Glechoma hederacea (syn. Nepeta glechoma Benth., Nepeta hederacea (L.) Trevir.) is an aromatic, perennial, evergreen creeper of the mint family Lamiaceae. It is commonly known as ground-ivy, gill-over-the-ground, creeping charlie, alehoof, tunhoof, catsfoot, field balm, and run-away-robin. It is also sometimes known as creeping jenny, but that name more commonly refers to Lysimachia nummularia. It is used as a salad green in many countries. European settlers carried it around the world, and it has become a well-established introduced and naturalized plant in a wide variety of localities.
It is considered an aggressive invasive weed of woodlands and lawns in some parts of North America. Despite this, no biological control research has been conducted by the USDA. Instead, chemical herbicides are suggested, despite their drawbacks, particularly for woodland ecosystems. Hand-pulling is also suggested, although the plant is difficult to eradicate that way.
Glechoma hederacea can be identified by its round to reniform (kidney or fan shaped), crenate (with round toothed edges) opposed leaves 2–3 cm (0.79–1.18 in) diameter, on 3–6 cm (1.2–2.4 in) long petioles attached to square stems which root at the nodes. Like crabgrass, creeping charlie's root has a ball. It is a variable species, its size being influenced by environmental conditions, from 5–50 cm (2.0–19.7 in) tall.
Glechoma is sometimes confused with common mallow (Malva neglecta), which also has round, lobed leaves; but mallow leaves are attached to the stem at the back of a rounded leaf, where ground ivy has square stems and leaves which are attached in the center of the leaf, more prominent rounded lobes on their edges, attach to the stems in an opposite arrangement, and have a hairy upper surface. In addition, mallow and other creeping plants sometimes confused with ground ivy do not spread from nodes on stems. In addition, ground ivy emits a distinctive odor when damaged, being a member of the mint family.
The flowers of Glechoma are bilaterally symmetrical, funnel shaped, blue or bluish-violet to lavender, and grow in opposed clusters of two or three flowers in the leaf axils on the upper part of the stem or near the tip. It usually flowers in the spring.
Glechoma thrives in moist shaded areas, but also tolerates sun very well. It is a common plant in grasslands and wooded areas or wasteland. It also thrives in lawns and around buildings since it survives mowing. It spreads by stolons or by seed. Part of the reason for its wide spread is this rhizomatous method of reproduction. It will form dense mats which can take over areas of lawn and woodland and thus is an considered invasive or aggressive weed in suitable climates where it is not native.
A number of wild bees fly upon this plant, including Anthophora furcata, Anthidum manicatum, Anthophora plumipes, Anthophora quadrimaculata, Osmia aurulenta, Osmia caerulentes, and Osmia uncinata. The plant is also galled by several insects, including Rondaniola bursaria (Lighthouse Gall), Liposthenes glechomae or Liposthenes latreillei (Kieffer, 1898) (a gall wasp). Despite its name, it is not related to true ivy (Hedera).
Glechoma hederecea is gynodiecious, with genets being either female or hermaphrodite. The females depend upon pollen from hermaphrodites for pollination. Female flowers tend to be smaller than hermaphrodite flowers. There is disagreement among biologists as to whether hermaphrodite flowers can pollinate themselves. The plant spends the winter as either a small ramet or a small rosette. It produces flowers between April and July, the flowers are visited by many types of insects, and can be characterized by a generalized pollination syndrome. Each pollinated flower can produced up to four seeds, which are dispersed by the stem bending over and depositing the ripe seeds in the ground adjacent to the parent plant, although ants may carry the seeds further. The seeds germinate a few days after contact with moisture, although they can be stored dry. Dry storage for a period up to a month is thought to improve the germination rate.
The plant can also reproduce clonally, with the stems bending down to the earth and allowing roots to attach themselves. Singles clones can grow several metres across, although precise data is not currently available.
Cultivation and medicinal and culinary uses
Some people consider Glechoma to be an attractive garden plant, and it is grown in pots and occasionally as a groundcover. Easily cultivated, it grows well in shaded places. A variegated variety is commercially available; in many areas this is the dominant form which has escaped cultivation and become established as an aggressive, adventitious groundcover.
Glechoma was also widely used by the Saxons in brewing ale as flavoring, clarification, and preservative, and later by the English, before the introduction of hops into brewing which changed the ale into beer, in the late 15th century. Thus the brewing-related names for the herb of, alehoof, tunhoof, and gill-over-the-ground.
Glechoma hederacea has been used in the traditional medicine of Europe going back thousands of years: Galen recommends the plant to treat inflammation of the eyes. John Gerard, an English herbalist, recommended the plant to treat tinnitus, as well as a "diuretic, astringent, tonic and gentle stimulant. Useful in kidney diseases and for indigestion." It has also been used as a "lung herb". Other traditional uses include as an expectorant, astringent, and to treat bronchitis. The essential oil of the plant has been used for centuries as a general tonic for colds and coughs, and to relieve congestion of the mucous membranes. In the traditional Austrian medicine the herb has been prescribed for internal application as salad or tea for the treatment of a variety of different conditions including disorders associated with the liver and bile, gastrointestinal tract, respiratory tract, kidneys and urinary tract, fever, and flu.
Although it has been used as a salad green and in herbal medicines for thousands of years, the safety of Glechoma hederacea has not been established scientifically, and there is sufficient evidence to warrant caution with its use because it is toxic to cattle and horses. Glechoma hederacea is known to contain terpenoids; terpene-rich volatile oils are known to irritate the gastrointestinal tract and kidneys. The volatile oil also contains pulegone, a chemical also occurring in pennyroyal, that is a known irritant, toxic to the liver, and also an abortifacient. The concentration of volatile oil in Glechoma is less than 1/30th that in pennyroyal.
As is often the case when a plant has this many familiar names, Glechoma is familiar to a large number of people as a weed, a property it shares with many others of the mint family. It can be a problem in heavy, rich soils with good fertility, high moisture, and low boron content. It thrives particularly well in shady areas where grass does not grow well, such as woodlands, although it can also be a problem in full sun.
Small infestations can be controlled through hand weeding; repeated weeding is required because the plant is stoloniferous and will continue to spread from its roots or bits of stem which reroot. Like crabgrass, creeping charlie's root has a tough-to-remove ball (un-belied by its delicate wide leaves). It can be picked out by hand but there are tools to aid in crabgrass ball removal.
There have been no biological control agents introduced by agencies such as the USDA in North America to help to reduce its spread. It even appears that no research has been done on the subject of biological control agents for this plant. Biological control, however, is the most ecologically effective type of control for plants such as this that invade woodland ecosystems, as it does not cause destruction of valued species, or pollution, as chemical control attempts do.
Glechoma is unusually sensitive to boron, and can be killed by applying borax (sodium tetraborate) in solution. The ratio is eight to ten ounces (225–275 g) of borax dissolved in four ounces (125 ml) of warm water, diluted to 2.5 U.S. gallons (10 l) of final solution, to be sprayed evenly over precisely 1,000 square feet (100 m2) of lawn. Borax is toxic to other plants and to animals at only slightly higher concentrations and it does not break down in the environment; therefore the long-term effects of this technique on soil or groundwater, although not well documented, can be assumed to be unfavorable. More recent research discounts the efficacy of borax, primarily because finding the correct concentration for a given area is difficult and the potential for damaging desired plants.
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