Tearthumbs are native weak-stemmed plants that sprawl in moist areas. In the Arrow-leaved Tearthumb, a native annual, the stems grow from 3 to 6 feet long by climbing on and over other vegetation. There are no tendrils and the stems do not twine. Stems are 4-angled, hollow, green to pinkish red when in full sun. Stem joints are swollen at the nodes. The stems and the leaf midrib underside are covered with tiny backward pointing (retrorse) prickles, 1 to 1.5 mm long, giving the common name "tearthumb", which happens if you run your fingers up the stem.
The leaves are alternate, entire, arrow shaped with downward pointing lobes that surround the stem with a heart shape base. An brownish cylindric ocrea forms at the stem where the leaf joins but is mostly visible only on the side opposite the leaf joint. (Ocreas are small sheaths the wrap the stem - replacing stipules that would normally be that position - common in grasses). The ocrea is without prickles. The leaf midrib underside, like the stem, is also covered with the tiny 1 to 1.5 mm long backward pointing (retrorse) prickles. The leaf surface may be smooth or with dense appressed short hairs.
The inflorescence consists of tight uninterrupted flower clusters on long stalks, either terminal or from the leaf axils. Cluster stalks may but normally do not have prickles.
The flowers are small, 3/8 inch long, and perfect, with either greenish-white or pinkish 5-parted perianth lobes. The petals and sepals of the perianth are combined as 5 tepals, united for at least 1/2 their length, each flower subtended by a small green bract. Tepals are broadly elliptic in shape with no noticeable veins. Flowers usually have 8 stamens, not united, with distinct white filaments, and white to pinkish anthers. Three styles appress together for half their length, forming a column.
Seed: Fertile flowers produce a dry achene that is somewhat 3-sided, smooth surfaced and varying in color from light brown to black.
Habitat: Arrow-leaved Tearthumb is an annual with a fibrous root system, growing each year by reseeding. The plant grows in moist areas that have lots of sunlight such as marsh edges, damp roadside ditches, shorelines, etc.
Names: The genus Persicaria is from the Latin and meaning 'peach-like', which was a name applied a millennium ago to Knotweeds (Smartweeds); it 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, sagittata is Latin for the arrow shape of the leaf. The genus Polygonum has recently been divided into several genera, one of which is Persicaria. This species was formerly classified as Polygonum sagittatum. Many references will still classify this species in the Polygonum genus. The author names for the plant classification are as follows: '(L.)' is for Carl Linnaeus (1707-1778), Swedish botanist and the developer of the binomial nomenclature of modern taxonomy. His work was updated by ‘H.Gross’ which refers to Hugo Gross (1888-1951) French botanist. He is listed as the authority on over 90 different species, principally in the Polygonaceae family, with about 28 species in the Persicaria genus alone. Two publications of his from 1913 concerning this family are: Remarques sur les Polygonées de l'Asie Orientale, Bulletin of Academie Internationale Géographie Botanique; and Beiträge zur Kenntnis der Polygonaceen. Published by Botanische Jahrbücher für Systematik
Comparisons: While there are 12 different species of Persicaria in Minnesota, none have the characteristics of this plant, particularly the retrorse prickles.
Above: The upper sections of several plants. The stems are usually erect at the terminal ends and sprawling across other plants at the lower ends.
Below: 1st photo - Both the terminal flower cluster and a leaf axil cluster are visible. 2nd photo - The leaf structure with the base clasping the stem, from which comes the name "Arrow-leaved".
Below: 1st photo - The ribbed stem and backward pointing stem prickles from which comes the name "tearthumb". Prickles are also on the underside leaf mid-vein. 2nd photo - Flower cluster detail showing one open 5-lobed flower.
Below: Two views of the leaf and stem ocreas, which do not have prickles.
Notes: Arrow-leaved Tearthumb is indigenous to the Garden; Eloise Butler catalogued it on Sept. 6, 1907. At that time it was classified as Polygonum sagittatum. Martha Crone's 1951 inventory of plants in the Garden at that time included it. It is native to Minnesota in wooded counties of the state, roughly those east of a diagonal running from Mower in the south to Marshall in the Northwest. In the U.S. it is found from the great plains east to the coast. In Canada from Manitoba eastward to the coast. There are 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.
Uses: It is not known that the plant was ever used for any purpose in North America but Mrs. Grieve (Ref.#7) reports that when it made it way to Europe the Irish are said to have used the plant medicinally to treat kidney pain and abdominal pains.
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.
Identification booklet for most of the flowering forbs and small flowering shrubs of the Eloise Butler Wildflower Garden. Details Here.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"