A Pesky Invasive That Can be Managed

Posted by: Nancy Klasky, Broomfield County Extension

You may have seen common mullein (Verbascum thapsus) scattered along roadways, mountain canyons, rangelands, or in forests. The tall stalky plant has yellow blooms gathered at the top and large sage-colored leaves at the base. It grows profusely after major disturbances like fire, construction, or flooding. Common mullein is an introduced species and is present in all 50 states and Canada. Originally brought to North America for medicinal purposes, it has been designated a C List Noxious Weed by the Colorado Department of Agriculture, which means it is up to private and public entities to manage. 

I got to learn a lot about mullein after the High Park Fire west of Fort Collins. This lightning- ignited fire started in June of 2012 and due to the hot, dry spring, it burned over 87K acres and destroyed 259 homes before being contained a month later. I worked on land in this area at the time and saw first-hand both the devastating effects of the fire and the renewal of the forest that followed. After all, when occurring naturally, fire is a cleansing process that actually keeps our forests healthy. If left unmanaged, accumulated duff (forest litter) and smaller shrubs and trees can create dense, overgrown forests that can cause cataclysmic wildfires. Some regular burning keeps this in check.

Common mullein before mitigation work.

Tiny seeds find their way into crevice of rocks and form first year rosette. 

The land I worked on is a mixed forest just over 8,000 feet in elevation. It’s primarily ponderosa pine, with some Douglas fir and aspen. Ponderosa pine evolved with the wildfires. Thick bark protects them from destruction by low-intensity fires, and natural loss of lower branches reduces “ladder fuel” that allows fire to climb to the crown of the tree. Much of the surrounding land was turned into a moonscape by the High Park Fire, with 100% mortality of trees and other plants. On the managed land, many ponderosa survived, but most other vegetation was destroyed. This created the ideal environment for common mullein to grow, and boy did it grow. Where once there was an abundance of wildflowers, kinnik-kinnik, and common juniper shrubs, there was now a forest of mullein.

Mullein seeds will survive a fire and can remain viable for 100 years! They may have been in the area for decades, unable to germinate until this disturbance. Worse yet, each of these plants can produce 100,000 to 250,000 seeds per terminal spike. I had my work cut out for me, but I was determined to see this beautiful property restored to its natural state. 

Common mullein is a biennial plant, which means it takes two years to grow to its mature height and bloom. The first-year plant is a basal rosette that stays low to the ground and does not flower. If the plant lives to the next year it will bolt up and bloom. Management techniques that work with this lifecycle are important to stopping the spread of this invasive plant and not exacerbating the problem. 

Last years deadheaded stalks with no new mullein growing!

Like I mentioned mullein likes disturbed ground, so how do you get a plant out of the ground without creating more disturbance? First, you only want to remove the plant if it is a first year rosette. The roots are shallow and usually easy to pull up. It’s important to try and press down the soil as you pull the plant out. Yanking the plant out will give the seeds you know are there a better chance. If the plant has made it to the second year, which was the case with many plants by the time we got to them, you want to cut the bloom off before it starts to dry and drop its seeds. With the help of great groups of volunteers we were able to make many trips to the property to work on the mullein issue. Each year we saw less of the plant come back, and each year we worked to remove what did pop up. With persistence and determination, we saw the native wildflowers, kinnick-kinnick, and juniper shrubs take back what was theirs! The beauty of the restoration is when the native vegetation grows back, that is enough to keep the mullein from growing. There are other management options to consider, but I found the mechanical removal of this plant the best one for this area. 

The property with it's natural vegetation having taken back over.

For more information go to the Colorado Dept. of Ag's page on Common Mullein.