Solidago

Solidago, commonly called goldenrods, is a genus of about 100 to 120 species of flowering plants in the aster family, Asteraceae. Most are herbaceous perennial species found in open areas such as meadows, prairies, and savannas. They are mostly native to North America, including Mexico; a few species are native to South America and Eurasia. Some American species have also been introduced into Europe and other parts of the world.
Description.
"Solidago" species are perennials growing from woody caudices or rhizomes. Their stems range from decumbent (crawling) to ascending or erect, with a range of heights going from to over a meter. Most species are unbranched, but some do display branching in the upper part of the plant. Both leaves and stems vary from glabrous (hairless) to various forms of pubescence (strigose, strigillose, hispid, stipitate-glandular or villous). In some species, the basal leaves are shed before flowering. The leaf margins are most commonly entire, but often display heavier serration. Some leaves may display trinerved venation rather than the pinnate venation usual across Asteraceae. The flower is also the state flower of Kentucky.
The flower heads are usually of the radiate type (typical daisy flower heads with distinct ray and disc florets) but sometimes discoid (with only disc florets of mixed, sterile, male and bisexual types). Only ray florets are female, others are male, hermaphroditic or entire sterile. Head involucres are campanulate to cylindric or attenuate. Floret corollas are usually yellow, but white in the ray florets of a few species (such as "Solidago bicolor"); they are typically hairless. Heads usually include between 2 and 35 disc florets, but in some species this may go up to 60. Filaments are inserted closer to the base of the corolla than its middle. Numerous heads are usually grouped in complex compound inflorescences where heads are arranged in multiple racemes, panicles, corymbs, or secund arrays (with florets all on the same side).
"Solidago" cypselae are narrowly obconic to cylindrical in shape, and they are sometimes somewhat compressed. They have eight to 10 ribs usually and are hairless or moderately hispid. The pappus is very big with barbellate bristles.
The many goldenrod species can be difficult to distinguish, due to their similar bright, golden-yellow flower heads that bloom in late summer. Propagation is by wind-disseminated seeds or by spreading underground rhizomes which can form colonies of vegetative clones of a single plant. They are mostly short-day plants and bloom in late summer and early fall. Some species produce abundant nectar when moisture is plentiful, or when the weather is warm and sunny.
The section "Ptarmicoidei" is sometimes treated as a separate genus "Oligoneuron", and is distinguished by flat-topped to rounded corymbiform flowerheads.
Taxonomy.
"Solidago" is in the family Asteraceae (also known as Compositae), a diverse and widespread clade containing approximately 23,000 species and 12 tribes, which inhabit all continents except Antarctica. Within Asteraceae, Solidago is in the tribe Astereae and the subtribe Solidagininaeae.
The genus "Solidago" is monophyletic as indicated by morphological characters and molecular evidence. All "Solidago" species are herbaceous perennials, growing from approximately 2-cm to 2.5-m tall. Yellow to white, pistillate ray flowers and yellow, perfect disc florets are characteristic of "Solidago" inflorescences, which have a wide range of shapes. Molecular studies using nuclear rDNA have hypothesized boundaries on the genus "Solidago", but there have been difficulties in parsing out evolutionary relationships at the sub-genus scale, and defining which should be included and separated from "Solidago".
"Solidago" and related taxa.
Related Asteraceae genera such as "Chrysoma", "Euthamia", and "Oreochrysum" have been included within "Solidago" at one point or another, but morphological evidence has suggested otherwise. In a study comparing morphological characters of "Solidago" and related subgroups, the authors consider the subjectivity of classifying a genus, and how to define it within broader tendencies concerning the taxonomy of North American Asteraceae. Little to no differences were observed between "Solidago" and the subgroups in terms of karyotype. However, external morphological characters such as habit, or the general appearance of the plant and how a suite of traits contribute to its phenotype; pappus size; and the point of freeing of stamen filaments from the corolla tube, are useful classification schemes for "Solidago", since they are applied to differentiating between Asteraceae taxa. While one school of Asteraceae taxonomy thought unites all taxa sharing similar floral head structure and subsequently ignores deviation from this morphology; while another places greater weight on these morphological deviations. The authors argue that the latter opinion should be applied. Since there is no theoretical foundation for relative taxonomic importance of traits, they assert that habit should be a central trait when defining taxa, and subsequently that all the subgroups considered in their study ("Brachychaeta", "Chrysoma", "Euthamia", "Oligoneuron", and "Petradoria") should be segregated from "Solidago".
Results from a leaf anatomy study comparing differences in mesophyll, bundle sheath extensions, and midvein structure, among others in a suite of leaf traits, are incongruent with those in an earlier study. Based on the lack of bundle sheath extensions, it is suggested that "Chrysoma", "Euthamia", "Gundlachia", and "Petradoria" should be distinct taxa and outside of "Solidago". However, "Brachychaeta", "Brintonia", "Oligoneuron", "Oreochrysum", and "Aster" should be considered as components of "Solidago". To summarize, the relation of "Brachychaeta" and "Oligoneuron" to "Solidago" is inconsistent based on these results. Both support the separation of "Chrysoma", "Euthamia", and "Petradoria" from "Solidago". A study reviews the taxonomic position of "Oligoneuron" relative to "Solidago", as based on taxonomic evidence, treats it as separate from "Solidago", similarly to Kapoor & Beaudry (1966). The first molecular phylogeny based on chloroplast DNA, treats "Brachychaeta, Brintonia, Oligoneuron", and "Oreochrysum" as constituents of "Solidago""." Using consensus trees from ITS data, another study found support for "Oligoneuron" as part of "Solidago""," and the findings of Zhang (1996). More recently, an analysis of combined ITS and ETS data provided additional support for the inclusion of "Oligoneuron" as part of "Solidago".
Until the 1980s, the genus "Euthamia" was largely considered to be a part of the "Solidago" due to morphological similarities between species in both genera, and a history of synonymy of "Solidago lanceolata" and "Euthamia graminifolia". As mentioned, the lack of bundle sheath extensions in "Euthamia" compared to "Solidago", and deviations in floral morphology present evidence for separation of these taxa. A taxonomy of "Euthamia" as a genus was presented, providing a detailed description of distinguishing external morphological characters, such as fibrous-roots, sessile leaves, and mostly corymbiform inflorescences.
Evolutionary relationships within "Solidago".
Chromosome counts and advances in molecular systematics have enabled greater understanding of evolutionary relationships within "Solidago". At the time a taxonomy of "Solidago" was published"," related taxa causing contention, such as "Chrysoma, Euthamia, Oligoneuron", and "Petradoria" were excluded from this genus. The number of "Solidago" species has remained relatively stable, around 120 species, with approximately 80 in North America. Due to monophyletic support for the New World taxa and taxonomic difficulties with Old World taxa, the taxonomy provided in the 1990s only includes North American taxa and thus treats "Solidago" as non-monophyletic. Existing molecular-based phylogenies provide monophyletic support for "Solidago", given its inclusion of "Oligoneuron".
Chromosome counts have proven to be a valuable character in "Solidago" taxonomy and in elucidating the cytogeographic history of "Solidago". Similar chromosome counts may indicate close evolutionary relationships while different chromosome numbers may suggest distant relationships through reproductive isolation. Chromosome counts have been studied extensively in North America; all "Solidago" species have a base chromosome number of x=9, but the following ploidy levels have been observed: 2x, 3x, 4x, 6x, 8x, 10x, 12x, and 14x.
Though negligible differences in karyotype among "Solidago" and related genera were found, "Solidago" taxa with multiple cytotypes are more common than those with one. Although chromosome count is a useful metric for differentiating among "Solidago" taxa, it may be problematic due to the frequent variation in ploidy levels. Cytogeographic patterns in the "Solidago gigantea" complex, with tetraploids occurring in eastern North America and hexaploids in Oregon and Washington were observed. Cytogeographic patterns are also observed in the "Solidago canadensis" complex; hexaploids within "S. canadensis" have been observed east of the Great Plains and are treated as "Solidago altissima," and diploids and tetraploids occurring in the Great Plains are treated as "Solidago gilvocanescens". The taxonomic status of "Solidago ptarmicoidei" created an extensive debate due to frequency hybridization of "S. ptarmicoidei" with members of the "Ptarmicoidei" section of "Solidago". It was asserted that "S. ptarmicoides" should be united with Solidago rather than the genus Aster due to external morphological features such as similar pappus length as well as the same chromosome base (x=9). Information about chromosome number is still a crucial part of current understanding and phylogenies of "Soldiago".
Use and cultivation.
Young goldenrod leaves are edible. Native Americans used the seeds of some species for food. Herbal teas are sometimes made with goldenrod.
Goldenrod often is inaccurately said to cause hay fever in humans. The pollen causing this allergic reaction is produced mainly by ragweed ("Ambrosia" sp.), blooming at the same time as the goldenrod and pollinated by wind. Goldenrod pollen is too heavy and sticky to be blown far from the flowers, and is pollinated mainly by insects. Frequent handling of goldenrod and other flowers, however, can cause allergic reactions, sometimes irritating enough to force florists to change occupation. Goldenrods are attractive sources of nectar for bees, flies, wasps, and butterflies. Honey from goldenrods often is dark and strong because of admixtures of other nectars. However, when honey flow is strong, a light (often water-clear), spicy-tasting monofloral honey is produced. While the bees are ripening the honey produced from goldenrods, it has a rank odor and taste; the finished honey is much milder.
Goldenrods are, in some places, considered a sign of good luck or good fortune. They are considered weeds by many in North America, but they are prized as garden plants in Europe, where British gardeners adopted goldenrod as a garden subject long before Americans did. Goldenrod began to gain some acceptance in American gardening (other than wildflower gardening) during the 1980s.
They have become invasive species in other parts of the world, including China; "Solidago canadensis", which was introduced as a garden plant in Central Europe, has become common in the wild, and in Germany is considered an invasive species that displaces native vegetation from its natural habitat.
Goldenrod species are used as a food source by the larvae of many Lepidoptera species. The invading larva may induce the plant to form a bulbous tissue mass called a gall around it, upon which the larva then feeds. Various parasitoid wasps find these galls and lay eggs in the larvae, penetrating the bulb with their ovipositors. Woodpeckers are known to peck open the galls and eat the insects in the center.
Cultivated species.
Cultivated goldenrods include "S. bicolor", "S. caesia", "S. canadensis", "S. cutleri", "S. riddellii," "S. rigida", "S. shortii", and "S. virgaurea".
A number of cultivars have been selected, including several of hybrid origin. A putative hybrid with aster, known as ×"Solidaster" is less unruly, with pale yellow flowers, equally suitable for dried arrangements. Molecular and other evidence points to ×"Solidaster" (at least the cultivar 'Lemore') being a hybrid of "Solidago ptarmicoides" and "Solidago canadensis", the former now in "Solidago", but likely the "aster" in question.
The cultivars 'Goldenmosa'
and "S." × "luteus" 'Lemore' have gained the Royal Horticultural Society's Award of Garden Merit.
Industrial use.
Inventor Thomas Edison experimented with goldenrod to produce rubber, which it contains naturally. Edison created a fertilization and cultivation process to maximize the rubber content in each plant. His experiments produced a plant that yielded as much as 12% rubber. The tires on the Model T given to him by his friend Henry Ford were made from goldenrod. Like George Washington Carver, Henry Ford was deeply interested in the regenerative properties of soil and the potential of alternative crops such as peanuts and soybeans to produce plastics, paint, fuel and other products. Ford had long believed that the world would eventually need a substitute for gasoline, and supported the production of ethanol (or grain alcohol) as an alternative fuel. In 1942, he would showcase a car with a lightweight plastic body made from soybeans. Ford and Carver began corresponding via letter in 1934, and their mutual admiration deepened after George Washington Carver made a visit to Michigan in 1937. As Douglas Brinkley writes in "Wheels for the World," his history of Ford, the automaker donated generously to the Tuskegee Institute, helping finance Carver's experiments, and Carver in turn spent a period of time helping to oversee crops at the Ford plantation in Ways, Georgia.
By the time World War II began, Ford had made repeated journeys to Tuskegee to convince George Washington Carver to come to Dearborn and help him develop a synthetic rubber to help compensate for wartime rubber shortages. Carver arrived on July 19, 1942, and set up a laboratory in an old water works building in Dearborn. He and Ford experimented with different crops, including sweet potatoes and dandelions, eventually devising a way to make the rubber substitute from goldenrod, a plant weed commercially viable. Carver died in January 1943, Ford in April 1947, but the relationship between their two institutions continued to flourish: As recently as the late 1990s, Ford awarded grants of $4 million over two years to the George Washington Carver School at Tuskegee.
Extensive process development was conducted during World War II to commercialize goldenrod as a source of rubber. The rubber is only contained in the leaves, not the stems or blooms. Typical rubber content of the leaves is 7%. The resulting rubber is of low molecular weight, resulting in an excessively tacky compound with poor tensile properties.
Traditional medicine.
"Solidago virgaurea" is used in a traditional kidney tonic by practitioners of herbal medicine to counter inflammation and irritation caused by bacterial infections or kidney stones. Goldenrod is also used in some formulas for cleansing of the kidney or bladder during a healing fast, in conjunction with potassium broth and specific juices. Some Native American cultures traditionally chew the leaves to relieve sore throats, and the roots to relieve toothaches.
Medicinal exploration.
In various assessments by the European Medicines Agency with respect to Solidago virgaurea, non-clinical data shows diuretic, anti-inflammatory, antioxidant, analgesic and spasmolytic, antibacterial, antifungal, anticancer and immunomodulatory activity. However, as no single ingredient is responsible for these effects, the whole herbal preparation of Solidago inflorescences must be considered as the active ingredient.
Cultural significance.
The goldenrod is the state flower of the U.S. states of Kentucky (adopted 1926) and Nebraska (adopted 1895). "Solidago altissima", tall goldenrod, was named the state wildflower of South Carolina in 2003. The sweet goldenrod ("Solidago odora") is the state herb of Delaware. Goldenrod was the state flower of Alabama, but it was later rejected in favor of the camellia.
In the Midwestern United States, the blooming of goldenrods in August is a reminder that it will soon be time for children to go back to school after summer vacation.
Diversity.
 
Numerous species formerly considered members of "Solidago" are now regarded as better suited to other genera, including "Brintonia, Duhaldea, Euthamia, Gundlachia, Inula, Jacobaea, Leptostelma, Olearia, Psiadia, Senecio, Sphagneticola, Symphyotrichum, Trixis, Xylothamia"