Source: California Invasive Plant Council


URL of this page: http://www.cal-ipc.org/ip/management/ipcw/pages/detailreport.cfm@usernumber=87&surveynumber=182.php

Invasive Plants of California's Wildland

Verbascum thapsus
Scientific name   Verbascum thapsus
Additional name information:   L.
Common name   common mullein, wooly mullein, great mullein, mullein, Jacob’s staff, flannel leaf, velvet plant, candlewick plant, lung wort, felt wort
Synonymous scientific names   none known
Closely related California natives   0
Closely related California non-natives:   3
Listed   CalEPPC List B,CDFA nl
By:   Michael Pitcairn
Distribution  
verbascum-map

HOW DO I RECOGNIZE IT?
Distinctive features:  
Common mullein (Verbascum thapsus ) is a biennial or annual herb that sends up a large stalk, three to six feet tall, topped with yellow flowers closely attached to the stalk. It spends the first year as a rosette close to the ground. The leaves are large, six to twelve inches, densely woolly, and soft to the touch. Leaves are largest at the base and gradually become smaller up the stalk. Leaves on the stalk occur alternately, and the base of each leaf extends a short way down the stalk. Yellow flowers, three-quarters to one and a half inches in diameter, consist of five circular petals, and occur on the uppermost portion of the stalk.
verbascum-illus

Description:  

Scrophulariaceae. Biennial or annual herb. Stems: 1.5-6 ft (50-200 cm) tall when mature. Leaves: first-year plant is a rosette of large, woolly, gray-green leaves. Leaves 2-4 times longer than wide, 2-16 in (5-40 cm) long, alternate, the largest leaves at the base of the plant, smallest near the top; leaf pedicle short with leaf base extending a short way down the stalk to form wings. Plants usually bolt in second year and have a single stem covered with overlapping, woolly leaves from base to inflorescence. Inflorescence: a spike extending upward from top of stalk. Usually 1 inflorescence is produced, but occasionally a second or third can form with branches occurring where inflorescences begin. Flowers: densely packed along inflorescence, youngest near top; bracts 0.5-0.7 in (12-18 mm), flower pedicels short, <0.08 in (<2 mm), generally fused to stalk. Calyx deeply 5-lobed; 0.3-0.4 in (7-9 mm) long, several times longer than wide. Corolla (petals) 0.6-1.0 in (15-25 mm) wide, 5-lobed, circular, nearly regular, sulphur-yellow. Seeds: held in a 2-celled capsule 0.24 in (6 mm) diameter, covered with short, branched hairs. Seeds brown, irregular, oblong, 0.02-0.03 in (0.5-0.7 mm) long, with wavy ridges alternating with deep grooves. Each capsule holds numerous seeds. Flowering occurs from June through October (from Munz 1959, Gross and Werner 1978, and Hickman 1993).

 

WHERE WOULD I FIND IT?  

Common mullein occurs throughout California, but is particularly abundant in dry valleys on the eastern side of the Sierra Nevada. High population densities have been observed in moist meadows and creek drainages near Mono Lake and Owens Valley. It prefers disturbed habitats with little other vegetation, especially on dry, gravelly soils. It is common along roadsides, rights-of-way, and river banks and in forest cuts, meadows, pastures, and waste areas (Gross and Werner 1978). It is an early colonizer and may be the first plant to colonize bare soil. It is found in all forty-eight contiguous states and in Hawaii. In Canada it is reported to grow abundantly in soils with a pH range of 6.5-7.8 (Gross and Werner 1978). It is found from sea level to 8,000 feet (2,440 m) elevation.

 

WHERE DID IT COME FROM AND HOW IS IT SPREAD?  

Common mullein is native to Asia, but it probably was introduced to the United States from Europe. It was valued for its medicinal properties and has been carried with immigrants throughout the world. It has been used as a remedy for coughs and lung diseases, diarrhea, burns, and earaches (Mitich 1989). It probably was introduced several times into North America as a medicinal herb as well as accidentally. The earliest recorded intentional introduction was in the 1700s in the Blue Ridge Mountians of Virginia (Gross and Werner 1978). It apparently naturalized and spread rapidly, for it was erroneously described as a native by Eaton (1818) and was present as far west as Michigan in 1839 (Gross and Werner 1978). It was first recorded in California in 1880 as being widely naturalized in old fields in Siskyou County (Watson 1880). It spreads by prodigious seed production and maintains its presence by long-lived seeds in the soil. Its seeds have no specialized structures for long-distance dispersal by wind or animals. Movement of soil for highway and building construction may have assisted in its dispersal.

 

WHAT PROBLEMS DOES IT CAUSE?  

Common mullein is not a weed of agricultural crops, as it cannot tolerate cultivation. It is, however, thought to serve as a host for insects that are themselves economic pests, such as the mullein leaf bug, a pest of apples and pears in the eastern United States and Canada (Maw 1980). Common mullein is not often a significant weed of most wildlands and natural areas, as it is easily crowded out by grasses or other competing vegetation. It is a problem, however, in the sparsely vegetated soils of the eastern Sierra Nevada. In moist meadows and drainages near Mono Lake and Owens Valley, common mullein can become abundant and has invaded pristine meadows with undisturbed soils, displacing native herbs and grasses. It has also been observed to rapidly establish following forest fires in the western Sierra Nevada. High densites of rosettes appear to prevent the reinvasion of native herbs and grasses in burned areas, but eventually these give way to a developing shrub canopy. In this situation, mullein appears to disrupt the normal sequence of ecological succession.

 

HOW DOES IT GROW AND REPRODUCE?  

verbascum-large2

(click on photos to view larger image)

Common mullein reproduces solely by seed. Flowering occurs in summer, usually July through October in the eastern Sierra and June to August in the Sacramento and San Joaquin valleys. Flowers are borne on an inflorescence that occurs as an upward extension of the stalk. Individual flowers are short-lived, opening before dawn and closing

verbascum-small2

by mid-afternoon the same day. Flowers are visited by a diverse array of insects, but only the short- and long-tongued bees are effective pollinators. Flowers are also self-fertile, with self-fertilization occurring when the flower closes at the end of the day if cross-pollination has not occurred (Gross and Werner 1978).

Seeds are contained in a capsule with two cells. Field studies show that single plants produce 200 to 300 capsules with 500 to 800 seeds per capsule. Thus, seed production can be 100,000 to 240,000 seeds per plant. When dry, the capsule splits open and releases the seeds. Seeds are not adapted to dispersal by wind or animals and usually fall to the ground. Field studies report that seeds will disperse as far as eleven meters, but 75 percent fall within one meter of the parent plant (Gross and Werner 1978).

Common mullein seeds do not appear to undergo dormancy or require a period of after-ripening. They germinate rapidly under appropriate environmental conditions (Baskin and Baskin 1981). Seed germination can occur in continuous darkness (e.g., when buried) or in light. High germination rates in darkness are restricted to relatively hightemperatures (>30 degrees C). In contrast, high germination rates were observed at 0 degrees C in darkness alternating with 35-40 degrees C in light (Semenza et al. 1978). This indicates that germination is possible on soil surfaces where extreme diurnal fluctuations occur. Despite observations of seed germination in darkness in the laboratory, field studies of buried seeds show low germination rates(<15 percent) suggesting that factors other than darkness may play a role in preventing germination of buried seeds (Baskin and Baskin 1981).

Generally, only those seeds at or near the soil surface will germinate (Semenza et al. 1978). Mullein seeds can survive and remain viable for thirty-five to 100 years when buried (Gross and Werner 1978, Baskin and Baskin 1981). The presence of mullein plants immediately following soil disturbance is likely a result of the presence of a seedbank rather than dispersed seeds. In California common mullein seeds usually germinate in spring following snowmelt and in fall with the onset of rains.

Common mullein is a usually a biennial, forming a taproot and a rosette in the first year and a flowering stalk in the second year. Rosettes consist of a whorl of leaves from the root crown clustered at the soil surface. In the eastern Sierra Nevada, however, it can grow as a biennial or as a winter annual. If seed germinates in spring, the plant will remain a rosette through the first growing season and the following winter. It will then bolt in spring and flower in summer. If seed germinates in fall, the plant will enter winter as a rosette and bolt the following spring (Semenza et al. 1978). Regardless of its flowering pattern, common mullein spends the first half of its life as a rosette, producing a deep taproot before sending up a three- to six-foot stalk and producing flowers. Flowers may be produced until the first frost or snowfall in late fall.

 

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.