Polygonaceae
Polygonaceae, the buckwheat, knotweed, or smartweed family, is a moderately large family of eudicots in the order Caryophyllales, containing about 50-65 genera and over 1,100 species. Found worldwide but most diverse in temperate regions, members are typically herbs (sometimes shrubs or vines) characterized by swollen stem nodes and the presence of a unique stipular sheath called an ocrea encircling the stem above each node. Flowers are generally small, lack true petals but often have petaloid tepals, possess a superior ovary with a single basal ovule, and mature into a triangular or lens-shaped achene, frequently enclosed by the persistent perianth.
Overview
The Polygonaceae family is globally distributed, thriving in habitats ranging from wetlands and disturbed areas to alpine slopes and arid deserts (especially the genus Eriogonum in western North America). The single most defining feature of the family is the ocrea, a tubular sheath formed from fused stipules that surrounds the stem at the base of the petiole – its presence is nearly universal and highly diagnostic. Stems often appear jointed due to swollen nodes where the leaves (and ocreae) attach.
Economically, the family includes important food crops like buckwheat (Fagopyrum esculentum), cultivated for its grain-like seeds, and rhubarb (Rheum rhabarbarum), grown for its edible, tart petioles (leaf stalks). Many species are familiar wildflowers or weeds, including smartweeds (Persicaria), knotweeds (Polygonum sensu lato, including invasive species like Japanese Knotweed Reynoutria japonica formerly in Polygonum), docks, and sorrels (Rumex). Some Eriogonum species are cultivated as ornamentals for dry gardens. The sour taste of sorrel and rhubarb is due to high concentrations of oxalic acid, common in the family. Anthraquinone pigments also occur, providing red colors and sometimes having laxative effects (Rheum).
In Oklahoma (current location as of March 27, 2025), the family is well-represented, primarily by numerous native and weedy species of smartweeds/knotweeds (Persicaria, Polygonum) in moist or disturbed areas, docks (Rumex) in fields and roadsides, and potentially some Wild Buckwheats (Eriogonum) in drier western parts of the state. Buckwheat and Rhubarb are cultivated food plants, not native.
Quick Facts
- Scientific Name: Polygonaceae Juss.
- Common Name: Buckwheat family, Knotweed family, Smartweed family
- Number of Genera: Approximately 48-65
- Number of Species: Approximately 1,100-1,200
- Distribution: Cosmopolitan, especially Northern Hemisphere temperate regions.
- Evolutionary Group: Eudicots (Core Eudicot)
- Order: Caryophyllales
Key Characteristics
Growth Form and Habit
Mostly annual or perennial herbs, some are shrubs, vines (Fallopia), or rarely trees. Stems are often distinctly jointed with swollen nodes.
Leaves and Ocrea
Leaves are almost always alternate (rarely opposite/whorled), simple, and typically have entire margins (though sometimes lobed or crenate). Venation is pinnate. The most diagnostic feature is the presence of an ocrea (plural: ocreae): a sheath, typically membranous or papery (sometimes leathery or leafy), that surrounds the stem immediately above the node where the petiole attaches. It is formed from fused stipules. The ocrea's size, texture, persistence, and margin characteristics (e.g., entire, lacerate, fringed with bristles) are important for identification.
Characteristic Polygonaceae node: Alternate simple leaf with an ocrea (stipular sheath) encircling the stem above the petiole base.
Inflorescence
Flowers are generally small but often aggregated into conspicuous inflorescences. These are highly variable, including terminal or axillary racemes, panicles, spikes (often dense and narrow in Persicaria), or head-like clusters, often composed of smaller cymose units (fascicles). In Eriogonum, flowers are typically clustered within a distinct cup-like involucre composed of fused bracts.
Flowers
Flowers are small, usually bisexual (sometimes unisexual), typically actinomorphic (radially symmetric), and hypogynous. They lack true petals.
- Perianth: Composed of (3-) 5 or 6 tepals (sepal-like or often petaloid), arranged in one or two whorls (e.g., 5 in one whorl, or 3+3). Tepals are distinct or fused only at the base, usually greenish, whitish, pinkish, or reddish. A key feature is that the tepals are typically persistent and often enlarge, become winged, hooked, fleshy, or hardened around the developing fruit, playing a role in dispersal.
- Androecium: Stamens usually number 3-9 (often 6 or 8), typically arranged in two whorls. Filaments are distinct or slightly fused at the base, arising from the receptacle or perianth base. Nectar glands often form a disc around the base of the ovary.
- Gynoecium: Ovary is superior, composed of typically 2 or 3 fused carpels, forming a single locule containing a single basal ovule. Styles typically 2 or 3, distinct or fused below; stigmas variable, often capitate or tufted.
Left: Small flower with 5-6 tepals, 6-9 stamens, superior ovary. Right: Achene (triangular or lens-shaped) often enclosed by persistent, modified tepals.
Fruits and Seeds
The characteristic fruit is a dry, indehiscent, single-seeded achene (or nutlet). Reflecting the gynoecium structure, the achene is typically triangular (trigonous, if from 3 carpels) or lens-shaped (lenticular/biconvex, if from 2 carpels). The achene is frequently enclosed or subtended by the persistent and often modified perianth (tepals). These modified tepals can form wings (Rumex), hooks (Rumex sometimes), or become fleshy, aiding wind, water, or animal dispersal. The seed has a straight or curved embryo and abundant starchy endosperm (unlike the perisperm found in many other Caryophyllales families).
Chemical Characteristics
Many species contain oxalic acid or soluble oxalates, contributing to a sour taste (Rumex - sorrels, Rheum - rhubarb). Anthraquinones are common, especially in roots (Rumex, Rheum), providing reddish/yellow pigments and sometimes laxative properties. Tannins and flavonoids are also widespread. Unlike most core Caryophyllales families, Polygonaceae produce anthocyanin pigments (not betalains) for red/purple coloration, a trait shared with Caryophyllaceae and Molluginaceae within the order.
Identification Basics
Identifying Polygonaceae almost always relies on finding the characteristic ocrea at the nodes, combined with other features like alternate simple leaves, small flowers with persistent tepals, and achene fruits.
Primary Identification Features
- Ocrea Present: A membranous or papery sheath encircling the stem above the node (formed from fused stipules). This is the most diagnostic feature.
- Habit: Mostly herbs (sometimes vines or shrubs).
- Swollen Nodes: Stems often appear jointed or swollen at the nodes where leaves attach.
- Alternate Simple Leaves: Leaves typically alternate, simple, often entire.
- Small Apetalous Flowers: Flowers lack true petals; perianth of (3-)5-6 persistent tepals (often petaloid).
- Superior Ovary (1 Locule, 1 Basal Ovule): Leading to a single seed.
- Fruit an Achene: Dry, single-seeded, typically triangular or lens-shaped, often enclosed by persistent tepals.
Secondary Identification Features
- Ocrea Margin: Entire, torn (lacerate), or fringed with bristles.
- Persistent Perianth Modification: Tepals becoming winged, hooked, or fleshy around the fruit (Rumex).
- Achene Shape: Triangular vs. Lens-shaped.
- Inflorescence Type: Racemes, panicles, spikes, heads (sometimes involucrate in Eriogonum).
- Sour Taste: Often present in leaves/stems (Rumex).
Seasonal Identification Tips
- Growing Season: Ocreae, alternate leaves, and swollen nodes are visible. Flowering occurs spring through fall.
- Late Season/Winter: The persistent perianth enclosing the achene is highly diagnostic and often remains on dead stems through winter (especially prominent in Rumex). Dried stems may still show swollen nodes and remnants of ocreae.
- (Oklahoma Context: Knotweeds/Smartweeds (Polygonum, Persicaria) and Docks (Rumex) are common weeds flowering/fruiting through summer and fall; their distinctive fruits often persist. Look for ocreae on any suspect plant).
Common Confusion Points
- Amaranthaceae / Chenopodiaceae: Also Caryophyllales, often herbaceous weeds with alternate simple leaves, small apetalous flowers with persistent tepals, superior ovary with single basal ovule, achene/utricle fruit. Key difference: Amaranthaceae lack an ocrea. Nodes not typically as swollen; perisperm present in seed.
- Caryophyllaceae (Pink Family): Often herbaceous with swollen nodes, superior ovary. Key differences: Leaves usually opposite; lack ocrea; flowers have distinct petals (often notched); fruit a capsule with multiple seeds on free-central placenta.
- Other families with small flowers/alternate leaves: Check carefully for the presence of the ocrea at the nodes. This feature is largely unique to Polygonaceae.
The presence of the ocrea is the single most reliable field mark for identifying Polygonaceae.
Field Guide Quick Reference (Polygonaceae)
Look For:
- Ocrea (stipular sheath) at nodes
- Stems often with swollen nodes
- Leaves alternate, simple (usually)
- Flowers small, apetalous
- Perianth of (3-)5-6 persistent tepals
- Ovary superior, 1 locule, 1 basal ovule
- Fruit an achene (triangular or lens-shaped)
- Achene often enclosed by persistent perianth
- Herbaceous habit (mostly)
Key Variations:
- Ocrea margin (entire, bristly)
- Perianth modification in fruit (wings, hooks)
- Achene shape
- Inflorescence type (Eriogonum involucrate heads)
- Habit (herb, vine, shrub)
Representative Genera
The Buckwheat family includes important grains, edible plants, widespread weeds, and diverse wildflowers.
Polygonum (e.g., P. aviculare)
Knotweeds
Genus often treated broadly or narrowly (with Persicaria, Fallopia segregated). Typically annual or perennial herbs with swollen nodes and distinct ocreae. Flowers small, pinkish/whitish/greenish, borne in axillary clusters or terminal spikes. P. aviculare (Prostrate Knotweed) is a common mat-forming weed of paths and disturbed ground in Oklahoma.
Persicaria (e.g., P. pensylvanica)
Smartweeds / Lady's Thumbs
Often segregated from Polygonum. Typically herbs of moist habitats with swollen nodes and ocreae (often bristly-fringed). Flowers small, pinkish or white, densely packed in terminal, spike-like racemes. P. pensylvanica is a common native smartweed in wet areas in Oklahoma.
Rumex (e.g., R. crispus)
Docks and Sorrels
Perennial or annual herbs, often with large basal leaves (sometimes sour-tasting - sorrels). Ocreae often papery and disintegrating. Flowers small, greenish, borne in large terminal panicles. Distinctive fruit: a triangular achene enclosed by 3 persistent inner tepals which often enlarge and become winged or bear grains/tubercles (valves). R. crispus (Curly Dock) is a common weed in Oklahoma.
Eriogonum (e.g., E. annuum)
Wild Buckwheats
A very large genus (>250 spp.), especially diverse in arid western North America, with some species reaching western Oklahoma. Herbs or subshrubs, often hairy or woolly. Lack typical ocreae (stipules absent or basal). Flowers small, borne in cymose clusters subtended by a distinct cup-like involucre of fused bracts. Tepals 6.
Fallopia (e.g., F. scandens)
Climbing Knotweeds
Often segregated from Polygonum. Includes herbaceous vines that climb by twining stems. Leaves often heart-shaped or triangular. Ocreae present. Flowers small, greenish-white, in axillary or terminal racemes/panicles. Fruiting perianth often winged. F. scandens is native to eastern/central US including Oklahoma. Genus also includes highly invasive Japanese Knotweed (Reynoutria japonica, often placed in Fallopia).
Fagopyrum esculentum
Common Buckwheat
An annual herb cultivated as a grain crop (pseudocereal) and cover crop, originating from Asia. Features heart-shaped or triangular leaves and terminal clusters of small white or pinkish flowers. Ocreae present. Fruit is a large, sharply triangular achene, not enclosed by persistent perianth.
Rheum rhabarbarum
Rhubarb
A stout perennial herb cultivated for its large, fleshy, tart petioles used in cooking (leaf blades are toxic due to high oxalic acid). Native to Asia. Forms large basal leaves; flowers small, greenish-white, borne in large terminal panicles. Ocreae large.
Phylogeny and Classification
Polygonaceae is placed in the large order Caryophyllales (Core Eudicots). Its exact position within the order is subject to some variation in phylogenetic analyses, but it is often placed within the core Caryophyllales clade, potentially sister to Plumbaginaceae (leadwort family) or part of a broader group including carnivorous families (Droseraceae, Nepenthaceae) and Tamaricaceae/Frankeniaceae. Notably, Polygonaceae is one of the few families within the core Caryophyllales (along with Caryophyllaceae and Molluginaceae) that produce anthocyanin pigments rather than the betalains characteristic of most other families in the core group (e.g., Amaranthaceae, Cactaceae, Aizoaceae).
The family itself is strongly monophyletic, clearly defined by the presence of the ocrea. Internal relationships have led to the recognition of several subfamilies, with Eriogonoideae (Eriogonum) often diverging early, lacking typical ocreae found in Polygonoideae (Polygonum, Rumex, Persicaria, Fallopia, Fagopyrum, Rheum, etc.). The evolution of the persistent and often modified perianth surrounding the achene fruit is a key theme related to diverse dispersal strategies (wind, water, animal attachment/ingestion) within the family.
Position in Plant Phylogeny
- Kingdom: Plantae
- Clade: Angiosperms (Flowering plants)
- Clade: Eudicots
- Clade: Core Eudicots
- Order: Caryophyllales
- Family: Polygonaceae
Evolutionary Significance
Polygonaceae is significant for its unique morphological synapomorphy, the ocrea, which clearly defines the family. Its phylogenetic position within Caryophyllales, possessing anthocyanins rather than the betalains typical of the core clade, provides important clues about pigment evolution within the order. The family demonstrates successful adaptation to a wide range of habitats, from wetlands (Persicaria) to deserts (Eriogonum), often characterized by weedy or colonizing strategies. The diversification of fruit dispersal mechanisms involving the persistent perianth highlights evolutionary ingenuity in seed propagation.
