Glechoma hederacea 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 also considered an aggressive invasive weed of woodlands and lawns in some parts of North America. In the absence of any biological control research conducted by the United States Department of Agriculture, herbicides are relied upon (despite their drawbacks) particularly for woodland ecosystems. The plant's extensive root system makes it difficult to eradicate by hand-pulling.
Description Glechoma hederacea can be identified by its round to reniform (kidney- or fan-shaped), crenate (with round-toothed edges) opposed leaves 2–3 cm (3⁄4–1+1⁄4 in) diameter, on 3–6 cm (1+1⁄4–2+1⁄4 in) long petioles attached to square stems that root at the nodes. The plant spreads either by stolon or seed, making it exceptionally difficult to eradicate. It is a variable species, its size being influenced by environmental conditions, from 5–50 cm (2–19+1⁄2 in) tall. It is creeping in habit, with its stems growing a long as 1 m in legnth. As a seedling, G. hederacea has glabrous kidney-shaped cotyledons with a small notch at the tip. The flowers of G. hederacea 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. The fruits are dry and indehiscent with small, dark brown, flat dry nuts 1.0 to 2.0 mm in length and 1.0 mm wide.
Reproduction Glechoma hederecea is gynodioecious, 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. Disagreement exists 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, which are visited by many types of insects, and can be characterized by a generalized pollination syndrome. Each pollinated flower can produce 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. Single clones can grow several metres across, although precise data are not available.
Similar species G. hederacea 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.
Distribution and habitat 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 also considered an aggressive invasive weed of woodlands and lawns in some parts of North America, being considered a non-native plant in the United States, and invasive in some U.S. states. It has invaded wild areas, sometimes choking out native wildflowers. G. hederacea 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. Part of the reason for its wide distribution is its rhizomatous method of reproduction.
Ecology A number of wild bees collect pollen from 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). It forms dense mats that can take over areas of lawn and woodlands, thus is considered an invasive or aggressive weed in suitable climates where it is not native.
Control A non-native invasive in North America, 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. Because the plant is stoloniferous and continues to spread from its roots or bits of stem which reroot, even small infestations resist repeated hand weeding. Like crabgrass, G. hederacea's root has a tough-to-remove ball (unbelied by its delicate wide leaves). No biological control agents are known to help to reduce its spread in North America. Commercial herbicides containing triclopyr are used to control the plant. G. hederacea is also unusually sensitive to boron, and can be killed by applying borax (sodium tetraborate) in solution. However, borax is toxic to ants and to animals at only slightly higher concentrations, and does not break down in the environment. In addition to adverse long-term effects on soil or groundwater, recent research discounts the very efficacy of borax treatment, primarily because finding the correct concentration for a given area is difficult and the potential for damaging desired plants is high.
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