Potentilla anserina L., Silverweed
Account Summary
Native, common and very widespread. Circumpolar boreo-temperate, but widely naturalised.
June 1862; Smith, T.O.; Ardunshin.
April to November.
Growth form and preferred habitats
This very variable, low-growing, carpet-forming, stoloniferous, rosette perennial is a common pioneer weedy colonist of open bare ground and of gaps in pastures. Silverweed is shade-intolerant but is quite frequent along moderately disturbed roadside verges, waste ground, heavily trampled soil around gateways and in compacted, otherwise more or less bare ground beside tracks and paths, on damp to perfectly dry, sometimes very stony soils. However, it definitely prefers and is most common and abundant in almost permanently damp, moderate to richly fertile, near neutral, base-rich or calcareous soils, where seasonal flooding inundates and nutrient-enriches grassland on lakeshores and riverbanks. The vigorous vegetative reproduction of the species allows it to invade vegetation gaps in overgrazed or moderately disturbed pastures and, once established, it can become very enduring.
While it is certainly able to perform well as a pioneer coloniser of bare, disturbed soil and vegetation gaps, in Sweden on a grazed Baltic seashore pasture, P. anserina was found to be competitively inferior to three common grass species, Agrostis stolonifera (Creeping Bent), Festuca rubra (Red Fescue) and Poa pratensis subsp. irrigata (Lindm.) H.Lindb. (= P. humilis) (Spreading Meadow-grass). Silverweed could only out perform these grasses on patches of cattle dung (Eriksson 1986b).
P. anserina is also common where persistent rain regularly produces temporary pools in hollows in wet pastures (Sinker et al. 1985; Hill et al. 1999). As it also features on sea coasts on salt marshes and sand dune grasslands, this is very definitely a species (or species aggregate) of wide ecological versatility and, consequently, it is widespread all around the northern hemisphere.
Fermanagh occurrence
Apart from very acid peaty uplands and regularly managed agricultural ground, P. anserina is common and widespread in Fermanagh and has been recorded in 376 tetrads, 71.2% of those in the VC, the main exceptions being the upland acid ground of the Cuilcagh and Lough Navar areas. It is less frequent or absent in the better agricultural ground east of Lough Erne.
In Fermanagh, Silverweed is most constantly present and abundant around the winter high water level on many local lakeshores, and by far the greatest local concentration of P. anserina is in the Upper Lough Erne basin. In part, this is certainly an artefact produced by the enormous amount of recording time taken by the government conservation team that surveyed this area in the mid-1980s. However, the eminently suitable terrain and the traditional water-meadow management regime employed by the local farmers, provides an ideal habitat for this stress-tolerant, moderately competitive, ruderal, vegetatively far-creeping, clonal species (Grime et al. 1988).
Flowering reproduction and genetic variation
While P. anserina flowers from May to August, it tends to do so relatively sparingly; each solitary bright yellow blossom, 2-2.5 cm in diameter, is formed either axillary on the mother rosette, or terminally on stolon internodes (Eriksson 1986b). Flower production is inversely related to plant vigour (Ockendon & Walters 1970). The dish-like, unspecialised flowers stay open for 2-3 days, are self-incompatible and are pollinated by various short-tongued insects. While this is the case, they still frequently fail to set seed and fruiting plants are often difficult to find (Ockendon & Walters 1970; Fitter 1987; Miyanishi et al. 1991).
The production of seeds in a Swedish study was reduced by limited pollen availability in the Baltic coastal population studied, mean seed set being less than ten per flower. Pollen availability varies from year-to-year and with prevailing weather conditions and it is particularly sensitive to low temperatures and precipitation, although the species does occur north of the Arctic Circle (Miyanishi et al. 1991). Pollen limitation will be most significant in populations dominated by few clones or incompatibility types (Eriksson 1987).
Another reason for fruiting failure, although probably only an extremely minor element, is that P. anserina exists in both tetraploid (2n=28) and hexaploid forms (2n=42), which are morphologically inseparable (Rousi 1965; Ockendon & Walters 1970). No diploid representatives have been found so far and Rousi (1965) concluded that they were unlikely in Europe or N America, but they might still survive in unexplored Asia. Hexaploids are probably autoalloploids arising in a single step from the fusion of reduced and non-reduced tetraploid gametes (Cobon & Matfield 1976). A cytological survey of P. anserina in Britain found hexaploids scattered throughout otherwise tetraploid populations (Ockendon & Walters 1970).
The tetraploid is the widespread, fertile and most probably ancient form of the plant in both B & I and around the world, while the hexaploid shows no discernible geographical pattern in these islands. It only rarely sets seed and thus is something of an evolutionary blind alley (Rousi 1965; Ockendon & Walters 1970). The fact that Erlandsson (1942) found two pentaploid Silverweed plants, however, proves that the hexaploids are not totally sterile. Having said this, no natural hybrids of P. anserina with any other Potentilla species have been reported, although it can cross with the Fragaria × ananassa Duch. (2n=56) the Cultivated Strawberry (Miyanishi et al. 1991).
Morphological variation, hybridization and gene flow
There is considerable variation in form within Silverweed populations throughout the species wide total range, and especially so further north. Taxonomists and plant geographers have named up to eight subspecies and/or varieties, treating P. anserina as a species aggregate (Rousi 1965; Hultén 1968). Hybrids have been recorded in Finland between some of the subspecies, particularly between subsp. anserina and subsp. egedii Wormsk. (a more arctic form found today in N & C Europe that is sometimes given full species status (Rousi 1965)). A number of other botanists are not convinced about the reality of some of these northern variants, eg Porsild & Cody (1980), who could find no distinctions between specimens identified as P. anserina and P. egedii subsp. yukonensis [in Arctic Canada] on the basis of their achene or bractlet characteristics.
On the basis of mainly glasshouse hybridization experiments between populations and observations of natural hybrids in populations on the coast of Finland, Rousi (1965) has argued that the two forms may have coexisted during the Late-glacial period, hybridized then, and transferred some subsp. egedii characters to subsp. anserina which still persist until today. Subsp. egedii is usually distinguished by its smaller size, its glabrous stolons and the lack of silvery pubescence on its leaflets (Miyanishi et al. 1991).
The fact that these two taxa (subsp. anserina and subsp. egedii) have managed to keep distinct from one another in the N Hemisphere for such a long period raises questions as to how they achieved isolation and avoided intermingling? Presumably this is due to differing ecological tolerances, but the fact that subsp. egedii is self-compatible must also help keep the two separate, their flowering periods only have a brief overlap and the geographical ranges have also parted company nowadays to a large extent (Rousi 1965).
The latest critical Flora of B & I distinguishes four varieties on the basis of the hairiness of the upper leaf surface and the size of various plant parts, including leaves, flowers and petiole length: these forms are given as Var. nuda Gray; Var. sericea Hayne; Var. anserina and Var. grandis Lehm. (Sell & Murrrell 2014). These taxa do not appear to coincide with the forms recognised by Rousi (1965).
Fossil history in B & I
The achenes of P. anserina are sufficiently distinguishable that the species has been recorded in Britain back as far as two stages in the Hoxnian interglacial 400,000 BP which lasted about 53,000 years. It has appeared in all glacial and interglacial periods since then, although there was a hiatus between Flandrian zone IV and zones VIIb & VIII, the latter falling within Roman and Bronze Age periods. This suggests vegetation or woodland clearances going on that created open, disturbed conditions more favourable for increase of the species. Periglacial outwash from retreating glaciers and bare river terrace deposits would favour the pioneering behaviour of P. anserina and, in more recent times, the species has spread with human culture thanks to the environmental habitat disturbance we create (Godwin 1975).
Vegetative reproduction
P. anserina spreads efficiently by its annual stolons, 30-100 cm long, with internodes 10-15 cm long at which daughter ramets are formed. The often reddish stolons wither each autumn and thereafter the rooted ramets live independently (Eriksson 1987). Dense mats of ramets can develop with up to 2,000 ramets per m² (meaning in this case, each ramet may occupy as little as 5 sq cm, although there could be some degree of overlap)(Eriksson 1986a). The vigorous vegetative reproduction shown by P. anserina is the major means of ramet recruitment to the population, far outstripping recruitment from seed and ramet mobility enables plants to find space and avoid competition (Eriksson 1986b). Each ramet consists of a short vertical rhizome bearing swollen adventitious roots containing starch. The ramets are perennial, all above ground tissues withering in the autumn, the plants overwintering simply as the short rhizome with winter buds developing on their uppermost part (Miyanishi et al. 1991). In the Swedish Baltic shore study, the half-life of an established ramet was 3.1 years (Eriksson 1986a).
Vegetative reproduction is regularly associated with polyploidy. Other examples of this property include Agrostis stolonifera (Creeping Bent), Potentilla reptans (Creeping Cinquefoil), Ranunculus repens (Creeping Buttercup) and Trifolium repens (White Clover) (Salisbury 1964, p. 335). Plants that grow very vigorously and reproduce asexually often reduce their investment in flower production and sexual reproduction. This appears to happen in many Silverweed populations, which may produce several stolons or 'runners' up to a metre in length in a single season, but which flower only sparingly and sporadically. These vegetatively produced clonal patches are potentially long-lived, perhaps sometimes ancient (Eriksson 1987).
Fruit production and dispersal
P. anserina plants that do manage to fruit successfully produce a head of between 6-50 (maximum 60) lightweight, dry achenes (single-seeded fruits). These are dispersed either by water flotation (the achene wall has a layer of air-containing cells enabling them to remain buoyant for up to 15 months), rain wash, adhesion in mud to other organisms, or by being eaten along with foliage and passing through birds or other animals (Ridley 1930; Salisbury 1964, p. 276; Miyanishi et al. 1991). Achenes remain viable for up to a year buried in soil (four reports), but there is only one report of long-term survival (ie longer than five years) so it is unlikely that there is a permanent seed bank (Thompson et al. 1997).
British and Irish occurrence
P. anserina is common and widespread throughout the whole of B & I except for inland parts of N Scotland. This pattern reflects the species avoidance of extreme acid soils and colder, high altitude sites. Around the coasts of B & I, P. anserina is also an almost universal component of the salt-tolerant maritime vegetation of the upper part of shingle or coarse sand beaches, where it forms a distinct pioneer zone below the turf of the true dry land vegetation (Preston et al. 2002).
Specimens from coastal Scottish populations were found to approach the more northern or arctic form P. anserina subsp. egedii, which is characterised by entire or shallowly toothed epicalyx segments and a low number of teeth on the terminal leaflet of leaves, seeming to have a mixture of anserina and egedii characters, similar to the situation described by Rousi in Baltic Sweden (Ockendon & Walters 1970).
Species origin
It is believed by some botanists that P. anserina originated on sea shores and other saline soils and that it subsequently developed a wider ecological tolerance of trampling, disturbance and nutrient status to become a common and widespread stress-tolerant weed of disturbed ground in cool temperate regions. Other evidence from the extent of variation points to the Himalaya, or somewhere in SE Asia as the centre of origin of the species or species aggregate (Rousi 1965, p. 106). At some unknown stage in history, tetraploid P. anserina spread around the globe in both hemispheres. As with other weedy ruderal species, man undoubtedly assisted its transportation, but despite the excellent review by Rousi (1965, pp. 106-9), it is difficult to be convinced that anyone knows the true native range of P. anserina (Rousi 1965; Hultén & Fries 1986, Map 1097; Miyanishi et al. 1991).
European & world occurrence
P. anserina is common and widespread throughout N, W & C Europe, extending south into the northern half of the Iberian peninsula and extending eastwards north of the Alps to N Greece and the Crimea (Kurtto et al. 2004, Map 3400). P. anserina s.s. is worldwide in distribution, although it is not yet possible to separate geographically the varieties that have been distinguished in this taxon. It is certainly most widespread and frequent in temperate parts of the N Hemisphere where it is a member of the circumpolar boreo-temperate element, but it has also been introduced and has become an invasive, ruderal weed in the S Hemisphere in Chile, SE Asia, New Guinea, S Australia and New Zealand (Hultén & Fries 1986, Map 1097).
Threats
None.