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.
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 flower 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.
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 sepals 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. This species was formerly classified as Polygonum persicaria. (More detail at bottom of the page.) The author name for the plant classification - ‘Gray’ 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.
Comparison: A Smartweed without the blotch on the leaf and which grows in a more wet 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.
Above: The inflorescence is one or two spike-like dense elongated racemes of small flowers, the raceme usually arching or jointed at the top. The flowers are 5-parted, 1/8 inch long, with a perianth that may be greenish-white at the tip to rose at the base or entirely rose-pink.
Below: 1st photo - The leaves have a distinct triangular to crescent shaped blotch on the upper surface. 2nd photo - There is a sheath-like structure called an ocreolae where the flower stem attaches. 3rd photo - There is a green to greenish-brown sheath where the leaf attaches - an ocrea.
Below: 1st photo - The lower portion of stems are often reddish to purplish.
Below: Drawing of Spotted Lady's-thumb seeds by Norman Criddle.
Notes: Spotted Lady's-thumb is native to Eurasia, Africa and Oceania, and is now naturalized across North America. In Minnesota it is found in most counties throughout the state. It was first collected in Minnesota in 1877 in Hennepin County. It is one of 12 species of Persicaria in Minnesota, 3 are introduced, 9 are native. All but two are found in Hennepin County and one of those two, Persicaria careyi, is on the Special Concern list as it was last collected in 1940.
Classification: The genus Polygonum has recently been divided into several genera, one of which is Persicaria. Spotted Lady's-thumb has had a number of previous scientific classifications, the most general being Polygonum persicaria L., which had some varieties associated with it. Some of the important authorities today, such as Flora of North America, refuse to recognize these older names and varieties and considered all the differences simply widespread variances of the same genus and species. They recognize some varieties that exist in Europe but not in North America. Both the U of M Herbarium and the MN DNR listings follow that line of reasoning. USDA still lists the plant as Polygonum persicaria L.
References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"