Its small, yellow flowers are densely grouped on a tall stem, which grows from a large rosette of leaves.

It is a common weedy plant that spreads by prolifically producing seeds, and has become invasive in temperate world regions.

[1] It is a minor problem for most agricultural crops, since it is not a competitive species, being intolerant of shade from other plants and unable to survive tilling.

[9] Flowers are pentamerous with (usually) five stamen, a five-lobed calyx tube, and a five-petalled corolla, the latter bright yellow and an 1.5–3 cm (0.59–1.18 in) wide.

[11] For the purpose of botanical nomenclature, Verbascum thapsus was first described by Carl Linnaeus in his 1753 Species Plantarum.

[19] The taxonomy of Verbascum has not undergone any significant revision since Svanve Mürbeck's monographs in the 1930s, with the exception of the work of Arthur Huber-Morath, who used informal grouping in organizing the genus for the florae of Iran and Turkey to account for many intermediate species.

Since Huber-Morath's groups are not taxonomical, Mürbeck's treatment is the most current one available, as no study has yet sought to apply genetic or molecular data extensively to the genus.

[21][22][23][24] As Verbascum thapsus is the type species of the genus the application of article 22 of the ICNafp gives sect.

[28] In V. t. crassifolium, the hairiness is less dense and often absent from the upper part of the anthers, while lower leaves are hardly decurrent and have longer petioles.

Some of the more whimsical ones included "hig candlewick", "Indian rag weed", "bullicks lungwort", "Adams-rod", "hare's-beard", and "ice-leaf".

[43] V. thapsus has a wide native range including Europe, northern Africa, and Asia, from the Azores and Canary Islands east to western China, north to the British Isles, Scandinavia, and Siberia, and south to the Himalayas.

[5] It has been introduced throughout the temperate world, and is established as a weed in Australia, New Zealand, tropical Asia, La Réunion, North America, Hawaii, Chile, Hispaniola, and Argentina.

[49] In the United States, it was imported very early in the 18th[note 3] century and cultivated for its medicinal and piscicide properties.

[51] In Canada, it is most common in the Maritime Provinces and southern Quebec, Ontario, and British Columbia, with scattered populations in between.

[19][52] Great mullein most frequently grows as a colonist of bare and disturbed soil, usually on sandy or chalky ones.

This ability to grow in a wide range of habitats has been linked to strong phenotype variation rather than adaptation capacities.

[10] This dormancy is linked to starch degradation activated by low temperatures in the root, and gibberellin application bypasses this requirement.

While it can also grow in areas where some vegetation already exists, growth of the rosettes on bare soil is four to seven times more rapid.

The flowering period of V. thapsus lasts from June to August in most of its range, extending to September or October in warmer climates.

[59] Other bird species have been reported to consume the leaves (Hawaiian goose)[60] or flowers (palila),[61] or to use the plant as a source when foraging for insects (white-headed woodpecker).

[68] It is also a potential reservoir of the cucumber mosaic virus, Erysiphum cichoraceum (the cucurbit powdery mildew) and Texas root rot.

Most of the pests found were western flower thrips (Frankliniella occidentalis), Lygus species such as the tarnished plant bug (L. lineolaris), and various spider mites from the family Tetranychidae.

[68] Other insects commonly found on great mullein feed exclusively on Verbascum species in general or V. thapsus in particular.

[70] The plant's ability to host both pests and beneficials makes it potentially useful to maintain stable populations of insects used for biological control in other cultures, like Campylomma verbasci and Dicyphus hesperus (Miridae), a predator of whiteflies.

[71][72] A number of pest Lepidoptera species, including the stalk borer (Papaipema nebris) and gray hairstreak (Strymon melinus), also use V. thapsus as a host plant.

[10] Phytochemicals in V. thapsus flowers and leaves include saponins, polysaccharides, mucilage, flavonoids, tannins, iridoid and lignin glycosides, and essential oils.

[75] Leaves were smoked to attempt to treat lung ailments, a tradition that in America was rapidly transmitted to Native American peoples.

[31][76] The Zuni people, however, use the plant in poultices of powdered root applied to sores, rashes, and skin infections.

[31] Topical application of various V. thapsus-based preparations was recommended for the treatment of warts,[78] boils, carbuncles, hemorrhoids, and chilblains, amongst others.