Weed Of The Week #11 Common Mullein (Verbascum thapsus)

Weed Of The Week #11
Common Mullein (Verbascum thapsus)
Noxious Under Alberta’s Weed Control Act
Submitted By Kelly Cooley, CoolPro Solutions
Common Mullein is a non-native, shallow tap-rooted biennial from the Figwort (Scrophulariaceae) Family. This species prefers disturbed, gravel-textured or sandy-silty soils, and is a huge issue on our gravel-bottomed creek and riverine riparian areas, displacing preferred native plants. It also does well in heavily grazed pastures, roadsides, gravel pits, old construction zones, and other disturbed areas. Seeds from Common Mullein move readily in seed-contaminated gravel and soil, including on equipment that works with these materials. Seeds will also spread in flowing water, depositing on the stream banks to germinate and spread.
Understanding Common Mullein’s biennial (two-year) life cycle is critical to maintain control of infestations! First year plants (flat to the ground rosettes) germinate from tiny seeds, and may escape detection as they establish taproots. These first year rosettes already feature this species’ distinctive soft and densely haired pale green leaves. First year Common Mullein plants do not flower, but the rosette leaves shrivel in the fall with killing frost, and the established taproot overwinters.
The following spring, second year Common Mullein plants regenerate from the overwintered taproot with much larger, overlapping, softly and densely haired pale green leaves. By June, a single, unbranched, torch-like solid and very straight center stem emerges with diminishing alternately placed leaves as it grows taller through the growing season. This spike-like stem is also densely and softly hairy to the touch. Stem height varies greatly, from a few inches tall to over 6 feet (nearly 2 metres) tall! This top third of the stem develops green, fuzzy densely packed buds which in July and August open to produce 5 petalled yellow flowers that open for just 1 day each. Flowers mature in July-August to form clusters of seed capsules, with each capsule containing up to 700 seeds, yielding up to 240,000 seeds per plant!
As Common Mullein matures to eventual death at the end of the second year, the formerly soft-hairy leaves and stems become yellow-brown, brittle, and irritating when handled with bare hands. The straight-standing, brown-black ‘skeleton’ of the dead plant may persist with seed capsules in place through the following winter and into subsequent seasons, with the stem eventually falling to the ground to break open the capsules and distribute the seeds. It is suspected that birds spread seeds from the broken capsules, but most germinate in the area around the parent plant.
Best results to control Common Mullein are with selective broad-leaved herbicide treatments of first year rosettes and early spring second year plants! If treating second year plants with herbicides, do so well prior to flowering, ensure the entire plant is thoroughly covered, and add a surfactant to help penetrate through the hairy leaves and stems.
Hand pulling and bagging second year plants is very effective, but make sure to get as much of the root as possible. Once the plant is well into flowering (August), control may be achieved by simply cutting off, bagging, and burning the flowering spikes that contain the clustered flowers and mature seed capsules. The remainder of the plant will decay and die with no risk of seed production.
Common Mullein seeds can remain viable for up to 100 years! The mistake often made is thinking one has beaten an infestation after one or two years of control efforts, because one may not see any flowering plants. More than likely, that’s because the infestation after a couple of years of control efforts is only producing first year (non-flowering rosette) seedlings. It is critical to maintain control pressure on the non-flowering rosettes in Common Mullein infestations, not letting any mature to second year flowering plants!

More information can be found in the Common Mullein Fact Sheet from the Alberta Invasive Species Council, and you can report this invader yourself using their free EDDMapS application on your mobile device. For local control options for Common Mullein, please contact our Agricultural Fieldman at 403-339-8741. 
Photo Credits:
Kelly Cooley, CoolPro Solutions

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Municipal District of Pincher Creek No. 9
Box 279
1037 Herron Avenue, Pincher Creek, AB T0K 1W0
Phone: 403-627-3130       Fax: 403-627-5070
Email: info@mdpinchercreek.ab.ca
Office Hours: By Appointment Only due to COVID-19

Municipal District of Pincher Creek No. 9
Box 279
1037 Herron Avenue
Pincher Creek, AB T0K 1W0
Phone: 403-627-3130
Fax: 403-627-5070
Email: info@mdpinchercreek.ab.ca
Office Hours: By Appointment Only due to COVID-19
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