provided by labeled products. Product performance also was not affected the adjuvant (NIS or COC) used.

 

We chose to conduct this herbicide trial in April as it is an ideal time to control common mullein. Had the herbicide applications been delayed to late May, or June, there may have been more differences in common mullein control between herbicide products. However, by keeping the application timely (i.e. early in the growing season), we were able to control common mullein with all labeled products, and even with dicamba which is not recommended for common mullein control.