Verbascum thapsus
(common mullein)

V. thapsus is a biennial herb native to Europe, north Africa and western and central Asia. It has become naturalized in most temperate regions of the world, where it can be locally abundant on roadsides, railroads, fence rows, old fields, pastures and other open, disturbed areas (PIER, 2014; USDA-NRCS, 2014). Once established, it exhibits vigorous growth and threatens native plants in meadows and forest gaps. Each individual can produce 100,000-175,000 seeds that can remain viable for more than 100 years, making established populations of V. thapsus difficult to eradicate (Gross and Werner, 1982; ISSG, 2014).

V. thapsus has a broad native range including Europe, north Africa and western central Asia. It has been introduced to Japan, Sri Lanka, Reunion, the Americas, Australia and New Zealand. It is naturalized in temperate regions, where it can become abundant in open, disturbed areas (PIER, 2014; USDA-NRCS, 2014).

V. thapsus is widely available as an ornamental and is still used as a herbal remedy, so escape is a continuing threat. However, the risk of introduction and reintroduction in managed areas may be minimal; it produces many seeds, but has no specialized mechanism for dispersal. In addition, the seedlings require open, bare ground and may not establish well in late successional communities.

In its native range, V. thapsus is commonly found on dry, rocky hillsides, disturbed areas and open woodland. It prefers dry sandy soil, but can be found in a variety of well-drained soils in meadows and forest gaps. It also grows well in open, disturbed areas such as pastures, old fields, along fence rows and roadsides, and in urban areas (Hoshovsky, 2001; ISSG, 2014).

In Hawaii, where it is introduced and invasive, it can be found in 'open sites, cinder cones, subalpine woodland and shrubland to alpine desert, 1,550-2,350 m' altitude; in New Zealand, it is found on 'poor stony or gravelly pastures, screes, stony river beds, roadsides and railways, shingle river banks and beds, dry waste places, sea level to c. 1000 m' altitude; and in Chile it grows from coastal areas up to the tree line, 2000 m altitude, particularly in dry, arid areas with 6-10 month drought periods and 100-800 mm of annual precipitation (PIER, 2014).

Extracts from V. thapsus were reported to have some allelopathic effects on wheat seedlings, but soil cultivation practices such as ploughing usually prevent the establishment of V. thapsus in active agricultural areas (Gross and Werner, 1978).

Genetics

The chromosome number commonly reported is 2n = 36 (30, 34) (IPCN Chromosome Reports, 2014; Missouri Botanical Garden, 2014).

Reproductive Biology

V. thapsus is cross-pollinated by short and long tongued bees, but flowers can also be autogamous. If cross-pollination has not occurred, V. thapsus will self-pollinate when the flower closes at the end of the day. It is a prolific seed bearer and a single plant may produce 100,000-175,000 seeds (Gross and Werner, 1978).

Physiology and Phenology

Seedlings are pioneer species on bare or disturbed sites. Seedling emergence and survival is limited by available light (ISSG, 2014). They emerge in early spring and form rosettes that grow into late autumn. In the second spring, the rosette forms a flowering stalk. Flowering occurs June through August, but may extend into early October. The stalk may remain erect past senescence until disturbance from wind or a large animal releases the seeds from the capsule (Gross and Werner, 1978).

Kumschick et al. (2013) found genotypes of V. thapsus from its introduced range grew larger than native genotypes under more arid conditions. Increased biomass, a strong response to increased water availability, and low root to shoot ratios suggest that V. thapsus in its introduced range exhibits a fast-growing, weedy phenotype instead of root adaption to low-water environments. Genotypes from its introduced range were also more resistant to herbivory.

In another study of invasive V. thapsus, young leaves had 6.5 times the concentration the toxic iridoid catalpol present in old leaves. This greatly reduced herbivory from generalists on young leaves, resulting in minimal losses of high-quality tissue and increased performance (Alba et al., 2013). This may help explain its success in its introduced range.

Conversely, Seipel et al. (2015) found that V. thapsus does not consistently exhibit increased performance in its invaded range. Instead, plant height, number of flowering branches and population density were more strongly influenced by local climatic differences among regions. The authors suggested that studies on this species comparing native and non-native ranges should include both plant and population characteristics from multiple regions in both ranges, as well as along environmental gradients.

Longevity

V. thapsus is biennial, rarely surviving a third (vegetative) year. Seeds can remain viable for more than 100 years in soil seed banks, and viable seeds have been found in the soil samples dated to 1300 A.D. (Gross and Werner, 1982).

V. thapsus is susceptible to several fungi and coleopteran larvae (Gross and Werner, 1978). Powdery mildew (Erysiphe cichoracearum) and root rot (Phymatotricum omnivorum) can infect V. thapsus, but they also affect agricultural species (Hoshovsky, 2001; ISSG, 2014). The larva of the mullein moth (Cucullia verbasci) usually feeds specifically on V. thapsus, but may also feed native plant species (Hoshovsky, 2001; Remaley, 2005).

Materials

• Dyestuffs

Medicinal, pharmaceutical

• Traditional/folklore

Ornamental

• Seed trade

Verbascum blattaria (moth mullein) is similar to V. thapsus. The two plants have overlapping distributions and habits, and may hybridize (Gross and Werner, 1978). However, V. blattaria is not as robust as V. thapsus and it does not have the dense pubescence.

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Physical/Mechanical Control

Small populations of V. thapsus can be removed by hand, and this is recommended in sensitive areas where other control methods may cause a greater disturbance. Often large-scale removal creates areas of bare ground that allows the establishment of new V. thapsus populations. However, normal cultivation can control the plant. Plants are best controlled during the early rosette stage when tap roots are shallow and seeds have not formed (Remaley, 2005; ISSG, 2014; PIER, 2014). For second year plants, mechanical removal before the seed capsules mature significantly reduces viable seed production and reduces changes of re-establishment (Wilbur and Hufbauer, 2012).

Cultural Control

V. thapsus seedlings are not shade tolerant and bare ground is required for emergence. Sowing invaded sites with early successional native species may decrease germination and emergence of V. thapsus seedlings. Late successional native species planted among V. thapsus invasions may also outcompete it for light and space.

Due to the dense pubescence, V. thapsus is unpalatable to cattle and sheep. However, goats will graze on it and may be useful in controlling dense infestations. It has been suggested that chickens may deplete the seed bank where mature plants have been removed (ISSG, 2014).

Biological Control

The European curculionid weevil (Gymnetron tetrum) is a seed predator specific to V. thapsus. It was introduced to North America as a biological control with some success. The larva of this beetle matures in the seed capsules and can destroy up to 50% of the seeds (Gross and Werner, 1978).

Chemical Control

Spot application of 2% glyphosate, triclopyr or tebuthiuron mixture applied to first year rosettes can control V. thapsus. However, the dense epidermal hairs can reduce the effectiveness of aqueous solutions, and a follow-up treatment at half concentration may be needed to achieve long-term control (Hoshovsky, 2001; Remaley 2005; ISSG, 2014). Broadcast foliar herbicides are effective in dense populations of V. thapsus where exposure to non-target species is minimal (Hoshovsky, 2001). Rosettes are particularly sensitive to metsulfuron (PIER, 2014).

30/03/15 Original text by:

Jeff Masters, University of Louisville, USA