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Alliaria petiolata (M. Bieb.) Cavara & Grande, Garlic Mustard

Account Summary

Native, locally frequent. European temperate, but widely naturalised.

1872; Stewart, S.A.; Enniskillen Town.

Throughout the year.

Growth form and preferred habitats

This tall winter annual, but more generally rosette-forming monocarpic, tap-rooted biennial, or occasionally short-lived perennial can reach a height of 120 cm in woods or along ± open, but semi-shaded, occasionally disturbed, linear habitats, often forming dense stands. It is often most abundant on damp, shaded soils although it can cope with a very wide range of soil moisture content (Cavers et al. 1979). The white racemes of flowers are extremely conspicuous in April, May and early June. Garlic Mustard has variable, round or heart-shaped, mainly stalked, bluntly-toothed leaves up the stem, which smell strongly of garlic when crushed.

A. petiolata appears most abundantly on roadsides, hedgerows and woodland margins, where the more fertile the soil, the denser and taller the stand it develops. It is interesting that A. petiolata can also tolerate very much drier and less fertile conditions in sand or gravel quarries and on scree slopes. The inherent disturbance of these types of unstable sites is clearly significant, and presumably it must assist the species survival by limiting the growth of potentially vigorous competitors.

Moderate disturbance is experienced by A. petiolata when roadsides are occasionally mown, and along stream and river banks where the ground is kept sufficiently open by seasonal flooding. The species is probably completely excluded only from the most acidic or the very wettest soils, or from greatly disturbed situations of ploughed, closely grazed or heavily trampled agricultural ground (Cavers et al. 1979; D.A. Pearman, in: Preston et al. 2002).

Flowering reproduction

The biennial form of the plant typically spends its first year of growth slowly developing a basal leaf rosette. However this is followed in the very early spring of the second year by a period of rapid flowering shoot elongation (ie bolting), of up to 1.9 cm/day. It has been suggested that the presence of glucosinolates and a range of similar compounds in the tissues of A. petiolata may permit the species to grow vigorously at near freezing temperatures in late autumn, and again in early spring (Cavers et al. 1979).

In the spring of the second year of growth, plants initiate inflorescence production, and they flower from early April, reaching a peak of flowering around mid-May. Flowers remain open for two, or very rarely three days, but they produce nectar and are visited by insects mainly in day one. The species is capable of both cross- and self-pollination, and pollen release and stigma receptivity occur before the flower bud opens. Thus, while flowers are visited by a range of midges, short-tongued bees and flies for food and pollination, autogamy might appear to be the most likely breeding system (Anderson et al. 1996). Self-pollination can occur as early as mid-afternoon on day one. Older flowers have little or no nectar present, which deters insect visitors (Cruden et al. 1996). However, self-pollination is not automatic, and indeed there is evidence that selfing is biologically delayed, presumably to favour out-crossing. A high degree of out-crossing would enable rapid adaptation of the population to local growing conditions (Clapham et al. 1962; Cavers et al. 1979; Cruden et al. 1996).

Mature plant size and seed production varies enormously within each type of habitat the species occupies. The long slender fruit capsules (siliques), ripen and split to release seed from the middle of July onwards to around early October. All the flowering plants die off in the autumn, and dormant seed overwinters to germinate in early spring (usually concentrated in April).

Seed production

Mature plant size and annual seed production varies enormously within each type of habitat the species occupies, but it is considerable when, as they often do, plants grow in dense stands. Measured seed production ranges between 15,000 in woodland to 107,000 seeds/sq m in more open conditions (Cavers et al. 1979).

Seed dispersal

There is no specialised seed dispersal mechanism. Wind dispersal is very ineffective and the seed does not float well either. However, seeds stick together when damp and they readily adhere to small crumbs of soil. Thus it is very probable that the major means of seed dispersal involves epizoochory (ie external adherence on passing animals), including man and his machines (Cavers et al. 1979).

The high rates of seed set and production typical of the species often enable A. petiolata populations to colonise fresh sites in suitable vegetation around existing individuals. In several parts of N America it has become a major alien invader, ousting native species from what appears to be relatively undisturbed woodlands (Cavers et al. 1979; Anderson et al. 1996). On the basis of similar evidence in England, Grime et al. (1988) classified its reproductive strategy as a 'competitive-ruderal' species.

Seed dormancy and germination

Dormancy is controlled by physiological features of the embryo. Cold stratification at temperatures between 1° - 6 °C for a period between 60 - 97 days is required to break dormancy (Lhotská 1975). Most seeds germinate in the spring after they are produced following exposure to winter conditions, but some viable seeds are retained in the seedbank and germinate over a period of 3 to 4 years (Cavers et al. 1979; Baskin & Baskin 1992). In several studies the range of seedlings surviving to flowering maturity varied from 2 - 7.5 %, the great majority of plants dying off at the seedling stage during dry periods in May (Cavers et al. 1979; Anderson et al. 1996).

Vegetative reproduction

Salisbury (1964) mentions that A. petiolata, like several other English hedgerow plants including Hypericum perforatum and Pimpinella major, is capable of a certain degree of vegetative reproduction. Should the flowering stem become damaged early on in the second season of bolting growth mentioned above, the plant may form secondary shoots from adventitious buds on the roots to replace the injured one. The frequency of this occurrence is probably not very great, and its significance should not be overestimated, indeed, from the evidence reported by Cavers et al. (1979) in Canada, it may not happen at all in some populations.

Fermanagh occurrence

In Fermanagh, A. petiolata is locally frequent and has been recorded in 93 tetrads, representing 17.6% of the area. Garlic Mustard is most commonly found scattered throughout the VC in lowland, shaded, somewhat disturbed ground involving moderate to high exposures of bare soil. It grows best in fertile, moist situations. The fact that ten scattered tetrads have pre-1976 records only is not regarded as significant in view of the biology and ecology of the species.

British and Irish occurrence

In Ireland, A. petiolata is much more thinly scattered, eastern and central in its distribution in comparison with the much more widespread occurrence in Britain (Perring & Walters 1976; Preston et al. 2002). Allen (1984) described the plant as calcicole in Ireland, but although it is more widespread on the limestone districts in the south of Co Dublin (Colgan 1904), little else seems to support this notion, and the current author prefers to consider it a species of fertile, moderately base-rich soils.

The New Atlas hectad map shows that in Britain A. petiolata is much less frequent or absent from the more strongly acid conditions prevalent in the N & W of Scotland, and on wet, infertile, heavily leached high ground throughout.

European and world occurrence

A. petiolata is native, common and widespread throughout Europe and eastwards to Asia Minor and the Himalaya. It is also considered native along the N coast of Africa. The European distribution extends in an increasingly scattered manner northwards up the west coast of Norway to around 68°N, and it thins markedly southwards in Greece. Garlic Mustard is absent from Crete, the Balearic Isles and Sardinia, although it is present in Sicily (Jalas & Suominen 1994, Map 2139). It also extends (native or not), to India and Sri Lanka, and is a definite introduction in NE parts of Canada, the United States and Australiasia (Cavers et al. 1979; Anderson et al. 1996; Rich 1991). A. petiolata is considered a rather invasive alien weed in the eastern states of N America. It was also introduced to one site in New Zealand at the end of the 19th century, but it only survived there for about ten years (Webb et al. 1988).

Uses

Like other wild crucifers, Garlic Mustard has been used as a condiment, boiled and eaten with mutton, as a salad, or to make various sauces, so that one of the fifteen alternative English names listed by Grigson is 'Sauce Alone', apparently originating in Somerset (Grieve 1931; Grigson 1987). The leaves and top of the plant just before flowering have a higher value of vitamin C, on a weight basis, than oranges, and the leaves at all times of year contain more vitamin A than spinach (which has the highest level of all the widely marketed garden vegetables) (Zennie & Ogzewalla 1977). The leaves are also reported to contain unusually high concentrations of N, P, Ca, Fe and Na (Survey of Inorganic Elements in Foliage: J.P. Grime & S.R. Band unpublished - quoted in Grime et al. 1988, p. 70).

In herbal medicine, A. petiolata leaves were used internally as a sudorific (ie to promote sweating), and as a deobstruent to relieve obstructions of the intestinal tract. It was also applied externally as an antiseptic treatment for gangrene and ulcers. Leaf juices taken alone, or boiled into a syrup with honey, were also used to treat dropsy (Grieve 1931).

The plant is the specialist food plant of the caterpillars of both the Orange-tip and Green-veined white butterflies (Garrard & Streeter 1983).

Names

The genus name 'Alliaria' was given to the plant by the herbal writer Fuchs, referring to the garlic-like smell of the crushed plant, and is derived from 'Allium' or 'Alium', the classical name for Wild garlic (Gilbert-Carter 1964). The Latin specific epithet 'petiolata' translates as 'furnished with a leaf-stalk', and usually means the plant involved possesses a particularly long petiole (Stearn 1992).

The English Common name 'Garlic Mustard' is given on account of the fact that its rather variable leaves do smell rather strongly of garlic when they are crushed, as indeed does any part of the plant. There is no odour unless the plant is crushed. Another well known name is 'Jack-by-the-hedge', and there are eight similar references to the hedgerow habitat in the names listed from around B & I by Grigson (1987). One of the more interesting names given by Britten and Holland (1886) is 'Swarms' from Yorkshire, possibly referring to the dense, tall stands the plant regularly produces on fertile ground.

Threats

None.