Tuesday, 30 January 2018

Silverweed (Potentilla anserina L.)


(Potentilla anserina L., synonyms: Argentina anserina; Dactylophyllum anserine; Fragaria anserina.)

Family: Rosaceae
[‘RR’ rarity rating, Stace 2010:  254]
Silverweed, shown here at the Farlington Marshes,
is easily recognised by its silver-green tufted leaves
and small yellow flowers, which bloom betweenJune
and August.  (Farlington Marshes, August 1998)
Silverweed roots are one of the most carbohydrate-rich and palatable of our indigenous British root-foods. This species is widespread throughout the British Isles, often observed growing in dense swards on damp grassy areas, waste lands and sand dunes (for distribution maps see Online Atlas of the British and IrishFlora). Common in the temperate, alpine and cooler zones of the northern latitudes of Eurasia and North America, where it has frequently been reported to be used as food, this plant has an extensive range, occurring from arctic to almost subtropical latitudes (Rousi 1965, 47). It has a broad ecological versatility, being tolerant of frost as well as of marine exposure, and although preferring saltier and wetter environments, it grows on a range of soil types (Plants ForA Future [PFAF] database).

The leaves, stems, young shoots and roots of silverweed are edible although it is the carbohydrate-rich storage roots that have most often been reported to be eaten. Silverweed, alongside burdock root (Arctium lappa L.), has the highest concentrations of assimilable carbohydrate of any of the wild British root foods that we have analysed thus far.  In fact, silverweed and burdock roots have greater concentrations of carbohydrate than that of most starch staple foods consumed in the UK today (e.g. potato, see below) making them both remarkably sustaining foods.

Distinguishing Features of the Plant

This creeping, hardy perennial is easily recognised by its silver-green tufted leaves.  It has occasional thin red stolons and small (c. 2 cm.) 5-petalled yellow flowers that bloom between June and August. In addition to the normal, inedible fibrous roots, each mature plant produces between one and six starch-rich secondary roots, which store the plant’s carbohydrate energy reserves.
Close-up of the five-petalled
 floret. (Horseye Level, May 2008)

The secondary roots are long and narrow, rarely branching. When fresh they are yellow ochre to dark brown in colour and a have a transversely wrinkled surface. They typically measure between three to seven mm in diameter (thickness) and from 15 to 30 cm long.  On rare occasions, the storage roots are found with thicknesses as great as 10-12mm in diameter, which is most likely why the silverweed storage root is sometimes incorrectly described as a tuber[i]

Silverweed can form extensive stands such as in this
sward on the River Medway August 2006.
In common with the secondary root tissue of other dicotyledons, silverweed root is formed of concentric rings of tissue, the most external being the periderm, followed by a ring of phloem/parenchyma, internal to which is a cambium band and then the xylem. Hather (1988; 1993) examined silverweed anatomy under SEM and observed that parenchyma (storage) cells form the majority of silverweed secondary root tissue (for more details about the anatomy, see Hather 1993, 50-51).

Nutrient Values

Despite their scrawny appearance, these roots are highly palatable, raw or cooked.  They have a pleasant nutty taste and crunchy consistency, due to the rich starch content and absence of stringy fibres. Because of their starch content, silverweed roots are substantially more digestible if processed with heat and water, whether by steaming, boiling, light roasting, or made into "dampers" (a type of unleavened bread made from a flour and water and baked in the ashes of a fire) after being dried and ground.
Individual plants from a
heavily-trodden path. (March 2009)

The results of a recent nutrient analysis of silverweed roots at the King’s College London Division of Nutrition Sciences (Leeds and Hillman, unpublished notes[ii]) show that silverweed roots contain exceptional concentrations of assimilable carbohydrate. With 31g per 100g of edible portion, they provide higher levels of carbohydrate than most "starch staple foods" presently consumed in the UK, e.g. cultivated potato (Solanum tuberosum), which has a maximum of 26.5g per 100g FSA 2014).  Similar remarkable levels of carbohydrate are recorded for the roots of other Potentilla species. Pacific silverweed (Potentilla pacifica, (syn. Argentina egedii and Potentilla anserina. ssp. pacifica)), a species indigenous to the Northwest Coast, contains 29.5 g per 100 g of edible portion (Kuhnlein et al. 1982).

The principal carbohydrate in P. anserina root is starch, at 3.3% (per 100g dry weight (Wolfred and Fisher 1944, cited in Lloyd 2011, 129). Although various analyses of the nutrient profile of Potentilla spp. have been made, no analysis of specifically the starch content was found in the literature (a recent analysis of the related North American Pacific silverweed, Potentilla pacifica (synonym. Argentina egedii) was inconclusive; Lloyd 2011, 130).
Pacific silverweed also contains remarkably high concentrations of important essential vitamins and minerals in including Ca, Mg, Fe, Cu, Mn and Zn (Kuhnlein et al.1982).
Starch-rich secondary (storage) roots of P. anserine.
(Weir Wood Reservoir, August 2006)

Traditional Food uses of Silverweed roots in the British Isles in  recent times

The 19th-century Scottish folklorist Alexander Carmichael (1900) observed that Silverweed roots were counted as one of the ‘sevenbreads of the Gael’, which up to the arrival of the potato, were cultivated as well as gathered from the wild. He also reported that it was claimed in the Outer Hebrides island of North Uist that silverweed productivity was so great that a man could survive on the roots from a small plot. The abundance of silverweed in North Uist today gives credence to Carmichael’s claims (Ray Meares, Pers. Comm. 2011). At Clunie in Perthshire, people in the late 18th c. consumed silverweed roots as one of just two caloric staples, the other being the pignut (Conopodium majus) (Darwin and Bewick 1996).  Svanberg (2012) notes their consumption in the Shetlands and Johnson (1862), in his Useful Plants of Great Britain, recorded the consumption of these roots in times of famine in the Hebrides.

Traditional uses of Silverweed Root in Other Regions within the Northern Hemisphere

Various Silverweed species are reported to for their food uses (or medicine, see below) for many different societies throughout the Northern Hemisphere. The roots were eaten, for example, in Iceland (Svanberg (2012), parts of Scandinavia, e.g. Norway, the Faroe Islands, eastern Europe, (Łuczaj and Szymański 2007) and across Asia, particularly among Tibetan pastoralists (Boesi 2014).  In his remarkable Geschichte unserer Pflanzennahrung, Maurizio (1927), for example, describes the roots as a favourite food of the Tanguts of the Tsaidam Depression in NE Tibet. Hunter-gatherer groups in the Russian Far East, such as the Chukchi hunter-gatherers in the Russian Far East, also gathered silverweed roots, which they ate in soups (Strecker 2014).  
The most complete ethnobotanies and ethnohistoric descriptions of silverweed harvesting and processing are for the hunter-gatherer peoples in north western Canada, particularly the area that is present-day British Columbia.
Silverweed Food Uses by Coastal and Interior Plateau Hunter-Gatherers in British Columbia, Canada
Two Potentilla species were eaten by Hunter-gatherer peoples in this province:  P. anserina in the interior and P. pacifica on the coast.
In the relatively arid Southern Interior of the province, P. anserina is common in alkaline soils around the edges of lakes, ponds, mud flats, marshes and meadows (Turner 1997). It is one among many wild plants with carbohydrate-rich edible storage organs that are abundant in the Southern Interior. The Secwepemc, Upper Stl'atl'imx, Nlaka'pamux and Okanagan First Nations dug the roots in late summer and autumn, and as with other edible roots, roasted or pit-cooked them before eating (Turner 1997). In some Southern Interior communities, carbohydrates, primarily from USOs (plant Underground Storage Organs), comprised more than 50% of the diet.
Unlike the Interior Plateau, the diet of Coastal First Nations was restricted to primarily marine foods as sources carbohydrate were less available (Deur 2005). Therefore, the edible roots of the Pacific silverweed (P. pacifica) and Springbank clover (Trifolium wormskjoldii)) were highly important staple sources of carbohydrates for coastal First Nations. The most detailed information on Silverweed harvesting and processing that is found in the literature is on this species P. pacifica (see Kuhnlein et al. 1982; Lloyd 2011 Turner 1995; Turner and Kuhnlein 1982).
Also known as Pacific cinquefoil, Pacific silverweed is unique to the Northwest Coast. It grows in estuarine environments including river tidal flats, salt marshes, beaches and dunes, from southern Vancouver Island to Alaska (Turner 1995). From north to south, the Haida, Nuxalk, Kwakwaka'wakw, Comox, Sechelt, Ditidaht (Nitinaht), Straits Salish, as well the Makah of Washington are reported to have eaten Pacific silverweed (Turner 1995).
Native peoples typically managed wild plots of Pacific Silverweed and also the rhizomatous Springbank clover (a species that is also unique to the Northwest Coast). These two plants are found in the same types of habitats and often occur together.  Both were managed by Coastal people by weeding, selective harvesting and replanting. Some communities, most notably the Kingcome Inlet Kwakwaka'wakw on the central coast and Ditidaht (Nitinaht) of southern Vancouver Island, created root gardens through by transplanting and cultivating these plants on purpose-built mounds or terraces (Deur 2005; Lloyd 2011; Kuhnlein et al. 1982; Turner 1995).
Silverweed roots were of such importance that individuals, particularly chiefs, claimed ownership of the wild and cultivated stands, which were passed on through inheritance. Both species were also highly valued trade items and feast foods, and the best specimens were reserved for high-status individuals and given away at Potlatches (Turner 1995).
Pacific silverweed roots were not eaten raw because of their inherent bitterness, which is reduced but not always entirely removed, by steaming (Turner 1995).  The roots were gathered in large quantities in September and October and cooked in steaming pits or bentwood boxes (and in recent times, kettles). To cook the roots, pit ovens or boxes were lined with grasses and fuelled by hot rocks placed in the bottom; vegetation, including leaves and branches, were added in layers, onto which the food to be cooked was placed; water was then added to create steam and the pits were covered to retain moisture and heat (Kuhnlein et al. 1982; Lloyd 2011). Among the vegetation used for steaming Pacific silverweed roots, Kuhnlein et al.  (1982, 90) list salal leaves (Gaultheria shallon), red alder branches (Alnus rubra), and the fronds of the ferns Polystichum munitum, Droyopteris expansa and sometimes bracken fern (Pteridium aquilinum).

Detailed descriptions of Canadian Native uses of these two species can be found in ethnobotany papers by Deur et al. (2005) Kuhnlein et al. (1982) Lloyd (2011), Turner (1997) and Turner and Kuhnlein (1982).
Medicinal Uses
Across Eurasia, written and oral histories on the medicinal uses of Potentilla, for both internal and external medical applications, have significant time-depth. In Chinese traditional medicine, particularly on the Tibetan Plateau, it is claimed that P. anserina roots and leaves have been used for thousands of years to treat a range of illnesses, including hepatitis-B, certain cancers, diabetes mellitus infections as well as external problems such as wounds (Tomczyka and Latte 2009; Zhaoa et al. 2008).

Several ancient European texts, some dating from over 1,000 years ago, describe the use of Potentilla. Among these is the Materia Medica written by Pedanius Dioscorides, a Greek physician and botanist, which describes a concoction of the roots of a Potentilla to treat mouth ulcers and facial eczema (Tomczyka and Latte 2009, 185). In Germany, P. anserina has been employed medicinally since at least medieval times as a tea for treating menstrual disorders, oral inflammations, diarrhoea and other gastrointestinal illnesses (Kombal and Glasl 1985). Sami reindeer herders of the Scandinavian far north are also reported to use a Potentilla medicinally (Rautio 2014).

On the Interior Plateau of British Columbia, decoctions of roots, stems, florets and/or leaves of several Potentilla species were used to treat both internal and external problems. For example, the Okanagan-Coleville people drank a tea from the pounded, fresh or dried plants to treat diarrhoea and applied a poultice made from various plant parts directly to sores and wounds (Turner et al. 1980, 126-127).

There is an increasing scientific interest in the phytochemical and pharmacological properties of Potentilla species. The results of recent controlled laboratory analyses (Wang et al. 2013; Zhao 2008) demonstrate that the positive medicinal effects of P. anserina are primarily due to high levels of hydrolysable and condensed tannins, flavonoids and triterpenes, which are present throughout the plant. It was also found to contain substantial types and amounts of various steroids, phenolic acids and coumarin compounds (Kombal and Glasl 1985).  The beneficial medicinal effects include immunity-enhancing, anti-oxidant, anti-inflammatory anti-fatigue, anti-viral and antimicrobial activities, cardiomyocyte and liver protection (Wang et al. 2013; Tomczyka and Latte 2009).
A comprehensive review of the phytochemical and pharmacological properties of Potentilla is provided in Tomczyka and Latte (2009).

The Root Harvest

The best times to gather Silverweed roots are early spring and early autumn. In the autumn the plants can be harvested when the leaves have begun to turn a rusty brown, as by this time of year a new generation of starch-rich roots are available. Although the roots are edible throughout the winter, until new aboveground shoots appear in the spring, the plant is impossible to locate after the leaves have died back in the autumn

In spring, new shoots sprout from the tops of the last year’s roots. These are best harvested early, while the silvery new leaves are still barely visible above the soil surface, and the roots are still sweet, crunchy and highly palatable. However, the carbohydrate reserves quickly become depleted because they are required to fuel new growth, including the aboveground stems, leaves and florets, and newly sprouting belowground roots. The new, thin, white roots (often mauve- or purple-tinged) sprout from the central stock and develop starch and other nutrients. Eventually they turn a streaky yellowish-brown, and are fit to eat again in the autumn.

Roots of P. anserina harvested from a
20 X 20 cm plot. (Weir Wood Reservoir, October 2006)
Digging up wild roots is often hard work, and Silverweed root is no exception, even though the roots are shallow compared with those of most other USOs. It is especially difficult on the heavy, sticky clay soils in which inland populations of Silverweed so often grow, and requires a spade, mattock or digging stick. Turner and Kuhnlein (1982) reporting on the harvesting of Springbank clover (which is similar to Pacific silverweed in this regard), estimate that a person might obtain 0.5-1 kg in an hour if digging a productive patch; they further observed that it had taken a Native colleague as much as five hours to obtain enough clover ‘roots’ for three large servings. Nevertheless, compared with other wild food plants, particularly those that need detoxifying or grinding, the processing end of the sequence is significantly less labour-intensive than other wild root foods. 

Archaeological record
Silverweed seeds (achenes) have been recovered from waterlogged Mesolithic sites in the UK, Ireland and other European archaeological sites (ASDU 2009), as well as Iron Age, Roman and Medieval sites but the roots have yet to be found. These finds verify that Silverweed was widely available, even though they are not evidence that the roots contributed to local diet.

Given the likelihood that these roots were consumed in the prehistory of Britain, Ireland and northern Europe, their absence in the archaeological record is probably due to preservation problems that are typical of soft plant tissue (see Colledge and Conolly 2014). The roots may in future be found preserved in waterlogged contexts or in sites where they were charred before deposition. However, Hather (1988: 156, 216-8), who described in detail the anatomy of these roots, also subjected them to charring experiments and found that the central vascular tract typically disappeared when charred, leaving unidentifiable debris.
Summary and Conclusion
Although the roots of Silverweed roots have yet to be found in Mesolithic sites, given its edibility and high nutritional composition, it likely played a role in the diet of the hunter-gatherers in Britain. In the damp conditions of the Mesolithic Britain there would have been many more inland habitats with wild meadows suitable for Silverweed. The benefits are that they can be harvested in quantity, are an extremely palatable energy-packed food, require minimal processing and cooking and can be eaten raw, and once dried, they can be stored for long periods. Furthermore, they area functional food that, in addition to providing starch, energy and significant levels of beneficial vitamins and mineral, contain phytochemical components that support good health and disease-prevention.
References Cited
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Colledge, S. and Conolly, J. 2014. Wild plant use in European Neolithic subsistence economies: a formal assessment of preservation bias in archaeobotanical assemblages and the implications for understanding changes in plant diet breadth. Quaternary Science Reviews 101: 193-206.

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Liu, Z.J., Bai, Y., Guo, L. and Wang, S. 2015 Research progresses on chemical constituents of the root of Potentilla anserina L. and its pharmacological activities.  Journal of Food Safety and Quality 6: 3569-3574.

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Wang, J., Zhang, J., Zhao, B., Wang, X., Wu, Y., & Yao, J. (2010). A comparison study on microwave-assisted extraction of Potentilla anserina L. polysaccharides with conventional method: Molecule weight and antioxidant activities evaluation. Carbohydrate Polymers, 80, 84–93 doi:10.1016/j.carbpol.2009.10.073 [Accessed January 26, 2018].

Zhao, Y.-L., Cai, G.-M., Hong, X., Shan, L.-M., Xiao, X.-H., 2008. Anti-hepatitis B virus activities of triterpenoid saponin compound from Potentilla anserine L.  Phytomedicine 15 253–258.

[i] Some readers may find the distinction of "root" from "tuber" to be merely semantics, but recognising the anatomical differences between root and tuber tissue are critical steps in archaeological identification.
[ii] Anthony Leeds is now a Visiting Senior Fellow in the Faculty of Health and Medical Sciences, Surrey University.

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