Minnesota Flowering Plants
Introduced species

Spotted Lady's-thumb is an introduced and naturalized erect to sprawling annual forb, growing on mostly smooth green stems from 1/2 to 2-1/2 feet tall with many branches, often red or purplish toward the base. Roots may form from the lower stem nodes.

The leaves are alternate, narrowly lance-shaped, about 6x longer than wide, with pointed tips, and a short stalk that has a light green to greenish-brown cylindric sheath (an ocrea), that encloses the stem. The ocrea is truncate at the edges, sometimes with fine hair, and may be fringed with short bristles. The leaves of this species have a distinct triangular to crescent shaped blotch on the upper surface that may be distinctly dark to faint and barely perceptible. Leaves have a tapered or wedge shaped base and taper to fine tip. Leaf margins are entire but margins have stiff forward pointing hairs (antrorse hairs).

The inflorescence is one or two spike-like dense elongated racemes of small flowers, the raceme usually arching or jointed at the top. These are terminal or from the upper leaf axils. Where the stem of the inflorescence attaches there is a sheath-like structure called an ocreolae. It has bristly margins. The flowers are in clusters (fascicles) and the clusters may be interrupted toward the bottom of the raceme as shown in photos below. There will be 4 to 14 flowers per fascicle on the raceme and the fascicles are closely spaced except for the interruptions.

The flowers are 5-parted, 1/8 inch long, with a perianth that may be greenish-white at the base to rose at the tip or entirely rose-pink. Sepals and petals are combined as 4 or 5 tepals that open to a cup shape; the tepals are united for about 1/3 of their length, have entire margins and prominent veins, but not veins ending in an anchor shape. Usually there are 4 to 8 stamens with white to pale yellow anthers, that do not protrude beyond the tepals and 2 or 3 styles that are united at their bases.

Seed: A fertile flower produces a dry ovate 3-angled shiny brown-black achene that tapers to a beak and is wind dispersed.

Habitat: Spotted Lady's-thumb is a weedy plant of moist waste places to cultivated places. It grows from a shallow root system and as an annual, it does not produce rhizomes but reproduces from seed.

Names: The genus Persicaria is from the Latin and meaning 'peach-like', which was a name applied a millennium ago to Knotweeds (Smartweeds) and is derived from the Latin persica for 'peach' and aria, meaning 'pertaining to', and thought to be referring to the leaves being similar to one of the peaches. The species, maculosa, is Latin meaning 'spotted' or 'mottled' referring to the blotch on the upper leaf surface. The genus Polygonum has recently been divided into several genera, one of which is Persicaria.

Author names for the plant classification: This species was originally classified by Linnaeus as Polygonum persicaria. (More detail at bottom of the page.) The current name was applied in 1822 by ‘Gray’ which is for Samuel Frederick Gray (1766-1828) British botanist and pharmacologist whose most important work was a Supplement to the Pharmacopoeia in 1818 and The Natural Arrangement of British Plants in 1821-22 in which this species is listed.

Comparison: A Smartweed without the blotch on the leaf and which grows in a more moist environment is Water Smartweed - Persicaria amphibia (L.) Gray. Another that likes moist environments is Dotted Smartweed, Persicaria punctata (Elliott) Small, which is without the blotch on the leaf, the flower raceme is interrupted, nodding, and the flowers are greenish-white.