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Galium aparine L., Cleavers

Account Summary

Native, very common and widespread throughout. European temperate, so widely naturalised it is now circumpolar temperate or almost cosmopolitan.

1881; Stewart, S.A.; Co Fermanagh.

Throughout the year.

Growth form and preferred habitats

A vigorous and tenacious annual scrambler that readily invades bare ground or gaps in vegetation and seeds itself profusely, G. aparine is a winter- to summer-annual, capable of germinating in either autumn or spring. It produces climbing stems that cling to bushes and hedges by means of short, hard, recurved, hook- or prickle-like hairs present on the four angles of the square stems and the margins and midrib of the six to eight whorled leaves and stipules, indistinguishable from one another (Salisbury 1964). Both leaves and stipules are linear or linear-oblanceolate, tapering to the base, up to 6.0 × 1.8 cm and ending in a long, slender, sharp point (Hutchinson 1972).

G. aparine thrives in a wide variety of habitats including hedgerows, scrub, cultivated ground, waysides, soil heaps and waste places, plus in more natural habitats including on river banks, tall-herb fen, shingle and limestone scree (W.R. Meek, in Preston et al. 2002). In the past it sometimes was, "a seriously abundant weed of arable land and especially in wheat crops on heavy soils." (Salisbury 1964).

Although seedlings occur on a very wide range of soils from pH 3.5-8.0 (Grime et al. 1988, 2007), the species undoubtedly performs best in soils that are mildly acidic (pH 5.5 and above) and damp to moderately wet. It also thrives where animal droppings have locally enriched the phosphate and nitrogen environment, and it tolerates drought and shade remarkably well. These habitat requirements are very similar to those of Urtica dioica (Common Nettle) and the two are very commonly associated (Pigott & Taylor 1964).

The wide range of habitats G. aparine succeeds in indicates that it can survive competition in relatively undisturbed, stable, ± closed vegetation, clambering over and through tall, dense herbage, up to 3 m or so in height. In these situations it reaches for the light using the multitude of tiny hooks on both its four-angled stems and its whorled leaves for attachment. However, as with other annual or biennial weedy species, irrespective of its vigorous growth, once established, ultimately populations of G. aparine depend on the availability of bare soil gaps permitting seedling establishment in the first instance.

The properties which allow G. aparine to compete in B & I on wood margins, tracks and clearings, scrub, hedges and in tall-herb and fen communities are: a. the ability to germinate after a chilling requirement is met, at any time during the growing season; b. rapid seedling development; c. an ability to grow during mild weather in winter; d. early flower initiation after a short period of vegetative growth; e. excellent climbing and clambering ability using 'sticky' hook-clad foliage; f. self-compatibility of the flowers with consequent full seed-set; and g. effective dispersal of 'sticky' burr seed (Grime et al. 1988, 2007; Malik & vanden Born 1988).

Given this range of capabilities, G. aparine is also able to thrive as a cosmopolitan weed in more heavily disturbed, open, fertile situations, including waysides, waste ground and fields in arable cultivation and particularly in cereal, Sugarbeet and Rapeseed crops (Grime et al. 1988, 2007; Malik & vanden Born 1988). Since arable agriculture is almost extinct everywhere in Fermanagh, Cleavers does not really trouble local farmers.

G. aparine is essentially a lowland species in B & I, decreasing progressively in abundance with altitude to an upper limit of 290 m in Ireland and 440 m in Cumbria (Wilson 1949; W.R. Meek, in: Preston et al. 2002).

Weed control

When they are abundant, the stems of G. aparine form matted masses which are very difficult to control or remove: the strong four-angled stems covered with many small backward curved prickles readily tangle in strimming machines making them very tricky to unravel. Removal from vegetation is further frustrated by the plant being resistant to commonly used selective herbicides, including phenoxyacetics, MCPA and 2,4-D. Chlorosulfuron applied at 20 g per ha, however, does provide season-long control in cereal fields (Malik & vanden Born 1988).

Fermanagh occurrence

G. aparine reaches its greatest frequency in Fermanagh around Upper Lough Erne, but it is very common throughout the VC in a wide variety of habitats and is abundant in many. It has been recorded in 403 Fermanagh tetrads, 76.3% of those in the VC. Local habitats range from wood margins and rides, scrub, hedges and field edges to rough grassland, stony lakeshores, tall herb fen and as a garden weed. It is also occasionally found on shaded cliffs and on limestone screes. Cleavers or Robin-run-the-hedge is absent from ericaceous heathland, wet bogland and aquatic situations involving submerged soils.

Flowering reproduction

The tiny, star-like, white or greenish-white bisexual flowers are produced in small paired clusters, each a two to five flowered cyme in the axils of the upper stem leaves from May to September. Pollination is said to be by small insects which visit sparingly, but if this fails, automatic self-pollination occurs (Clapham et al. 1962; Malik & vanden Born 1988; Taylor 1999). Only one of the numerous accounts of the flower the current author (RSF) consulted, that of Malik and vanden Born (1988), suggests that they are fragrant (although presumably due to one of those happy misprints given to us by modern spell-checkers, it actually describes them as, "flagrant"!). Presumably insect visitors are also rewarded by nectar, secreted by a disk at the base of the inferior ovary as in G. verum, although RSF has yet to see it mentioned anywhere in print (Hickey & King 1981).

The plants of G. aparine that flower earliest in the year also tend to produce the most flowers. After flowering, the plant foliage senesces and dies while the fruit is still ripening (Malik & vanden Born 1988). Fruiting takes place from June to October: the purplish-green globular fruit, covered with white hooked bristles consists of two large one-seeded mericarp portions which separate when they are fully ripe (Salisbury 1964). Salisbury (1942) gave the average weight of the individual mericarp as 0.0227 g, which is two hundred and fifty times the seed weight of G. anglicum Hudson (= G. parisiene L.) (Wall Bedstraw), a related unspecialised annual of more open situations found on sandy soils. Each Cleavers plant produces on average somewhere between 300 and 400 seeds or mericarps per year.

Seed dispersal

The dead stems persist throughout the following winter bearing these burrs which readily attach themselves to passing animals and clothing, or they may adhere to mud, and over the next six or seven months, they continue to disperse the seed. In waterside habitats, dispersal may also be water borne, floatation of pairs of fruits being facilitated by a small air space trapped between them (Ridley 1930; Malik & vanden Born 1988). G. aparine is also included among a long list of seeds that are ingested and redeposited unharmed by foraging ducks, presumably in tall herb fens and by river and stream sides (Gillham 1970). The seeds do not lose viability when ingested by animals and indeed the germination percentage actually increases. In addition to birds as vectors, viable seeds have been recovered from the droppings of horses, cattle, pigs and goats (Holm et al. 1977).

The efficiency of the burr dispersal process seems obvious as the species is particularly well represented in linear habitats along roadside verges, hedges, river and stream banks. Despite this, in January, a considerable number of burrs still hang on last year's dried stems. Clearly chance plays a huge part in determining dispersal by attachment to animals walking past, or connecting though their browsing, and the dispersal mechanism becomes very much less efficient or completely ineffective in dense, ± impenetrable vegetation. This is especially the case if the vegetation contains species which deter animals, such as stinging nettles (which die down completely in winter), and clinging brambles (which do not)!

Germination and seedling growth

Germination requires seeds to be covered in soil since they are inhibited by light, occurs either in the autumn, or as early as January in the following year, but viable seeds can persist in the soil for one or two years. A study in Japan, where the plant is a serious pest weed of drained paddy fields, showed that seminal roots of germinating seed grew to a length of 3 cm before the aerial part of the plant broke the soil surface. By the time the first leaves appeared, the roots were 5-6 cm in length. The fact that the roots elongated faster than the shoots was suggested as one of the reasons why it is difficult to kill even small G. aparine seedling with herbicides (Noda et al. 1965).

The root system of G. aparine spreads extensively in the upper soil horizons and utilizes the available water and nutrients from over a wide area (Noda et al. 1965). Interestingly, the root may or may not develop with the assistance of a vesicular-arbuscular (VA) mycorrhiza, and seedlings clearly can demonstrate their characteristic rapid growth without any fungal partner. The rapid seedling growth of G. aparine using the resources contained in its large seed contrasts very strongly with the almost snail-like progress of Arum maculatum (Lords-and-Ladies) seedlings which also emerge from large seeds and which do possess a VA mycorrhizal root partner (Prime 1960).

Late autumn or early winter germinated seed can make appreciable growth during mild periods in the winter months, an ability that allows them to compete with pre-vernal species in shaded habitats and to keep pace as tall, supporting perennial herbs grow.

Lodging and reattachment

Being a scrambling climber rather than a twining one, G. aparine is sometimes blown off its supporting plants by high winds, damage referred to as 'lodging', a term perhaps more prevalent in arable farming when cereal stems are flattened by adverse weather, but appropriate here also. Observation and experimentation has shown that the species recovers from this type of lodging by making corrective growth which is strictly limited to the curvature of the nodes. This pattern of growth response to the effect of gravity in plants is normally confined to the culms of grasses and it is most notably demonstrated by the familiar well-developed 'knees' of Alopecurus geniculatus (Marsh Foxtail). Heathcote (1987), who studied the phenomenon, found that the uppermost node developed a curvature, accompanied by a thickening of the node 1 or 2 mm either side of the point of leaf insertion. This localized region of the stem surrounding the node appeared to act as a specialised motor organ, rather like a pulvinus (ie the cells at the base of a leaf stalk which can move to adjust the orientation of the leaf blade). Stem curvature confined to the node ultimately led to a correction of the direction of growth, allowing the plant to climb back up onto its support.

Variation

G. aparine is an extremely variable species combining phenotypic plasticity and genetic variability and having a range of chromosome numbers from 2n=22-88, although 64 and the hexaploid 66 are the most common, certainly so in the British Isles (Malik & vanden Born 1988; Taylor 1999). Five climatic races or ecotypes have been recognised in Europe, probably forming a huge cline of variation (ie a gradient in the geographic pattern of a character) (Böcher et al. 1955). This pattern of variation includes a spring-germinating summer annual in S Europe, an autumn-germinating winter annual further north and an area including the British Isles where these two overlap. A biennial form also exists in Spain.

Toxicity

Although the foliage and fruit of G. aparine are grazed by a range of animals, as noted above (and horses avidly seek it out), the species does contain noxious substances which one would imagine make it less than ideal fodder. Galium species contain anthraquinones, which are said to have low systemic toxicity to mammals, but may cause skin irritation. They are also said to contain mollugin, coumarins and three iridoid glucosides, some or all of which have a bitter somewhat astringent taste (Grieve 1931; Batra 1984).

Taylor (1999) reported that Cleavers contains three phenolic compounds plus an isoquinoline-type alkaloid called protopine. G. aparine is reputed to cause internal inflammation when ingested by livestock, and it has a diuretic effect (Long 1960) yet, despite all this, none of the Rubiaceae are sufficiently toxic to feature in Poisonous plants and fungi in Britain (Cooper & Johnson 1998). In confirmation of this, at least 40 plant eating insect species have been identified on G. aparine and nine of these are monophagous (ie they subsist on one species of food plant only) (Taylor 1999).

Fossil record and status

Fossil fruits of G. aparine have been found in the long distant Cromer Forest Bed series and tentatively from the Late Weichselian zones II/III in Essex. All fossil records from the current interglacial (the Flandrian in England, the Littletonian in Ireland) are from archaeological sites however, right up until the mediaeval period (Godwin 1975).

In B & I, this abundant weedy plant is strongly associated with heavily disturbed or man-made habitats, but it does also occur in more natural situations, particularly on coastal shingle and on screes, so perhaps we may still regard it as possibly native, although with status unproven and likely to remain so.

British and Irish occurrence

G. aparine is an abundant, highly successful weed throughout the whole of B & I except on the wettest and most exposed sites. It is probably still expanding the range of ground it can colonise (W.R. Meek, in: Preston et al. 2002).

European and world occurrence

Common and abundant in temperate parts of Europe, N Africa, and N & W Asia – virtually around the whole N Hemisphere in fact, although its presence in inland states of N America is greatly reduced compared with coastal regions, and probably it is native in N America only in these latter areas (Malik & vanden Born 1988; Taylor 1999).

Uses

G. aparine is used by some horsemen as a tonic for the animal's lymphatic system and for treating horse skin conditions. Herbalists and homoeopaths use G. aparine for a wide variety of human ailments, including psoriasis and other skin diseases, for scalds, burns and blisters, for piles, and as a soothing tea for insomnia (Grieve 1931; Grigson 1955, 1987). The coffee tree, Coffea arabica, also belongs to the Rubiaceae, and G. aparine seeds are described by Grieve (1931) as, "one of the best substitutes for coffee; they require simply to be dried and slightly roasted over a fire, and so prepared have much the flavour of coffee. They have been so used in Sweden.".

Names

The genus name 'Galium' was first used by Dioscorides for G. verum, and is derived from the Greek 'gala' meaning 'milk'. The connection with the plant is that G. verum was widely used to curdle milk for making cheese (Johnson & Smith 1946; Gilbert-Carter 1964). The specific epithet 'aparine' is the ancient Classical name for G. aparine in the writings of Theophrastus and Dioscorides, and it is derived from the Greek 'aparo' meaning 'to cling' or 'to seize', an obvious reference to the plant's methods of climbing and fruit dispersal (Grieve 1931; Gilbert-Carter 1964; Gledhill 1985).

Familiarly known as 'Cleavers', 'Goosegrass', 'Robin-run-the-hedge' or 'Sticky Billy', G. aparine has dozens of other variant names in different parts of the British Isles, most of which refer to the clinging nature of both foliage and burrs (Grigson 1987). The name 'Goosegrass' refers to the fact that geese keenly graze it, and in the past (and perhaps continuing – see Vickery (1995)), it was collected and fed to poultry. Some of these uses and associated names go back to Anglo-Saxon times. A 15th century leech-book is quoted by Grigson (1955, 1987), "an erb that is cald clyvers that yonge gese eten". The same practice is alluded to in the names 'Gosling-grass' and 'Gosling-scrotch'.

One of the most interesting names in RSF's view is 'Harif', 'Heiriff', 'Haireve' or 'Haritch', or in Middle English 'Hayryf', all of which are derived from the Anglo-Saxon (Old English) 'hegerife' or 'hege' meaning 'a hedge', and 'reafa', which means both 'a tax-gatherer' and 'a robber'. Possibly 'reafa' therefore refers to the way in which the plant plucked wool from passing sheep, or maybe it suggests that wool contaminated with burrs was useless, or valueless, to the shepherd. This name was originally applied to Arctium spp. (Burdock), and at some later stage became transferred to G. aparine (Prior 1879).

Threats

None.

References

Grime, J.P., Hodgson, J.G. and Hunt, R. (1988, 2007); Salisbury, Sir E. (1964); Clapham, A.R., Tutin, T.G. and Warburg, E.F. (1962); Grigson, G. (1955, 1987); Grieve, M. (1931); Malik, N. and Vanden Born, W.H. (1988); Ridley, H.N. (1930); Wilson, A. (1949); Pigott, C.D. and Taylor, K. (1964); Salisbury, E.J. (1942); Gillham, M.E. (1970); Holm, L.G., Plunckett, D.L., Pancho, J.V. and Herberger, J.P. (1977); Long, H.C. (1960); Batra, W.T.S. (1984); Noda, K., Ibaraki, D., Eguchi, S. and Ozawa, K. (1965); Cooper, M.R and Johnson, A.W. (1998); Heathcote, D.G. (1987); Prime, C.T. (1960); Prior, R.C.A. (1879); Vickery, R. (1995); Godwin, H. (1975); Hickey, M. and King, C.J. (1981); Gilbert-Carter, H. (1964); Gledhill, D. (1985); Johnson, A.T.and Smith, H.A. (1946); Preston et al. 2002; Hutchinson 1972; Taylor (1999); Bocher et al. (1955).