Lamium galeobdolon

Lamium galeobdolon (gah-lay-OB-dough-lon), the yellow archangel, is a species of flowering plant in the mint family Lamiaceae. It is native to Europe and western Asia but it is widely introduced in North America and elsewhere. It is the only species in the genus Lamium with yellow flowers. Another common name for this species is golden dead-nettle. In New Zealand, it is called the aluminium plant or artillery plant. The common names archangel and dead-nettle have been in use for hundreds of years, dating back to at least the 16th century.

Lamium galeobdolon consists of four closely-related subspecies, one of which is the variegated yellow archangel (Lamium galeobdolon subsp. argentatum), an invasive subspecies in several European countries. Lamium galeobdolon is also invasive in New Zealand, Canada, and the United States, but in those countries the subspecies is unknown.

Description
Lamium galeobdolon sensu lato is a large-leaved perennial plant with square stems growing from 15 to 60 cm tall. The paired opposite leaves are stalked with toothed margins. The leaves may or may not have silvery markings (an important character used to distinguish subspecies). The inflorescence is a pseudo-whorl of 4–16 flowers (called a verticillaster) clustered around the axil of a leaf-pair. Each flower has bilateral symmetry (zygomorphic). The calyx is five-lobed and the corolla is yellow with a prominent hood. The flower’s lower lip has three lobes with the central lobe often streaked with orange. There are two short stamens and two long ones. A pair of fused carpels give rise to a four-chambered schizocarp.

Identification
Subspecies of Lamium galeobdolon sensu lato are the only taxa in the genus with yellow flowers. All of the subspecies are stoloniferous except subsp. flavidum. The invasive subsp. argentatum is readily distinguished by its silvery white variegated leaves. Lamium galeobdolon subsp. galeobdolon and subsp. montanum can be difficult to distinguish but assuming the plant in question is in full flower, subsp. galeobdolon has a maximum of six (rarely seven) flowers per verticillaster while subsp. montanum averages ten flowers per verticillaster.

Lamium galeobdolon subsp. flavidum is characterized by the lack of stolons, frequently branched fertile stems with upright flowering lateral shoots, and 10–16 small flowers per verticillaster. An identification key for the remaining taxa follows:

Other taxa in the group can have marked leaves but those of subsp. argentatum are distinctive:

"Permanent silvery pattern on leaf lamina in L. argentatum always forms two flexuous bands along the midrib, mostly not interrupted by the lateral veins. They are distinctive particularly from the late autumn to the early spring when a contrast brownish-maroon colouring develops on the abaxial side and extends to the midrib-zone of the upper leaf side. Although variegated leaves were frequently observed in all other taxa in our study, they never matched the description above. Silvery-grey markings constituted merely separate splashes or flecks (small-sized in L. montanum and L. flavidum, larger in L. galeobdolon), not merged into continuous bands. Thus, a combination of two distinct silvery flexuous bands together with a strong chocolate-maroon zone along the midrib in winter can be regarded as specific L. argentatum markers."

In particular, a cultivar of subsp. flavidum known as 'Hermans Pride' has silver-spotted leaves, but even though the markings are unlike those of subsp. argentatum, the two taxa are sometimes confused.

The base chromosome number of taxa in genus Lamium is x = 9. Lamium galeobdolon subsp. flavidum and subsp. galeobdolon are diploid (2n = 2x = 18) while subsp. argentatum and subsp. montanum are tetraploid (2n = 4x = 36). A few triploid individuals have been reported. The triploids are thought to be natural hybrids between diploid subsp. galeobdolon and tetraploid subsp. argentatum.

Taxonomy
Lamium galeobdolon was first described as Galeopsis galeobdolon by the Swedish botanist Carl Linnaeus in 1753. Linnaeus himself placed Galeopsis galeobdolon in the genus Lamium in 1759, and therefore the correct name is Lamium galeobdolon (L.) L.

Most authorities with a global scope recognize four closely-related taxa, either as subspecies of Lamium galeobdolon, or as full species in genus Lamium, in which case the group of taxa is referred to as Lamium subgen. Galeobdolon. Some authorities segregate the species into other genera (either Galeobdolon or Lamiastrum) but genus Lamium remains monophyletic in any case.

Lamium galeobdolon subsp. argentatum was first described as Galeobdolon argentatum by the Czech botanist Miroslav Smejkal in 1975. In a comprehensive and influential treatment of genus Lamium published in 1989, the Dutch botanist Jacob Mennema reduced the species to forma and placed it in synonymy with Lamium galeobdolon subsp. galeobdolon. Hence subsp. argentatum was neglected for a long time and its invasion history is poorly documented.

Most North American authorities recognize a single taxon, referred to as either Lamium galeobdolon or Lamiastrum galeobdolon. Multiple taxa are not recognized in North America. Authors that follow Mennema's taxonomy refer to either Lamium galeobdolon sensu lato or Lamium galeobdolon subsp. galeobdolon.

There are a number of closely related taxa that hybridize with L. galeobdolon and in some cases are not unequivocally accepted as distinct species but considered subspecies or varieties by many authors. Most well known among these is variegated yellow archangel (subsp. argentatum), whose leaves often have variegation, showing as silver patches arranged as a wide semicircle. This, and in particular its large-flowered and even stronger-marked cultivar 'Variegatum', is the taxon most often met with as a garden escapee.

Etymology
Both the generic name Lamium and the specific name galeobdolon were used by Pliny the Elder in the first century AD. The name galeobdolon has several possible origins. It may come from the Latin words galeo meaning "to cover with a helmet" and dolon meaning "a fly's sting", or it may come from the Greek words galen meaning "weasel" and bdolos meaning "fetid smell". The latter interpretation is likely the source of the common name yellow weasel-snout.

The common names archangel and dead-nettle have been in use for hundreds of years. In 1578, the Flemish physician and botanist Rembert Dodoens observed that "Dead nettell groweth every where". The English herbalist John Gerard used the word "archangel" in 1633. Gerard believed the dead-nettles were so-named because their leaves resembled those of the true nettles in the family Urticaceae. The names "Arch-Angel" and "Dead Nettle" appear in the book The English Physitian (also known as the Complete Herbal) published by the English botanist and herbalist Nicholas Culpeper in 1652. Both Dodoens and Gerard believed the name "archangel" referred to members of genus Lamium with "clusters of hooded flowers at the nodes rising along the stems like a choir of robed figures". Others believe the name refers to the wing-like shape of its paired opposite leaves.

Distribution
Lamium galeobdolon sensu lato is native to Europe, western Asia, and portions of the Middle East. It ranges northeast of Europe as far as the West Siberian Plain and southeast to Iran. Lamium galeobdolon subsp. flavidum is native to the Alps, northern Italy, and the mountain ranges northeast of the Adriatic Sea. The remaining subspecies have a center of distribution in Central Europe.

Lamium galeobdolon sensu lato is widely introduced in Europe, New Zealand, Canada, and the United States, but authorities disagree about the distribution of introduced subspecies. , Plants of the World Online (POWO) claims that Lamium galeobdolon subsp. argentatum has been introduced in Great Britain, Ireland, and Italy, but since subsp. argentatum is reported to be introduced in the Netherlands and Switzerland, the list is incomplete. POWO also claims that Lamium galeobdolon subsp. galeobdolon has been introduced in Madeira, New Zealand (both North Island and South Island), and the United States. However, authoritative sources based in North America implicitly refer to Lamium galeobdolon sensu lato, so POWO's claim is unsubstantiated. The list of U.S. states where Lamium galeobdolon sensu lato is said to occur varies dramatically depending on the source.

Ecology
Lamium galeobdolon sensu lato (also known as Lamiastrum galeobdolon) has been widely introduced as a garden plant, especially in New Zealand, Canada, and the United States where it has escaped cultivation and become an invasive species. In New Zealand, it is listed by the 2020 National Pest Plant Accord and therefore banned from sale, propagation, and distribution throughout the country. It is also listed by the Invasive Species Council of British Columbia in Canada. In the U.S. state of Washington, it is listed as a Class B Noxious Weed and therefore banned from sale by state law. It is also a B-listed Noxious Weed in the state of Oregon.

The variegated yellow archangel (Lamium galeobdolon subsp. argentatum) has become an invasive subspecies in several European countries. In the Netherlands, subspecies argentatum was introduced as an ornamental ground cover, and by 1985 it had become naturalized and recorded in more localities than the native subspecies galeobdolon. It is also invasive in Britain where it spreads by stolons at the rate of 1–2 meters per growing season. In Switzerland, subspecies argentatum tripled its occurrence in four decades (1980–2020) while exhibiting "a higher growth rate and regeneration capacity" than the native subspecies galeobdolon.

Based on morphological characters, number of chromosomes, pattern of geographical distribution, and phytochemistry, early botanists hypothesized that the tetraploid subsp. montanum originated as a hybrid between the two diploid taxa, subsp. flavidum and subsp. galeobdolon. However, more recent phylogenetic data suggests that subsp. montanum originated from subsp. flavidum alone. Similarly, there is conflicting evidence regarding the origins of the tetraploid subsp. argentatum suggesting that it may or may not have originated from subsp. galeobdolon alone.