Spotted lady's thumb (Persicaria maculosa, Gray 1821)

Identity

Spotted lady's thumb (Persicaria maculosa Gray, 1821), also called lady's thumb or redshank, is an annual Polygonaceae in the genus Persicaria. Its dark, crescent-shaped leaf blotch and reddish stem help identify it early in growth on wet, disturbed ground in temperate Europe. Plants of the World Online accepts the name dating to 1821, when Gray published it from an earlier Linnaean basionym. ^{1 2}

Latin persica, meaning peach tree, gives the genus Persicaria its name: the lance-shaped leaves resemble peach foliage. The epithet maculosa, “spotted,” describes the dark leaf marking, which also explains the English name lady's thumb. ⁴

Recognition

Spotted lady's thumb develops a jointed stem, upright or spreading according to available space. It commonly reaches 20–60 cm (8–24 in), occasionally 1 m (3.3 ft) in rich, wet ground. Swollen nodes, often reddish, carry the membranous, short-haired sheaths typical of Polygonaceae. ^{3 4}

Almost stalkless, lance-shaped leaves extend along the stem and measure 1–18 cm (0.4–7 in) long. Most display a brown or blackish crescent-shaped blotch in the center. This is useful but not conclusive: some individuals lack the marking. When it is absent, the hairy sheath at each node remains the decisive character, present in all individuals. ⁴

Pink to whitish flowers, just 2–4 mm (0.08–0.16 in) across, form dense, upright spikes in the upper leaf axils. In the temperate Northern Hemisphere these appear from July to September. The spikes are much less showy than those of garden-grown Persicaria bistorta. Each flower produces a glossy black, three-sided achene hidden within the persistent perianth. ^{3 4}

Useful taxonomy

The name retained by Plants of the World Online is Persicaria maculosa Gray. Its basionym, Polygonum persicaria L., dates to Linnaeus in 1753. Several synonyms still occur in older floras and herbarium collections: ¹

• Persicaria mitis Delarbre (botanical synonym or infraspecific name)
• Persicaria persicaria (L.) Small, a tautonym rejected under nomenclatural rules (botanical synonym or infraspecific name)
• Polygonum ruderale Salisb. (botanical synonym or infraspecific name)
• Peutalis persicaria (L.) Raf. (botanical synonym or infraspecific name)

Long treated as a section of Polygonum, the genus Persicaria groups plants whose nodes carry a membranous ochrea. This sheath distinguishes Polygonaceae from other herbaceous plants with flowers clustered into spikes. ⁷

Habitat and range

Temperate Eurasia extending into Indochina, Macaronesia and North Africa form the native range of spotted lady's thumb. In France, where it is native, it remains common across almost the whole country, from riverbanks to cultivated fields. ¹

North American records date its introduction to the Great Lakes region in 1843. It has since spread across much of the continent, where the USDA PLANTS Database lists it as a naturalized weed. Spotted lady's thumb also occurs in Australia and New Zealand, outside its native range. ^{6 4}

Disturbed, cool to wet soils suit the plant: riverbanks, ditches, cultivated fields, lightly weeded gardens and waste ground beside paths. Dry, well-drained sites restrict its growth. Its seasonal temperature tolerance is broad, −15 to 35 °C (5–95 °F), but it establishes persistently only where soil remains moist. ³

Life cycle and reproduction

An entire annual life cycle fits within one growing season. In the temperate Northern Hemisphere, spring germination leads to rapid leafy growth, flowering from July to September and death with the first autumn frost. Once soil warms, germination takes 1–2 weeks, allowing quick colonization of freshly worked land. ³

Self-fertile flowers produce black achenes that fall to the ground at maturity rather than being ejected. Water, wind and seed-eating birds subsequently disperse them; birds consume them in autumn and winter. This movement helps explain the plant's frequent occurrence beside waterways. ⁴

Rather than relying mainly on seeds dormant for decades, spotted lady's thumb depends chiefly on abundant annual seed production. Removing plants before flower spikes develop therefore effectively limits self-seeding the following year. ³

Uses and risks

Young leaves and shoots can be eaten raw or cooked. Their mild flavor contrasts with the pungency of the related Persicaria hydropiper. Although technically edible, the seeds are seldom used because gathering them yields little practical return. ⁵

Modest amounts of oxalic acid occur in the leaves, together with astringent tannins used traditionally against inflammation and in poultices. No acute toxicity is documented for humans or domestic animals. Regular consumption is nevertheless discouraged for people with gout, arthritis or kidney stones. ⁵

Cherokee traditional practice used crushed leaves externally to relieve itching caused by poison ivy. This use is reported in North American ethnobotanical accounts. ⁴

Varieties, look-alikes and related plants

Spotted lady's thumb has no recognized ornamental cultivar. It grows spontaneously and has not been selected for gardens, unlike horticultural relatives such as Persicaria bistorta and Persicaria amplexicaulis. ³

A frequent source of confusion is waterpepper, Persicaria hydropiper. Its crushed leaf tastes distinctly pungent, while that of P. maculosa is mild. The sheaths and density of the flower spikes also help distinguish the two species before flowering. ⁵

Related smartweeds, including Persicaria lapathifolia and Persicaria pensylvanica, share an upright habit and sometimes spotted leaves. Examine the membranous sheath at the leaf axil closely: its length and the presence and length of marginal hairs vary between species. These small characters require close inspection rather than identification from a distance. ⁴

Main sources

• Plants of the World Online / Kew, Persicaria maculosa Gray
• International Plant Names Index, Persicaria maculosa Gray
• Royal Horticultural Society, Persicaria maculosa - redshank
• Go Botany / Native Plant Trust, Persicaria maculosa
• Plants For A Future, Polygonum persicaria
• USDA PLANTS Database, Persicaria maculosa - spotted ladysthumb
• Integrated Taxonomic Information System, Persicaria maculosa Gray