Pale smartweed (Persicaria lapathifolia, (L.) Delarbre 1800)

Identity

Pale smartweed (Persicaria lapathifolia (L.) Delarbre, 1800), also known as pale persicaria, willow weed or dock-leaved smartweed, is an annual herb in the genus Persicaria and the Polygonaceae family. Plants of the World Online accepts the combination made by Delarbre in 1800, based on Polygonum lapathifolium, published by Linnaeus in 1753. ^{1 2}

The epithet lapathifolia means “dock-leaved,” referring to the resemblance between this plant's foliage and that of Rumex, the docks. Swollen stem nodes are another conspicuous feature shared by Persicaria and related Polygonum species; they explain the French name for these knotweeds. ³

Recognition

Pale smartweed has an upright or ascending stem, often reddish, with distinctly swollen nodes. Height varies considerably with climate: 20–80 cm (8–31 in) in France and temperate Europe, but up to 1.80 m (6 ft) or even 2 m (6.6 ft) on well-watered tropical riverbanks in northern Australia. ^{5 6}

At every swollen node sits a tubular ochrea, the stipule sheath characteristic of Polygonaceae. In pale smartweed its upper margin lacks a fringe of hairs, whereas the sheath of Persicaria maculosa is clearly hairy. Examining this detail with a hand lens is more reliable than stem color or leaf size for distinguishing the two in the field. ^{7 4}

Its alternate, lance-shaped leaves measure 4–22 cm (1.6–8.7 in) long. Their undersides often carry small, stalkless glands, visible as dots under magnification and absent in Persicaria maculosa. Some leaves have a central dark blotch, but this variable feature never confirms the identification by itself. ^{6 4}

Narrow, dense flower spikes droop or arch distinctly at maturity, unlike the shorter, upright spikes of Persicaria maculosa. Each tiny flower measures 3–4 mm (0.12–0.16 in), with four pink, white or pale green tepals and five to six stamens bearing reddish anthers. ^{4 7}

Useful taxonomy

For this variable species complex, Plants of the World Online retains Persicaria lapathifolia (L.) Delarbre, published in 1800 from the basionym Polygonum lapathifolium L., 1753. Four documented infraspecific taxa and synonyms illustrate its morphological diversity without displacing the accepted name. ¹

Frequently documented infraspecific taxa and synonyms are:

• Persicaria lapathifolia subsp. brittingeri (Opiz) Soják (botanical synonym or infraspecific name)
• Persicaria lapathifolia subsp. pallida (With.) Á.Löve (botanical synonym or infraspecific name)
• Persicaria lapathifolia var. salicifolia (Sibth.) Miyabe (botanical synonym or infraspecific name)
• Polygonum nodosum Pers. (botanical synonym or infraspecific name)

Regional floras sometimes treat the complex as an aggregate rather than a uniform species because stable characters separating some local forms are lacking. The accepted name in Plants of the World Online remains the consistent taxonomic reference. ^{1 7}

Habitat and range

Across the temperate and subarctic Northern Hemisphere, pale smartweed is native from North America through Europe and Asia. Native extensions reach North Africa, Ethiopia, and northern and eastern Australia. South America and southern Africa fall outside its native range. ¹

French records in the INPN place this native species in wet habitats throughout metropolitan France, including Corsica. ²

Water shapes its typical habitat: pond margins, ditches, riverbanks, wet farmland and disturbed ground on sandy, dark or clay-rich mud. In northern Australia, pale smartweed grows mainly along intermittent streams and around artificial ponds. It has also become naturalized beyond its native range, including Western Australia. ^{5 6}

Life cycle and reproduction

Through most of its temperate range, pale smartweed behaves as an annual: spring germination is followed by rapid growth, summer flowering and death with the first frosts. In humid tropical climates it can persist longer and exceed 1.80 m (6 ft), compared with 20–80 cm (8–31 in) in temperate conditions, as documented in northern Australia. ^{6 1}

Temperate Northern Hemisphere plants flower from July to October, while tropical plants may flower almost year-round. Each spike produces numerous shiny brown, flat-convex or biconvex achenes, 1.5–3.2 mm (0.06–0.13 in) long. They fall near the parent plant or travel with floodwater. ^{4 6}

Moisture strongly controls seed germination. Dense carpets of seedlings appear on newly exposed mud after water levels fall, explaining the crowded stands often seen on drying riverbanks in late summer. ⁵

Uses and risks

Agricultural records chiefly describe pale smartweed as a weed of wet cultivated land and disturbed ground, without a recognized horticultural use. Current reference databases list no documented toxicity to humans or common domestic animals. ^{2 3}

Unlike its peppery relative Persicaria hydropiper, pale smartweed has no distinctly pungent taste. Field botanists use this contrast as an additional clue alongside morphological characters when distinguishing the two species. ⁴

Flower clusters and fallen riverside achenes contribute to wetland ecology. They provide a late-season resource for pollinating insects and replenish the seed stock characteristic of periodically flooded soils. ⁵

Varieties, look-alikes and related plants

There are no recognized horticultural cultivars of pale smartweed. It is a spontaneously growing plant that has not been selected for ornamental use. ⁷

The closest routine look-alike is spotted lady's thumb (Persicaria maculosa), with its hairy ochrea and shorter, upright spikes. Other possible confusions include Persicaria hydropiper, with pungent leaves and conspicuous flower glands, and Persicaria pensylvanica, whose thicker inflorescences bear flowers with five tepals. ^{4 7}

Wet habitats also support other Polygonaceae, including Rumex obtusifolius and Rumex acetosa, whose foliage is likewise described as dock-like, and the much more invasive Reynoutria japonica. None has pale smartweed's characteristic drooping flower spikes. ²

Confirm pale smartweed by combining three details: an ochrea without a fringe of hairs, stalkless glands beneath the leaves and a distinctly arched flower spike. Hairs along the ochrea's edge instead point toward Persicaria maculosa.

Main sources

• Plants of the World Online / Kew, Persicaria lapathifolia (L.) Delarbre
• INPN / MNHN, Persicaria lapathifolia (L.) Delarbre, 1800
• Tela Botanica / eFlore bdtfx, Persicaria lapathifolia (L.) Delarbre
• Go Botany / Native Plant Trust, Persicaria lapathifolia - dock-leaved smartweed
• FloraBase / Western Australian Herbarium, Persicaria lapathifolia
• eFlora NT / Northern Territory Government, Persicaria lapathifolia
• Wilcox M., 2021, Recording Persicaria maculosa and P. lapathifolia, British & Irish Botany