How do you get rid of Verbascum thapsus?

Answer 1

Here's something I found on a site and I think it may be really helpful.

HOW CAN I GET RID OF IT?

The best method for controlling common mullein depends on the size of the infestation, the topography of the site, and the resources available. Timing is critical for efficient control, and follow-up is essential.

 Physical control:

Manual/mechanical methods: Perhaps the most effective method of controlling common mullein is to cut plants with a weed hoe. Plants will not resprout if cut through the root crown below the lowest leaves (Gross and Werner 1978). Removing rosettes with a hand hoe can be easily accomplished by workers trained to recognize the plant. Hand hoeing can be selective and effective, and two workers may clear up to twenty acres of mullein in a few hours. Bolted plants can also be removed with a weed hoe. Sometimes bolted plants can be pulled out of sandy soil, especially following heavy rain. If plants have begun to set seed, cut off the flowering racemes with pruning shears just below the lowest seed pods and collect them in a bag to prevent seeds from being released during the hand removal operation. A second or third weeding may be necessary.

Mowing appears to be ineffective, as plants cut above the root crown do not die. Rather, the basal rosette will continue to enlarge, then later bolt and flower. Clipping the terminal flower stalk will not prevent flowering, but will cause increased growth of axillary branches, which will produce flowers later (Gross and Werner 1978).

Prescribed burning: Burning kills bolted plants and appears to kill rosettes, but creates open areas for reinfestation from seed germination. Individual bolted plants can be killed using a flame thrower, but its use is to be avoided during fire season.

 Biological control:

Insects and fungi: No insects or diseases have been approved for introduction as biological control agents against common mullein in North America. A curculionid weevil, Gymnaetron tetrum Fab., was accidentally introduced into Canada from Europe (Maw 1980). This weevil is specific to common mullein and is considered one of two natural enemies significantly impacting the plant in Europe. The larvae feed on seeds and other tissues in the seed capsules. Larvae are able to destroy all seeds in a capsule when present; however, usually not all seed capsules are infested. Gross and Werner (1978) report that up to 50 percent of seeds may be destroyed by the larval feeding of this weevil. Over time, G. tetrum has spread into California and has been collected from mullein plants throughout northern and eastern California since 1942. While its impact has not been investigated in California, it is unlikely to have much impact on common mullein populations. Despite the rate of seed destruction, too many seeds remain for it to have much effect in controlling common mullein populations.

Grazing: Grazing animals generally will not eat mullein because of its hairy leaves (Whitson 1992).

 Chemical control:

Common mullein is difficult to control with herbicides because the thick hairs on the leaves prevent the herbicide from reaching and penetrating the leaf surface. A surfactant is recommended for all liquid herbicides used to control this plant.

Zamora (1993) compared the effectiveness of several herbicides to control common mullein along a roadside in Montana. Herbicides were applied with a backpack sprayer calibrated to deliver 20 gal/acre at 42 psi. Treatments were applied to late rosette and bolting plants in late May. Plant height and number of plants surviving to flower were recorded at the end of July. Of the three compounds available in California, 2,4-D provided 66 percent kill at 1.9 lbs/acre; height of standing plants averaged nineteen inches. Glyphosate provided 100 percent kill; height averaged nine inches. Sulfometuron (as Oust®) also provided 100 percent kill; average plant height was six inches. For comparison, in the unsprayed control areas, all plants survived to flowering (zero mortality); average plant height was thirty-eight inches (1 m).

Another control method, recently developed by a forest weed manager, is to spray each rosette with glyphosate by putting the spray nozzle into the center of the rosette (DiTomaso, pers. comm.). The applicator touches the plant with the spray nozzle and gives it one good squirt. The key is to ensure that the herbicide penetrates the region of the plant where the growing point is located. If the nozzle is off-center, this method does not work. Only seedlings and rosettes are susceptible using this method. In treating individual plants, it is recommended that a dye be used in the herbicide mixture to mark treated plants and prevent re-treatment.

If you want to know everything about the plant except the getting rid of them part, please see the attached link. I got the above information from that very site.