Silverweed
Argentina anserina
Rose family (Rosaceae)

Description: This herbaceous perennial plant forms a rosette of spreading to ascending basal leaves. The basal leaves are up to 12" long and 4" across, although they are usually less than two-thirds of the maximum size; they are simple-pinnate with 5-11 pairs of sessile opposite leaflets and a sessile terminal leaflet. The petioles of basal leaves are relatively stout and more or less long-hairy, especially toward their bases; they are light green, whitish yellow, or red. The leaflets become gradually larger toward the tips of the compound leaves; they are up to 2" long and " across. The leaflets are elliptic-oblong or oblanceolate-oblong in shape and coarsely serrated along their margins; the teeth of the margins have narrow bristly tips. The upper leaflet surface is yellowish green to dark green, glabrous to sparsely hairy, and more or less shiny. The lower leaflet surface is white-canescent from dense minute hairs, and some longer white hairs may be present. Interspersed between these leaflets, there are often much smaller secondary leaflets with similar characteristics.



The rachises of the compound leaves are similar to their petioles, except there are grooved above and tend to be less hairy. From the center of the rosette of basal leaves, one or more widely spreading stolons are produced that are 1-3' long. At intervals along these stolons, there are leafy-bracted nodes about 4-6" apart. The stolons are red, terete, and glabrous to sparsely long-hairy. The leafy bracts of the nodes are simple-pinnate; they resemble the basal leaves, except they are much smaller in size (up to 4" long and 1" across). When the nodes have contact with moist ground, they are able to develop new roots, from which new rosettes of basal leaves can develop. Solitary flowers are produced from the nodes of the stolons on long naked pedicels (floral stalks) about 2-6" long. These ascending to erect pedicels are light green to red, terete, and nearly glabrous to long-hairy. Each flower spans about " across, consisting of 5 spreading yellow petals, 5 light green sepals, a ring of 20-25 yellow stamens, and a central head of 20-100+ yellow pistils. The petals are obovate or broadly elliptic in shape, while the hairy sepals are ovate, and often cuspidate (abruptly tapering into narrow pointed tips).



Directly underneath the sepals, there are 5 floral bractlets; these bractlets are similar to the sepals, except they are usually lanceolate and sometimes sparingly toothed. Both the sepals and bractlets are joined together at the base, and they are both shorter than the petals. The blooming period occurs during the summer and early autumn for 2-3 months. Flowering is more abundant during periods of active growth. Cross-pollination of the flowers is required for the production of fertile seeds. The seedheads are cupped along their sides by the persistent sepals and floral bractlets. Individual seeds are 2–2.5 mm. long, compressed-ovoid in shape, and grooved along the upper surface. Because the outer seed coat is somewhat corky from enclosed air pores, the seeds are able to float on water; they can also be blown about by the wind. The primary root system consists of a taproot that often branches. This plant forms clonal colonies by means of its stolons.



Cultivation:
The preference is full sun, wet to mesic conditions, and calcareous soil containing gravel or sand. This plant doesn't tolerate much competition from taller vegetation.

Range & Habitat: The native Silverweed is uncommon in NE Illinois, while in the rest of the state it is absent (see Distribution Map). Silverweed has a wide distribution in boreal areas of North America, and it also occurs in Eurasia. Habitats include sand prairies, moist sand flats (pannes), borders of interdunal swales, low areas along sandy ponds, small sand dunes, middle to upper beach areas, and gravelly or sandy areas along mowed paths. In Illinois, this plant occurs primarily along Lake Michigan. It is usually found in high quality habitats where there is some natural disturbance from water or wind, although it can also colonize open degraded areas that are moist, sandy, or gravelly. This is one of the pioneer plant species that helps to stabilize the sand in wetlands, low dunes, and beach areas.

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Faunal Associations:
The flowers are usually cross-pollinated by various bees and flies, including bumblebees, cuckoo bees (Stelis spp.), Andrenid bees, and Syrphid flies (Miyanishi et al., 1991; Krombein et al., 1979; Discover Life, 2015). Both nectar and pollen are available as floral rewards. A small number of insects have been reported to feed on the foliage, sap, or roots of Silverweed in North America. These species include a flea beetle (Altica browni) and several aphids (Chaetosiphon fragaefolii, Macrosiphum potentillae, and Rhopalosiphoninus latisiphon); see Clark et al, (2004) and Blackman & Eastop (2013). Foliage and flowers of Silverweed are occasionally browsed by White-tailed Deer and other hoofed mammalian herbivores, although they are not eaten by the granivorous American Bison. By surviving the passage through their digestive tracts, the seeds of this plant may be spread to new locations by these mammals.



Photographic Location:
A moist gravelly area along a path and a low area along a sandy pond at Illinois Beach State Park in NE Illinois.

Comments: A scientific synonym of Silverweed is Potentilla anserina. The description of this plant refers to the typical subspecies, Argentina anserina anserina. This is the only subspecies that occurs in Illinois. Other subspecies have been described that vary in such characteristics as the hairiness of the foliage, the general size of individual plants, the presence or absence of an upper groove on the seeds, the relative size of the sepals versus the floral bractlets, and other characteristics. It is relatively easy to distinguish Silverweed from similar species (mainly Potentilla spp.). Compared to these other species, Silverweed can be distinguished by the greater number of leaflets per compound leaf, brilliant white undersides on its leaflets, sharp bristly teeth along the margins of its leaflets, and conspicuous red stolons.

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