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, and a few species are native to Mexico, South America, and Eurasia. Some American species have also been introduced into Europe and other parts of the world.
Solidago species are perennials growing from woody caudices or rhizomes. Their stems can be decumbent to ascending or erect, ranging in height from 5 to 100 or more cm. Some species have stems that branch near the top. Some Solidago species are hairless others have strigose, strigillose, hispid, or short-villous hairs. The basal leaves in some species remain persistent through flowering, while in others the basal leaves are shed before flowering. The leaf margins are often serrated, and leaf faces may be hairless or densely hairy; the distal leaves are sometimes three-nerved, and hairless or sparsely to densely hairy with scabrous, strigillose, or villous hairs. In some species, the upper leaves are stipitate-glandular or sometimes resinous. The flowering heads usually radiate, or are sometimes discoid, with (1–)2 to 1500+ florets in racemiform (club-shaped or pyramidal), paniculiform, or corymbo-paniculiform, or sometimes secund arrays. The involucres are campanulate to cylindric or attenuate. The ray florets are pistillate and fertile. The corollas are yellow or rarely white and are usually hairless. The disc florets are bisexual and fertile and number two to 35 typically, but in some species there may be up to 60 florets. The corollas of the disc florets are yellow and the tubes are shorter than the throats. The fruits are cypselae, which are narrowly obconic to cylindrical in shape, and they are sometimes somewhat compressed. The cypselae have eight to 10 ribs usually and are hairless or moderately covered with stiff, slender bristles. The pappi are 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. Goldenrod is often unfairly blamed for causing hay fever in humans. The pollen causing these allergy problems is mainly produced by ragweed (Ambrosia sp.), blooming at the same time as the goldenrod, but is wind-pollinated. Goldenrod pollen is too heavy and sticky to be blown far from the flowers, and is thus mainly pollinated by insects. Frequent handling of goldenrod and other flowers, however, can cause allergic reactions, sometimes irritating enough to force florists to change occupation.
Solidago species are easily recognized by their golden inflorescences with hundreds of small capitula; some species have their flowers in spike-like inflorescences and others have axillary racemes. They have slender stems, usually hairless, but S. canadensis shows hairs on the upper stem. They can grow to a length between 60 cm and 1.5 m. Their alternate leaves are linear to lanceolate. Their margins are usually finely to sharply serrated.
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.
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 (Solidago canadensis) essential oil
Goldenrods are attractive sources of nectar for bees, flies, wasps, and butterflies. Honey from goldenrods often is dark and strong due to 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, but finished honey is much milder.
Goldenrods are, in some places, held as 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 long before Americans did as garden subjects. Goldenrod only 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 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.
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 12-foot-tall (3.7 m) 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.
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.
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 has also been used as part of a tincture to aid in cleansing of the kidney or bladder during a healing fast, in conjunction with potassium broth and specific juices.Native Americans chewed the leaves to relieve sore throats and chewed the roots to relieve toothaches.