Glechoma hederacea
(ground ivy)

G. hederacea is a low-growing perennial herb native to Europe and parts of Asia. Despite its inability to produce fruits, at least in some environments, G. hederacea has become a remarkably successful invader in many countries, especially the USA (Waggy, 2009). It has also been introduced to Hong Kong, Australia and New Zealand.

G. hederacea is native to Europe and parts of Asia. It has been introduced to Hong Kong, North America, Australia and New Zealand.

Seeds of G. hederacea and its variegated form (G. hederacea ‘variegata’) are freely available on the internet. The plant is attractive as a pot plant, in hanging baskets or as garden ground cover, so people may try to introduce it, legally or illegally. It occasionally escapes cultivation in south-eastern Australia (Richardson et al., 2006).

In its native environment, and also where it has become naturalised, G. hederacea tends to be a plant of shaded habitats, including woodlands, hedgerows, shady roadsides, clear-felled and coppiced woodlands, scrub, derelict woodlands and orchards. Apparently it increases rapidly from either existing clones or seed when forests or woodlands are coppiced or harvested (Hutchings and Price, 1999; Waggy, 1999). However, G. hederacea also seems to grow in grassland in more or less full sunlight, beside rabbit scrapes, on spoil heaps, wasteland, waysides, walls and the edges of pastures, and even dominating enriched areas at the margins of arable fields.

In Britain, Hutchings and Price (1999) described it as frequent on shaded roadsides, in clear-felled and coppiced woodland and in orchards. 

In southern Sweden it occasionally forms extensive patches covering over 100 m², probably consisting of individual clones (Widén and Widén, 1990).

In continental Europe G. hederacea is a characteristic species of wood-margins, hedge banks and scrub communities. However, it is also frequently found in more open grassland swards, and it occurs as a minor component in swamp or fen communities.

In North America it occurs in deciduous and riparian forests in the eastern and central USA and Canada, but can also be found in more open plant communities, among grasses in early successional woodland or shrubland, and in lawns (Mitch, 1994; Waggy, 2009).

Genetics

The diploid chromosome number (2n) for plants from Britain is 18, 24 or 36; 18 for horticultural plants; 18 or 36 for plants of wild European origin; 24 or 36 for plants from north-west Poland, and 18 or 36 for North American plants (Hutchings and Price, 1999).

Reproductive biology

Reproduction in G. hederacea is primarily vegetative, and seed production seems to be uncommon in many places, even though resources are generously allocated to flower and seed production (Waggy, 2009). The species is gynodioecious, with clones being either hermaphrodite or female with sterile anthers, although sometimes the same individual will produce both kinds of flowers (Widén, 1992). Hermaphrodite flowers are protandrous and considered self-compatible, although both fruit-set and seed-set are pollen-limited even in hermaphroditic and mixed populations; fecundity increased after hand-pollination (Widén, 1992). Despite being self-compatible, hermaphrodite flowers may still need an insect pollinator to visit for pollination to occur (Hutchings and Price, 1999).

Sex-expression may not be completely under genetic control in G. hederacea, for when Price (1991, quoted in Hutchings and Price, 1999) transferred eight apparently male-sterile clones to a glasshouse, four of them produced considerable numbers of hermaphrodite flowers within three weeks and about 90% hermaphrodite flowers after six weeks.

The flowers produce nectar to attract pollinators (Southwick et al., 1991). In Sweden bumble bees (Bombus spp.) were the most common visitors, but honey bees (Aphis mellifera), syrphids, beetles and ants were occasionally reported (Widén and Widén, 1990). In the northeastern USA honey bees were the main nectar feeders (Southwick et al., 1991). For female plants in Sweden pollination rates and therefore fruit-set and seed-set were negatively correlated with distance to pollen source: mean pollen dispersal distance was estimated at 5.9 m when based on fruit-set and 5.3 m on the basis of seed-set. Pollination of female clones may not happen at all if the distance to the pollen source in a hermaphrodite clone is over about 100 m (Widén and Widén, 1990).

According to Hutchings and Price (1999) and Widén and Widén (1990), many populations of G. hederacea produce large quantities of viable seed, although some authors have not seen seeds of the species, for example, in New Zealand (Webb et al., 1988) and China (eFloras, 2013), and elsewhere seed production can sometimes be very rare (Slade and Hutchings, 1989). Isolated male sterile clones may produce no seeds (Widén and Widén, 1990). Slade and Hutchings (1999) found that hermaphrodite ramets produced more seed than female ramets.

G. hederacea overwinters either as small, two-leaved ramets or as 8-10-leaved rosettes produced in autumn by rooted ramets. From April onwards (in spring) the rosettes develop two-leaved nodes along the new ascending stems. Flowers then emerge in the leaf axils between April and July in Britain and at similar dates in most of the United States (Hutchinson and Price, 1999; Waggy, 2009). After flowering the ascending stems continue their vegetative growth, arching over to touch the ground and enabling the newly produced nodes to form roots. The stolons can sometimes be 2 m long (Hutchinson and Price, 1999). In Sweden and presumably elsewhere, stolon connections between the ramets decay after the growing season (Widén and Widén, 1990).

Seeds are dispersed by gravity. The calyx bends down at seed maturation and the seeds are passively dispersed. Bouman and Meeuse (1992, cited in Hutchinson and Price, 1999) suggested seeds may be further dispersed by ants. On contact with water the nutlets produce moderate amounts of mucilage which further helps fix them to the soil surface (Ryding, 1992, cited in Hutchinson and Price, 1999). Hutchinson and Price (1999) expanded on this, adding that in vitro the seed becomes invested in a disc of mucilage about 1 cm in diameter.

Chancellor (1985) suggested that the seeds of G. hederacea remain viable in the soil for only a short time, but Hutchings and Price (1999) commented that the sometimes rapid colonisation of coppiced woodland suggests otherwise. Thomson et al. (1997) cited reports of G. hederacea having relatively small seed bank sizes of 53 to 280 seeds m^-2, and Stagg (1996) recorded some seeds in soil from ex-arable sites at several dates through the year, with a peak of about 80 m^-2 in the top 4 cm of earth.

Physiology and phenology

Although some freshly collected seeds (about 45%) of G. hederacea germinate readily, the proportion increases to 70% after dry storage for one month at 20^oC (Grime et al., 1981). In a different sample, dry stored at 20^oC, germination was 32% in the light and 36% in the shade, with no germination occurring in the dark. Unpublished work by CPD Birch (mentioned in Hutchings and Price, 1999) reported that germination of seed after 10 days storage at 20^oC was 84% in the light, 8% in the dark, and 80% in neutral shade (i.e. 40% of ambient photosynthetically active radiation). Under blue filters that transmitted 63% of ambient light at a red:far-red ratio of 0.71 (as opposed to a ratio of 1.15 in ambient light), germination percentage was 84, whilst under green filters with a ratio of 0.38, germination was 52%. On return to unshaded conditions all seeds (including those previously in darkness) gave germination of 80-88%. This implies some inhibition of seed germination under vegetation cover.

The special vascular anatomy and phyllotaxis in the Lamiaceae family mean that as new stolon branches develop they acquire the status of integrated physiological units that are largely physiologically independent of each other (Hutchings and Price, 1999). New axillary stolons develop from the leaf axils when nodes have been superseded by several younger nodes towards the stolon apex. These axillary stolons capture most of the resources from the subtending leaf and from the roots that supply the same vascular bundle. Once an axillary stolon can sustain its own maintenance and growth it retains most of the resources it produces and does not export them to other stolons (Price and Hutchings, 1992).

When genetically identical clones of G. hederacea were grown in competition with Lolium perenne, their total biomass was reduced significantly by the competition, but was not significantly affected by the height of L. perenne (Hutchings and Price, 1999). Although primary stolon growth was unaffected by competition, the number and length of secondary stolons were reduced, especially in long grass. Internode and petiole length were significantly greater in clones in long grass, but not in shorter grass. These responses, suggested Hutchings and Price (1999), tend to promote avoidance of competition in both the horizontal and vertical planes.

G. hederacea tends to be more productive when grown in heterogeneous habitats than in uniform habitats. Birch and Hutchings (1994) grew clones in heterogeneous and homogneous habitats, both supplying the same total quantity of nutrients, and found that the biomass of plants grown in the former habitat was over 2.5 times greater than when they were grown in the latter. In the heterogeneous environment, roots developed earlier and grew longer.

Longevity

The species is clonal, and its clones live long enough to grow into very large plants (Widén, 1992). In addition, plants can become fragmented and the fragments will readily develop into completely independent plants. There is little evidence of long-lived or large seed banks.

Activity patterns

Plants of G. hederacea begin to grow in spring and proliferate, flower and set seed over the summer months. After flowering, the upright branches continue to grow and bend to the ground where they root and spread horizontally. In autumn and winter the plants form ramets which can separate from the parent stolon and become green overwintering plantlets (Waggy, 2009).

Population size and structure

At least in some environments, individual clones of G. hederacea can grow into huge plants up to hundreds of square metres in size (Widén and Widén, 1990).

Nutrition

The species is typically found in fertile habitats where potential dominants are restricted by shade, disturbance or both (Grime et al., 1988). It is intolerant of salinity, and prefers damp, heavy, fertile and calcareous soils with a pH range of 5.5 to 7.5, but also occurs in soils with pH as low as 4.0. It is only likely to become abundant where phosphate, nitrate and calcium in the soil are adequate (Hutchings and Price, 1999).

Associations

Species commonly associated with G. hederacea in Britain include Ajuga reptans, Brachythecium rutabulum, Bryonia dioica, Galium aparine, Geranium robertianum, Geum urbanum, Hedera helix, Poa trivialis, Ranunculus ficaria, Sambucus nigra, Stachys sylvatica, Stellaria holostea, Tamus communis and Urtica dioica, all of which are species of hedgerows and neglected waste places. G. hederacea also occurs in several woodland communities in Britain (Hutchings and Price, 1999).

Environmental requirements

Typically a plant of shaded habitats, G. hederacea seems to be highly adaptable to different substrates (although it prefers moist, fertile substrates), temperate environments and species communities.

General

• Sociocultural value

Materials

• Poisonous to mammals

Medicinal, pharmaceutical

• Traditional/folklore

Ornamental

• Propagation material
• Seed trade

G. hederacea is sometimes confused with Malva neglecta and other creeping plants, but can be distinguished by its long, square stems that root at the nodes.

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Cultural control and sanitary measures

Kohler et al. (2004) conducted experiments on cultural and chemical control of G. hederacea in Indiana and found that application of nitrogen (as urea) at higher than 98 kg ha^-1 in spring, summer or autumn decreased the cover of the species by up to 40%.

Physical/mechanical control

Both physical and mechanical control are reported as being very difficult due to the ability of the plant to spread and root easily. Control by agencies in the USA has had limited success (Waggy, 2009).

Biological control

The impacts of Puccinia glechomatis, a rust fungus, on Glechoma hederacea are currently being studied (NYS IPM Program, 2013).

Chemical control

In Poa pratensis turf, isoxaben, applied pre-emergence, prevented the growth and development of G. hederacea, and the herbicides triclopyr + clopyralid, MCPP + 2,4-D + dicamba, 2,4-D amine, 2,4-D ester and fluroxypyr all reduced its incidence when applied post-emergence (Kohler et al., 2004). Herbicide combinations including triclopyr, 2,4-D or fluroxypyr gave best control, although the authors acknowledge that other authors have sometimes achieved different results with some of these materials.

Hatterman-Valenti et al. (1996) and others have used borax (sodium tetraborate) in solution as a control for G. hederacea to good effect, although it causes temporary damage to Poa pratensis turf. They achieved equally good control with 2,4-D + dicamba + dichlorprop.

More information on any effects of G. hederacea on vegetation in natural environments would help to clarify the role of this species in its invasion of natural areas.

: Original text by:

Ian Popay, consultant, New Zealand, with the support of Landcare Research.