Polygonaceae; the buckwheat / knotweed / smartweed / dock family, Part 2
- jessica36732
- Dec 7
- 7 min read
Updated: Dec 8
By Patsy Cotterill
Persicaria (smartweeds) and Polygonum (knotweeds) are the other two genera in the buckwheat or dock family whose representatives we commonly encounter in the Parkland, apart from Rumex, the docks. (See Part 1)
Persicaria – the smartweeds
Originally species of Persicaria and Polygonum were included in the single genus Polygonum, and the key characters used to separate the two are not distinctly different. However, the smartweeds tend to bear their flowers in dense spikes, and to prefer somewhat wetter habitats than the knotweeds, in which the flowers are more dispersed in smaller groups, often in the leaf axils. With the exception of some obscure knotweeds, the species of both genera are in most cases distinct from each other and can be recognized by eye once known. Four other species of Polygonum have also been transferred to two different genera: Bistorta and Fallopia.
Pale smartweed (Persicaria lapathifolia) is a robust, native annual of wet places that can colonize large areas quickly if muddy or marshy ground promotes seed germination. It grows 20-80 cm tall, and is often branched and spreading. The alternate leaves are 5-20 cm long, lanceolate and entire, and can be hairy, especially below, or hairless. They are dotted with glands and often have a conspicuous black blotch in the middle of the leaf. The ochreae (stipules fused to form a sheath surrounding the stem at the base of the leaf) are entire, or adorned with very short hairs. The glandular flower stalks bear spikes, 1-6 cm long, of densely packed, small flowers, which can be greenish-white or pink. The usually five sepals, which are joined at the base, are 2.5 mm long and strongly veined, enclosing six stamens and two styles. The fruits are lens-shaped achenes that are 2-3 mm long and shiny. Pale smartweed is widely distributed in Alberta but is less common towards the west.
Two other annual species of Persicaria are reported for Alberta, both native to Eurasia and weedy: marsh-pepper smartweed (P. hydropiper) and spotted lady’s thumb (P. maculosa). In contrast, their ochreae are fringed with obvious bristles and their tepals lack prominent veins. P. maculosa in particular is very similar to P. lapathifolia, even having similar dark blotches on the leaves, but can be distinguished from it by the long bristles fringing the ochreae, which are lacking in P. lapathifolia. P. hydropiper is more distinctive in having long, narrow, cylindrical spikes.
Water smartweed (Persicaria amphibia) is a northern hemisphere, native perennial species with a wide distribution across northern North America. It comes in two varieties in Canada: flanged smartweed (var. stipulacea), and long-root smartweed (var. emersa). Wikipedia provides an easy key to distinguish between the two: (https://en.wikipedia.org/wiki/Persicaria_amphibia)
var. stipulacea: plants aquatic with floating stems and leaves; ochreae with a flared collar (although this may not always be present); floating leaves virtually stalkless with heart-shaped bases and blunt tips; flower spike usually less than 4 cm long;
var. emersa: plants commoner in marshes or ditches (i.e., wet soils); ochreae lacking a flared collar; leaves stalked with pointed tips; spikes usually two and unequal, longer than 4 cm.
In addition, Flora of Alberta (1982) reports that the flower spike stalks lack hairs or glands in var. stipulacea but are hairy in var. emersa. (Some authorities give these taxa specific status, with var. emersa otherwise known as Persicaria coccinea.)
Both varieties are rhizomatous, with long stems, up to 300 cm, and alternate, lanceolate leaves that can reach 30 cm in length. Stems and leaves can be hairy or hairless, with hairiness increasing in plants growing in more terrestrial habitats. Growing in typical aquatic habitat, var. stipulacea can be recognized by its shorter, oblong flower spike compared to the more cylindrical, longer flower spike of var. emersa.
The flowers in both species are pink, and 4-5 mm long, with 5-6 stamens and two styles. They come in two forms: one type has long stamens protruding beyond the sepals and short stigmas; the other type has short stamens and long stigmas. One source says that the flowers can also be unisexual. The achenes in both are lens-shaped and brown or black.
Terrestrial form of water smartweed (Persicaria amphibia var. stipulacea) in Acheson Field Pond, showing hairy stem and leaves, and flanged ochreae. 2022-07-13. Photo: P. Cotterill.
Typical aquatic form of var. stipulacea of water smartweed (Persicaria amphibia), in Acheson Field Pond, showing blunt hairless leaves and short flower spikes. 2013-07-17. Photo: Jason Teare.
Aquatic form of var. stipulacea of water smartweed (Persicaria amphibia) showing hairless leaves and ochreae without ciliate flanges. Zeiner Campground, Pigeon Lake Provincial Park. 2009-07-19. Photo: P. Cotterill.
The thick, shiny, floating leaves of var. stipulacea could possibly be mistaken for the floating leaves of grass-leaved pondweed, Potamogeton gramineus, as both can occupy the same habitat in our area.
The maps in Flora of Alberta show var. stipulacea as having a wide and common distribution throughout the province, whereas var. emersa is less common and more confined to the southern half of the province. Given that var. stipulacea can assume more of the appearance of var. emersa, as wetlands dry up, it is necessary to make a careful distinction between the two varieties to determine their relative abundance and distribution. An interesting case is that of a seasonal wetland in the Devon dune in the Acheson Industrial Area. When open water was present in a season with sufficient precipitation, typical var. stipulacea could be identified towards the centre of the pond, while plants showing the characteristics of the terrestrial form predominated towards the periphery, a predominance that became exclusive as the pond dried up owing to water diversion. When the pond had dried up farther and become a marsh, large areas of it became covered with the annual pale smartweed (Persicaria lapathifolia) - a different species altogether!
Water smartweed is a widespread species in the northern hemisphere that has had a variety of human uses, both for food and medicine.
Polygonum – the knotweeds
The members of the Polygonum genus that we are likely to encounter in the Parkland comprise only four species and two of these are rare. As weedy annuals with tiny flowers, they would not win any medals for pulchritude, or merit inclusion in the native garden, but they do have ecological value as pioneering species and as food for insects and other animals. Their flowers are confined to the leaf axils, in contrast to Persicaria species, and their leaves are commonly smaller.
Prostrate knotweed (Polygonum aviculare) is a prostrate, much-branched, annual (or sometimes perennial) plant of waste places, gardens and trails. It exists in a complex of six subspecies in Canada with only the subspecies buxiforme (box knotweed) recognized as native in Alberta. Since four of these subspecies are present in Alberta but are considered introductions, and because we do not normally distinguish between subspecies, it is impossible to know if the specimens we are looking at are native or non-native! The branches can extend up to 80 cm long. The leaves are narrowly lanceolate to oblong-lanceolate, 5-15 mm long, with silvery, transparent ochreae that become torn (lacerate) and fringed at their base. The flowers grow in clusters of 1-5 in the leaf axils and consist of five sepals joined at the base which are 2-3 mm long and green with white or pink margins. The achenes are 1.5-3 mm long, brown and dull.
Specimen of prostrate knotweed (Polygonum aviculare) collected from shore of Lake Wabamun. 2002-09-08. Photo: P. Cotteril.
Close up of specimen of prostrate knotweed (Polygonum aviculare) collected on shore of Lake Wabamun, showing leaves, flowers in axils and silvery lacerate ochreae. 2002-09-08. Photo: P. Cotterill.
Leathery knotweed (aka striate knotweed) (Polygonum achoreum), is a native annual ranging throughout Alberta. It is also a denizen of waste places, including roadsides, but can also occur in saline marshes. Its ridged stems are branched and spreading, although they can also grow erect. The leaves are often longer (1-3 cm) than in P. aviculare, bright or bluish-green and broader, elliptic to obovate, and broadly rounded at the tip. The whitish ochreae are thin and transparent. The flowers clustered in the leaf axils have yellow-green sepals, 3 mm long, that are joined at the base into a cup-like calyx that narrows towards the tip giving a bottle-like appearance. The three outer sepals are keeled and cover the three shorter inner ones. The achenes are three-sided, about 2.5 mm long, yellow-brown and dull.
Close up of leathery knotweed (Polygonum achoreum), showing clusters of bottle-shaped flowers, keeled sepals and fringed ochreae. 2025-07-25. Photo: P. Cotterill.
Branched shoot of leathery knotweed (Polygonum achoreum) showing leaves and flower clusters. 2025-07-25. Photo: P .Cotterill.
Leathery knotweed (Polygonum achoreum) growing on mud, Acheson Industrial Area. 2025-08-23. Photo: P. Cotterill.
Bushy knotweed (Polygonum ramosissimum) has yellowish-green flowers like leathery knotweed, and ridged stems, but is much less common. There are two records for the Edmonton area for this native but otherwise it is more likely to be found further south on saline shores, mud flats and roadsides. It can probably be best distinguished from P. achoreum by its narrow leaves which are pointed at both ends, and its shining achenes.
Douglas’ knotweed (Polygonum douglasii) is by contrast a slender, erect native annual, that can reach 40 cm in height. Again, it is a rather uncommon species, probably of somewhat disturbed grasslands and slopes. (I found it, for example, growing alongside an ant mound in Gibbons Badlands Prairie.) The leaves are narrow, 1-4 cm long, almost stalkless, and often have revolute margins. The flowers are about 4 mm long, in 1-3-flowered, axillary clusters. A distinctive character is that they soon become bent down, and the black, shining, three-angled achenes, which are 3-4 mm long, are likewise reflexed.
Finally, mention should be made of one other former species of Polygonum which has now been transferred to the genus Fallopia, as it is a commonly encountered annual weed of gardens and fields.
Wild buckwheat or Eurasian black bindweed (Fallopia convolvulus) has long stems bearing long-stalked leaves, at distant intervals, that trail or twine, often around other vegetation. Leaf blades are 2-5 cm long and broadly ovate-arrow-shaped, with the upper ones somewhat narrower. The flowers are in axillary clusters or small spikes with sepals that are greenish, whitish or pinkish, and 4 mm long; the outer sepals become keeled in the fruits. The achenes are three-angled, black, dull and granular.
Portion of twining stem of wild buckwheat (Fallopia convolvulus) in a weedy area of Nisku Prairie, showing leaves and flowers. 2023-07-31. Photo: P. Cotterill.
Twining stems with developing fruits of wild buckwheat (Fallopia convolvulus) in Nisku Prairie. 2023-07-31. Photo: P. Cotterill.
