About Verbascum thapsus L.
Verbascum thapsus L. is a dicotyledonous plant that forms a leaf rosette during its first year of growth. Its leaves are large, reaching up to 50 cm in length. In the second year, plants typically grow a single unbranched stem, usually 1 to 2 m tall; however, in the eastern part of its range in China, it has only been reported to reach a maximum height of 1.5 m. Tall, pole-like stems end in a dense spike of flowers that can make up half the total length of the stem. All parts of the plant are covered in star-shaped trichomes, which are particularly thick on the leaves and give the leaves a silvery appearance. The chromosome number of this species is 2n = 36. On flowering individuals, leaves are arranged alternately along the stem. They are thick and decurrent, with considerable shape variation between upper and lower stem leaves, ranging from oblong to oblanceolate, and can reach up to 50 cm long and 14 cm across (19 inches long and 5 inches wide). Leaves become smaller higher on the stem, and are less strongly decurrent further up. The flowering stem is solid and 2 to 2.5 cm (nearly an inch) across, and it may occasionally branch just below the inflorescence, usually after damage. After flowering and seed release, the stem and fruits typically persist through winter, drying into dark brown, stiff structures holding densely packed ovoid dry seed capsules. Dried stems may remain standing into the following spring or even the next summer. The plant grows a shallow taproot. Flowers are pentamerous, usually with five stamens, a five-lobed calyx tube, and a five-petalled corolla; the corolla is bright yellow and 1.5–3 cm (0.59–1.18 in) wide. Flowers are almost sessile, with very short pedicels only 2 mm (0.08 in) long. The five stamens are of two distinct types: the three upper stamens are shorter, their filaments covered in yellow or whitish hairs and bearing smaller anthers, while the lower two stamens have glabrous filaments and larger anthers. The plant produces small, ovoid capsules 6 mm (0.24 in) long that split open along two valves to release seeds. Each capsule holds large numbers of tiny brown seeds less than 1 mm (0.04 in) in size, marked with longitudinal ridges. A white-flowered form, V. thapsus f. candicans, is known to exist. Flowering can last up to three months from early to late summer (June to August in northern Europe), starting at the bottom of the spike and progressing irregularly upward; each individual flower opens for only part of a day, and only a small number of flowers open at the same time around the stem. V. thapsus has a wide native range covering Europe, northern Africa, and Asia, extending from the Azores and Canary Islands east to western China, north to the British Isles, Scandinavia, Siberia, and Kashmir, and south to the Himalayas. In northern Europe, it grows from sea level up to an altitude of 1,850 m, while in China it grows at altitudes between 1,400 and 3,200 m. It has been introduced across the temperate world, and is established as a weed in Australia, New Zealand, tropical Asia, La Réunion, North America, Hawaii, Chile, Hispaniola, and Argentina. It has also been reported to occur in Japan. It was imported to the United States very early in the 18th century and cultivated for its medicinal and piscicide properties. By 1818, it had spread so widely that Amos Eaton believed it was a native plant. It was already recorded in Michigan in 1839, and in California in 1876. Today it is commonly found across all U.S. states. In Canada, it is most common in the Maritime Provinces, southern Quebec, Ontario, and British Columbia, with scattered populations between these regions. Great mullein most often grows as a pioneer colonizer of bare, disturbed soil, usually preferring sandy or chalky soil. It grows best in dry, sandy, or gravelly soils, though it can grow in a variety of habitats including banksides, meadows, roadsides, forest clearings, and pastures. Its ability to grow in such a wide range of habitats is linked to strong phenotype variation rather than distinct adaptation capacities. Great mullein is a biennial that generally requires winter dormancy before it can flower. This dormancy is tied to starch degradation activated by low root temperatures, and application of gibberellin can bypass this requirement. Seeds almost exclusively germinate in bare soil, at temperatures between 10 and 40 °C. While seeds can germinate in complete darkness if other conditions are suitable, with tests recording a 35% germination rate under ideal dark conditions, in the wild they only germinate when exposed to light, or when positioned very close to the soil surface, which explains the species' observed habitat preferences. Even though it can grow in areas that already have some vegetation, rosette growth on bare soil is four to seven times faster. Seeds germinate in spring and summer. Seeds that germinate in autumn produce plants that overwinter if they are large enough, while rosettes smaller than 15 cm (6 in) across die over winter. After flowering, the entire plant usually dies at the end of its second year, but some individuals, especially in the northern parts of its range, require a longer growth period and flower in their third year. Under favorable growing conditions, some individuals even flower in their first year. Triennial individuals have been found to produce fewer seeds than biennial and annual individuals. While the year of flowering and plant size are linked to environmental conditions, most other characteristics of the species appear to be genetic. Each individual flower is open for only one single day, opening before dawn and closing in the afternoon. Flowers are self-fecundating and protogynous, with female parts maturing first, and will self-pollinate if they have not been pollinated by insects during the day. While many insect species visit the flowers, only some types of bees actually achieve pollination. In most of its range, V. thapsus flowers from June to August, extending into September or October in warmer climates. Common flower visitors include halictid bees and hoverflies. The hair on the lower stamens may act as a foothold for visiting pollinators. V. thapsus seeds can retain their ability to germinate for decades, with some studies recording germination after up to 100 years. Combined with the fact that the plant is extremely prolific at producing seeds, with each plant growing hundreds of capsules that each hold up to 700 seeds, for a total of up to 180,000 or 240,000 seeds per plant, this means seeds remain viable in the soil seed bank for very long periods. Seeds can sprout from apparently bare ground, or shortly after forest fires, long after any previous plants of the species have died. Its typical population pattern consists of an ephemeral adult population followed by a long period of dormancy as stored seeds. Great mullein rarely colonizes new areas without human intervention, because its seeds do not disperse very far. Seed dispersion requires the stem to be moved by wind or animal movement; 75% of seeds fall within 1 m of the parent plant, and 93% fall within 5 m. Megachilid bees of the genus Anthidium use the plant's hair, alongside hair from other woolly plants, to construct their nests. Seeds are generally too small for most birds to feed on, though American goldfinch have been recorded consuming them. Other bird species have been reported to eat the leaves (Hawaiian goose) or flowers (palila), or to use the plant when foraging for insects (white-headed woodpecker). Additionally, deer and elk eat the plant's leaves. Although V. thapsus has long been used in herbal medicine, no pharmaceutical drugs are manufactured from its components. Dioscorides first recommended the plant as a folk remedy for pulmonary diseases 2000 years ago. Leaves were smoked to try to treat lung ailments, a tradition that was quickly transmitted to Native American peoples in North America. The Zuni people use powdered root from the plant in poultices applied to sores, rashes, and skin infections, and use a root infusion to treat athlete's foot. All preparations intended for consumption must be finely filtered to remove the irritating trichomes. Flower oil was used to treat catarrhs, colics, earaches, frostbite, eczema, and other external conditions. Topical application of various V. thapsus-based preparations was recommended for treating warts, boils, carbuncles, hemorrhoids, and chilblains, among other conditions. Glycyrrhizin compounds with in vitro bactericide effects have been isolated from V. thapsus flowers. The German Commission E describes uses of the plant for respiratory infections. It was also included in the National Formulary of the United States and United Kingdom. The plant has been used in attempts to treat colds, croup, sunburn, and other skin irritations. Roman soldiers are said to have dipped the plant's stalks in grease to use as torches. Other cultures have used the leaves as wicks. Native Americans and American colonists lined their shoes with the plant's leaves to block out cold. Mullein may be cultivated as an ornamental plant. As noted by Pliny the Elder in his Naturalis Historia, great mullein was historically associated with witches, though the relationship was generally ambiguous, and the plant was also widely believed to protect against curses and evil spirits. The seeds contain several compounds (saponins, glycosides, coumarin, rotenone) that are toxic to fish, and have been widely used as a piscicide for fishing. Because it can become weedy, the plant is not often cultivated, unlike other species in the Verbascum genus such as V. phoeniceum.
