Polygonum aviculare L. agg./s.l., Knotgrass

Account Summary

Growth form and preferred habitats

The P. aviculare species agg., or P. aviculare s.l. is genetically very variable and phenotypically extremely plastic with respect to environmental growing conditions. British material of this weedy ruderal annual was split into six separately named, geographically and/or ecologically distinct taxa by Styles (1962), four of which are accepted today. Along with the species aggregate these are separately mapped in the New Atlas. Only two of these segregated species have proven to be common and widespread throughout most of Britain and Ireland and they alone are known to be present in N Ireland, ie P. aviculare s.s. and P. arenastrum (Equal-leaved Knotgrass).

Fermanagh occurrence

The majority of recorders operating in Fermanagh since Styles's study have regularly distinguished P. arenastrum from P. aviculare agg. However, with just one solitary exception (a NI Lakes Survey record), Irish field botanists (including ourselves), for reasons that are not immediately obvious, have continued to field record this weedy little annual in the broad sense, listing it as, 'P. aviculare agg.' or 's.l.', rather than 'P. aviculare s.s.'.

The aggregate or broad species taxon thus appears to combine records of both these weedy summer annuals and as a result the broad combination is more commonly and more widely recorded in Fermanagh than the only distinguished constituent species, P. arenastrum. At the time of writing (December 2010), the Fermanagh Flora Database incorporates records of the broad taxon, Polygonum aviculare agg. or s.l., from a total of 127 tetrads (24.1%) widely scattered in the lowlands, while for P. arenastrum there are records listed from 81 tetrads.

Reproduction

Although by definition P. aviculare s.s. and P. arenastrum are distinguishable, they are rather similar in both their form and biology. Furthermore, while they differ slightly but significantly in their ecological preferences and tendencies, they share an essentially ruderal, stress-tolerant, pioneer colonist nature. Consequently, and unsurprisingly, their distributions often overlap, forming mixed populations in open, disturbed more or less fertile ground. Despite this overlap and close physical proximity, hybrids do not occur between the two forms. The tiny bisexual flowers of both of these small, weedy annuals fail to attract insect visitors and they therefore habitually self-fertilise.

British and Irish occurrence

The New Atlas hectad distributions for P. aviculare s.s. and P. arenastrum in Britain are remarkably similar in both frequency and pattern. In Ireland, P. aviculare s.s. is less well distinguished and recorded than could be expected in comparison with P. arenastrum. Thus P. aviculare s.s. appears patchy and sparse or absent in large areas of C Ireland and in parts of the west (notably the Burren, Connemara and Donegal). No obvious climatic or soil factor seems to explain such a pattern, so it might reflect some degree of under-recording.

The New Atlas editors map not only the four segregated British species, but also include P. aviculare agg. This is shown to be virtually ubiquitous in lowland Britain and Ireland, with just a few missing hectads scattered in C & W Ireland and in NW Scotland.

European and world occurrence

Since the species aggregate includes forms like P. aviculare s.s. which are significant arable weeds of both cornfield and broad-leaved crops, it is not surprising that it has travelled around the globe with man into far distant lands, including New Zealand, the South Sea Islands, S America and C & S Africa (Hultén & Fries 1986, Map 645). The spread of the plant might also be partially due to its use and reputation in herbal medicine, since the fibrous roots were used as a quinine substitute in N & C Africa, and the seeds are emetic and cathartic (Mitich 1998).

Names

The genus name 'Polygonum' is a combination of two Greek words, 'polys', many or much, and either 'gonos', meaning offspring or seed, an allusion to the numerous seeds characteristic of the plants, or 'gony', knee-joint, an allusion to the swollen joints of the stems. The Latin specific epithet 'aviculare' means 'of or pertaining to small birds' or 'eaten by small birds' from ' avicula', diminutive of 'avis', bird, as Sparrows and finches feed on the fruits (Gilbert-Carter 1964; Gledhill 1985).

The English common name 'Knotgrass' was coined in the 16^th century by Turner (1538) who also called it 'Swyne grys' (ie Swine's grass) as it was used in herbal medicine for ulcers and other sores, and it was, according to Gerard (1597), fed to pigs "when they are sicke, and will not eat their meate." Grigson (1987) lists no less than 30 English common names from around Britain and Ireland for this familiar plant.

Threats

None.

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