Knotgrass (Polygonum aviculare L.): A Comprehensive Reference
1. Identity, Taxonomy, and Botanical Description
Scientific and Common Names
Polygonum aviculare L., commonly known as knotgrass, is a plant related to buckwheat and dock.
It is also called prostrate knotweed, birdweed, pigweed, and lowgrass, and is an annual found in fields and wasteland, bearing white flowers from June to October.
The pharmacopoeial herbal drug is known by its Latin designation Polygoni avicularis herba. Additional common names include wireweed, beggarweed, doorweed, bird knotgrass, sparrow tongue, cowgrass, crawlgrass, centinodia (Spanish), bian xu (Chinese), Vogelknöterich (German), centinode (French), and trampört (Swedish).
The genus name Polygonum is derived from the Greek words poly, meaning "many," and gony, meaning "knees," referring to the plant's jointed stems.
The species epithet aviculare comes from the Latin aviculum, meaning "little bird," likely suggesting its history of being eaten by birds or its prevalence in areas frequented by them.
Taxonomy
Polygonum aviculare is an annual plant that belongs to the knotweed family (Polygonaceae).
According to the Flora of North America, up to seven subspecies are recognized, which accounts for the variability of this species.
Morphology and Habitat
Common knotgrass is an annual, low-prostrate herb with semi-erect, branching stems, which forms patches up to about 2 m across as it matures.
It has alternate linear-lanceolate leaves which are longer (7–15 mm) on the main stems than on the branches (3–5 mm).
The plant is drought resistant, possessing a single taproot that can penetrate into the ground up to 45 centimetres, and its spreading stems can reach up to one metre long while staying low to the ground.
Its flowers occur as small clusters of tiny green, pink, or purple flowers with no petals in the axil of leaves near the stem end, blooming mid-summer to early fall; the fruit is a three-angled achene.
The plant is native to Eurasia, extending throughout Europe, Asia, and North Africa.
It is widespread across many countries in temperate regions, apparently native to Eurasia, and naturalized in temperate parts of the Southern Hemisphere.
It grows in fields and wetlands, in both high and low elevations, throughout the world and in all 50 states of the United States.
It is especially in Eastern Europe that the herb is collected for medicinal purposes.
Pharmacopoeial Status and Regulatory Standing
Polygoni avicularis herba is used in herbal medicinal products for the treatment of mild catarrhs of the respiratory tract and inflammatory changes of the pharyngeal and oral mucosa; its quality is laid down in the European Pharmacopoeia (Ph. Eur.).
Knotgrass herb is the common name for the flowering aerial parts of the plant Polygonum aviculare L., and the HMPC conclusions cover only preparations obtained by drying and comminuting (reducing into tiny pieces) the flowering aerial parts.
The European Medicines Agency's (EMA) Committee on Herbal Medicinal Products (HMPC) issued a full European Union herbal monograph on Polygoni avicularis herba in April 2016 (EMA/HMPC/143658/2015), assigning the herb to "traditional use" status. The Italian Ministry of Health includes Polygoni avicularis herba in its list of herbal substances allowed in food supplements, with indications covering regulation of intestinal transit, functionality of the digestive system, and functionality of the upper airways.
2. Common Forms and Preparations
Herbal medicines containing knotgrass herb preparations are usually available as herbal material that is used to make herbal teas to be drunk or added to water to produce a liquid (decoction) to be applied to the lining of the mouth.
Knotgrass herb preparations may also be found in combination with other herbal substances in some herbal medicines.
- Dried aerial herb (herba): The primary pharmacopoeial form. The dried, comminuted flowering aerial parts are the basis of all recognized preparations.
- Herbal infusion (tea): Tea is the most common entry point, and is the form most people use for cold-related support or general short-term internal use.
- Decoction: A somewhat stronger water preparation, often chosen when the herb is being used for urinary flushing or as the base for a rinse.
- Gargle / mouth rinse: Gargles and mouth rinses are especially logical as they bring the astringent constituents directly to the tissue being supported.
- Poultice / topical wash: In ancient texts and folk herbal manuals, knotgrass herb is referenced for its use in herbal baths, and its fibrous stems and leaves were sometimes used externally in poultices or washes.
- Powdered herb: The dried herb is also available in powdered form, suitable for capsules or direct incorporation into blends.
- Combination herbal teas: Polygoni avicularis herba is recognised within EMA-assessed herbal tea combinations, such as species diureticae, with the indication of being traditionally used to increase the amount of urine to achieve flushing of the urinary tract as an adjuvant in minor urinary complaints.
Parts of the plant are used medicinally, and the leaves are sometimes used to produce blue or green dye.
The seeds are edible raw or can be ground into a flour.
Knotgrass formed a traditional ingredient in porridge consumed by Germanic peoples of western Europe and has been found in numerous autopsies of peat bodies, including the Tollund Man.
3. Traditional and Historical Use
Europe
The most frequently cited traditional indications for Polygoni avicularis herba refer to mild catarrhs of the respiratory tract, inflammation of the mouth and upper respiratory tract, and renal elimination of water; the traditional use in the European Union in the forms of infusion and decoction for more than 30 years is supported by the literature as well as by medicinal products.
The herbal substance is mentioned in well-known handbooks as an expectorant and secretolytic to treat cough, bronchial catarrh, and is also recommended for use in inflammation of the mouth and upper respiratory tract, with references appearing in Hagers Handbuch (1927), Madaus (1938 and 1976), Kommission E Monographie (1987), and numerous other major European phytotherapy references.
Empirical European medicine also used it as an expectorant and secretolytic for coughs and bronchial catarrh, against night sweats in tuberculosis, as a diuretic, and as a hemostatic.
In European folk medicine, knotgrass is used for hypertension, dysuria, and jaundice, often prepared as decoctions or infusions to support urinary tract health and hemostasis.
Several monographs and handbooks described the usage of knotgrass for the treatment of urinary tract infection with difficult painful urination, kidney stones, gallstones, and gout, as well as its diuretic effect.
The plant is used in Swedish traditional medicine to treat inflammatory diseases and/or wounds, and is also used in Polish traditional medicine.
The herb's earlier use as a treatment for tuberculosis now has only historical interest.
The herbal substance knotgrass is included in many handbooks and compendia, such as Hagers Handbuch (Frerichs et al., 1927), Lehrbuch der Biologischen Heilmittel (Madaus, 1938, 1976), Drogenkunde (Hoppe, 1942), Farmakopea Polska (1970), and Gift- und Arzneipflanzen von Mitteleuropa (Gessner and Orzechowski, 1974).
The herbalist Nicholas Culpeper cited knotgrass in his Complete Herbal (1653) for its cooling, drying influence on "hot humours."
China and East Asia
In China, knotgrass (known as Bian Xu) has been used in traditional medicine since ancient times as a diuretic, for throat and bronchial ailments, and to treat skin problems.
Other usages in Chinese medicine include treatment for gonorrhoea, jaundice, and tapeworm in children.
In Chinese medicine, the leaves and stems are regarded as diuretic, anthelmintic, and antidiarrheal, and are externally used as an emollient and astringent for hemorrhoids, pruritus, and chancroid; in traditional Korean medicine it is used to treat obesity and symptoms associated with hypertension.
India and Ayurveda
Ethnomedicinal records from India highlight knotgrass's application for kidney stones, hyperglycemia, and anti-inflammatory purposes in digestive and urinary infections.
Polygonum aviculare is prized in Ayurvedic and folk medicine traditions for its diuretic, astringent, and wound-healing properties.
North Africa and the Mediterranean
Polygonum aviculare is an herb commonly distributed in Mediterranean coastal regions in Egypt and used in folkloric medicine.
Ethnomedicinal records from Morocco similarly highlight its application for kidney stones and anti-inflammatory purposes in digestive and urinary infections.
General Traditional Preparations
Across Europe and Asia, knotgrass has been employed as a diuretic and astringent, particularly for treating respiratory ailments such as coughs, skin conditions including wounds and burns, and digestive issues like diarrhea and ulcers.
In traditional herbal medicine, it has been used to treat conditions such as diarrhea, hemorrhoids, and skin disorders; the leaves were often employed to create soothing poultices for insect bites and minor skin irritations.
4. Key Constituents and Active Compounds
High phenolics and flavonoid content present in the leaves and stem of Polygonum aviculare L. is responsible for the most important pharmacological effects of the plant.
Flavonoids
Flavonoids are present at concentrations of 0.1–1% (rarely 2.5–3%), as derivatives of kaempferol, quercetin, and myricetin. Key flavonoids include avicularin (quercetin-3-O-arabinoside, approximately 0.2%), juglanin (kaempferol-3-O-arabinoside), hyperoside, quercitrin, quercetin-3-galactoside, vitexin, isovitexin, rutin, astragalin, isoquercitrin, miquelianin, spiraeoside, orientin, myricitrin, luteolin, taxifolin, isorhamnetin, and apigenin.
Quercetin is the most detected compound in the genus Polygonum.
Phenolic Acids and Coumarins
The plant also contains gallic, caffeic, oxalic, silicic, chlorogenic, and p-coumaric acids, as well as tannins including catechin.
Coumarins are present in the form of umbelliferone and scopoletin.
Silicic Acid
Knotgrass contains approximately 1% silicic acid, which is partly water-soluble (0.08%) in boiling water.
Because knotgrass contains silicic acid, it may be helpful in strengthening the connective tissue of the lungs; in Europe it has been used as a remedy for pulmonary tuberculosis and chronic bronchitis on this basis.
Tannins
Tannins are present at approximately 4% of the dry herb.
These condensed and hydrolysable tannins are responsible for the plant's characteristic astringent properties and contribute to its hemostatic and antidiarrheal effects in traditional use.
Lignans and Sterols
The plant also contains lignans including a lignan glycoside and aviculin, as well as sterols, mainly β-sitosterol.
Additional Constituents
Knotgrass additionally contains mucins, traces of an essential oil, and vitamin C.
In plants from the Mediterranean coastal regions of Egypt, tannins, saponins, flavonoids, alkaloids, and sesquiterpenes have been reported.
5. Established and Proposed Mechanisms of Action
Astringency and Hemostasis
The flavonoids exhibit astringent properties and are found to decrease capillary fragility and have a cortisone-like effect.
The tannin content underlies the plant's well-documented astringent activity, which accounts for its traditional use in hemorrhage, diarrhea, and wound healing.
Anti-inflammatory Activity
Significant anti-inflammatory, antioxidative, and antibacterial effects of Polygonum aviculare L. herba extracts (water, acetone, ethanol, and chloroform) and isolated constituents (including panicudine, flavonol glucuronides, and avicularin) have been demonstrated in experimental models in vitro, ex vivo, and in vivo.
Flavonoids found in the plant have many biological effects including anti-allergic, anti-carcinogenic, anti-inflammatory, antidiabetic, antiviral, and anti-aging activities.
Antioxidant Activity
P. aviculare L. extract exhibits an obvious antioxidant effect, and the high phenolic content of its extract is likely connected to its efficiency as an antioxidant, with hydroxyl groups in the phenolics as the probable cause.
The antioxidant potential of eleven flavonol glucuronides obtained from P. aviculare, eight of which were identified for the first time in the Polygonum species, was examined in vitro; all substances considerably reduced the formation of reactive oxygen species (ROS) at physiologically acceptable doses between 0.5 and 10 μM.
Diuretic Activity
The diuretic effect traditionally attributed to knotgrass is considered plausible by the EMA HMPC based on the constellation of compounds present. The HMPC conclusions on throat inflammation and minor problems affecting the urinary tract are based on "traditional use," meaning that although there is insufficient evidence from clinical trials, the effectiveness of these herbal medicines is plausible and there is evidence that they have been used safely for at least 30 years, including at least 15 years within the EU.
Airway Smooth Muscle Relaxation
Researchers screening several herbs found that Polygonum aviculare L. contains ingredients that inhibit the precontraction of mouse and human airway smooth muscle (ASM); channel-mediated currents were inhibited by compounds present in P. aviculare extracts, suggesting this inhibition is mediated by L-type voltage-dependent Ca²⁺ channels (LVDCCs), TRPC3, and/or STIM/Orai channels.
These channel-mediated currents were also inhibited by quercetin, which is present in P. aviculare extracts, and quercetin inhibited acetylcholine-induced precontraction in ASM.
Pancreatic Lipase Inhibition
From among 34 kinds of methanolic crude extracts of Korean medicinal plants, Polygonum aviculare L. showed the highest pancreatic lipase (PL)-inhibitory activity (63.97 ± 0.05% of inhibition); solvent fractionation and LC/MS analysis identified flavonol-3-O-glycosides, flavonol-3-O-(2″-galloyl)-glycosides, and flavonol aglycones as the active constituents.
Wound Healing (Wnt/β-catenin Pathway Activation)
P. aviculare extract was identified as a Wnt/β-catenin pathway activator, and it accelerated the migration of HaCaT keratinocytes without significant cytotoxicity; P. aviculare extract also efficiently re-epithelialized wounds generated on mice.
Ingredients of P. aviculare extract such as quercitrin hydrate, caffeic acid, and rutin also accelerated the motility of HaCaT keratinocytes with activation of Wnt/β-catenin signaling.
6. Scientific Evidence by Area of Use
6.1 Respiratory Tract (Cough, Catarrh, Bronchitis)
Traditional / regulatory status: The German Commission E monograph supports internal use for mild catarrh of the airways and external use for inflammatory changes in the mucous membranes of the mouth and throat.
The EMA HMPC similarly identifies mild catarrhs of the respiratory tract and inflammation of the mouth and upper respiratory tract as the most frequently cited traditional indications.
Pre-clinical evidence: In vitro data indicate that the ethyl acetate fraction of P. aviculare and quercetin can inhibit Ca²⁺-permeant LVDCCs, TRPC3, and STIM/Orai channels, which inhibits the precontraction of ASM; these findings suggest that P. aviculare could be used to develop new bronchodilators to treat obstructive lung diseases such as asthma and COPD.
Clinical evidence: There is insufficient evidence from clinical trials; the effectiveness of these herbal medicines is considered plausible, and evidence exists that they have been used safely for at least 30 years (including at least 15 years within the EU). No high-quality randomized controlled trials in humans are documented for respiratory endpoints in the EMA assessment report. Evidence strength is therefore traditional use only, with supporting in vitro mechanistic data.
6.2 Urinary Tract (Diuresis, Flushing, Minor Complaints)
Traditional / regulatory status: The HMPC monograph recognizes the following traditional internal uses: banal cold, increasing urine volume, flushing of the urinary tract, and mild urinary complaints.
Polygoni avicularis herba is also recognized within EMA-assessed diuretic herbal tea combinations, traditionally used to increase urine amount to achieve flushing of the urinary tract as an adjuvant in minor urinary complaints.
Pre-clinical evidence: The diuretic effect is considered pharmacologically plausible based on the plant's constellation of flavonoids and phenolic acids. It promotes elimination of urine, contributing to urinary wellness and helping to expel bacteria from the urinary tract due to its ability to inhibit bacterial adhesion in the mucosa. This mechanism has been proposed but is not yet confirmed in rigorous human trials.
Clinical evidence: No controlled human clinical trials specifically documenting the diuretic or urinary-flushing effects of knotgrass as a monotherapy are cited in the EMA assessment. Evidence strength is traditional use only.
6.3 Oral Mucosa / Gingivitis
Clinical evidence: One study observed a possible effect in decreasing gingivitis, but information on how the study was conducted is incomplete, so firm conclusions could not be drawn; the HMPC also considered laboratory studies supporting anti-inflammatory and antibacterial effects.
The HMPC traditional-use indication for external use specifically covers mild inflammation of the mouth and throat.
Evidence strength is preliminary; the existing gingivitis study is of insufficient methodological quality, and no further clinical trials were identified.
6.4 Anti-obesity and Metabolic Effects
Pre-clinical evidence — pancreatic lipase inhibition: The scope of the lipase inhibition study was limited to in vitro PL inhibition; however, the findings have implications for future practice, such as in vivo PL inhibition and anti-obesity functionality, and results could provide information aiding effective utilization of P. aviculare as an anti-obesity agent based on PL inhibition.
Pre-clinical evidence — animal study: In a murine model, mice were fed a high-fat diet supplemented with a P. aviculare ethanol extract (PAE) at 400 mg/kg/day for 6.5 weeks; the increased body weights, adipose tissue weight, and adipocyte area, as well as serum total triglyceride, leptin, and malondialdehyde concentrations, were decreased in PAE-treated mice relative to untreated obese controls, and PAE significantly suppressed mRNA expression of key adipogenesis-related genes (SREBP-1c, PPARγ, FAS, aP2) in white adipose tissue.
In addition, PAE treatment of 3T3-L1 cells inhibited adipocyte differentiation and fat accumulation in a dose-dependent manner.
Other effects not related to the indications proposed in the EMA monograph but demonstrated in non-clinical studies include anti-obesity activity, antifungal, vasorelaxant, anti-atherosclerotic, and anticancer effects. Evidence strength for anti-obesity effects in humans is non-existent; all data are pre-clinical (in vitro and animal studies only).
6.5 Wound Healing
Pre-clinical evidence: In a screen of 350 plant extracts, P. aviculare extract was identified as a Wnt/β-catenin pathway activator; it accelerated keratinocyte migration without significant cytotoxicity, efficiently re-epithelialized wounds in mice, and its active ingredients — quercitrin hydrate, caffeic acid, and rutin — also accelerated keratinocyte motility via Wnt/β-catenin activation.
Evidence strength: pre-clinical only; no clinical wound-healing trials are available.
6.6 Antimicrobial Activity
Pre-clinical evidence: Organic and aqueous solvent extracts and fractions of P. aviculare were investigated for antimicrobial activities against several microorganisms including bacteria and fungi using the paper disc diffusion method; extracts were active against both Gram-negative and Gram-positive bacteria, and phytochemical studies revealed the presence of tannins, saponins, flavonoids, alkaloids, and sesquiterpenes.
Evidence strength: in vitro only; clinical antimicrobial data are not available.
6.7 Hepatoprotective Activity
Pre-clinical evidence: The methanolic extract of the plant showed high protection against CCl₄-induced hepatotoxicity in mice, and this activity is attributed to the presence of flavonoid glucosides. Evidence strength: animal model only; no human data available.
6.8 Vasorelaxant / Cardiovascular Effects
P. aviculare extracts have been reported to induce vasorelaxation of precontracted rat aortic tissues.
Vasorelaxant and anti-atherosclerotic effects were demonstrated in non-clinical studies, but no human cardiovascular trials are available. Evidence strength: pre-clinical only.
7. Body Systems and Health Areas Associated with Knotgrass
- Respiratory system: Traditionally used for mild cough, bronchial catarrh, and sore throat; supported by Commission E monograph and EMA traditional-use designation; pre-clinical data suggest airway smooth muscle relaxation mechanisms.
- Urinary system: Traditionally used as a diuretic and to flush the urinary tract; recognized by EMA HMPC for mild urinary complaints.
- Gastrointestinal system: Traditional use for diarrhea, hemorrhoids, and ulcerative colitis, attributable to tannin-derived astringency.
- Oral mucosa: Traditionally used as a gargle for mouth and throat inflammation; one incomplete clinical observation in gingivitis.
- Integumentary system (skin/wound): Pre-clinical evidence for wound healing via Wnt/β-catenin pathway; traditional use in poultices for skin disorders.
- Metabolic system: Pre-clinical evidence for pancreatic lipase inhibition and anti-adipogenic effects in cell and animal models.
- Cardiovascular system: Pre-clinical evidence for vasorelaxation and anti-atherosclerotic effects; no human evidence.
- Immune / antimicrobial: In vitro antibacterial and antifungal activities demonstrated.
8. Dosage Forms and Reported Dosages
Dosage depends on the specific purpose. The best practical ranges come from traditional monograph-style guidance rather than modern clinical trial dosing, because human trial data are limited.
- Herbal tea (infusion): One preparation recommendation suggests adding 1 teaspoon to a cup, pouring boiling water, brewing covered for 10–15 minutes, straining, and drinking 2–3 times a day.
- Herbal tea combination (pectoral / diuretic blends): Based on information from marketed products, the single dose of herbal tea combinations is in the range between 1.5 and 5 g, with a mean dosage frequency of 3 times daily.
- For combination use (Species pectorales / diuretic teas): Polygoni avicularis herba can be given to adolescents, adults, and the elderly; the possible daily dosages in combination products are below the maximum posology given in the EU herbal monographs for the single active substances.
- Animal study dose (for context only — not applicable to humans): In the high-fat diet murine study, the ethanol extract was administered at 400 mg/kg/day for 6.5 weeks.
There is very limited information for paediatric use of herbal tea combinations in the therapeutic area of cough and cold; only some combinations including Althaea radix and Foeniculi amari fructus have been reported for use in children below 12 years of age. Knotgrass as a monotherapy has not been specifically evaluated for paediatric use in EMA assessments.
9. Safety Considerations and Notable Interactions
General Safety Profile
None of the reported non-clinical pharmacological studies described any cause for safety concern.
Animal studies indicated that Polygonum aviculare herba extract did not produce any pathological effects at the neuro-endocrine axis level or in the metabolism of studied organs.
Acute toxicity testing confirmed the extract's safety, with an LD₅₀ of greater than 2000 mg/kg in animal studies.
Contraindications
The EMA monograph lists hypersensitivity to the active substance(s) as a contraindication.
Pregnancy and Lactation
Excessive use of the herb should be avoided during pregnancy as it may have abortifacient properties.
No fertility data are available according to the EMA monograph. The combination herbal tea monographs for diuretic species similarly note the absence of relevant fertility data.
Overdose
No case of overdose has been reported, according to the EMA monograph information.
Drug Interactions
No contraindications, side effects, or interactions with conventional medicine or other medicinal herbs have been formally reported. The plant contains oxalic acid; chemical constituents of P. aviculare include oxalic acid, which could theoretically be relevant in individuals predisposed to oxalate kidney stones if consumed in large quantities over a prolonged period, though no clinical reports of this complication were identified in the EMA assessment.
Diuretic Interactions
Given the plant's traditional diuretic properties, caution is theoretically warranted when it is used concurrently with pharmaceutical diuretics, though no formal interaction data are reported in the EMA assessment report or in the peer-reviewed clinical literature.
Children
There is very limited information for paediatric use of herbal tea combinations in the therapeutic area of cough and cold; only some combination products have been reported for use in children below 12 years of age. The EMA monograph for Polygoni avicularis herba as a single-substance preparation does not establish a paediatric posology.
Quality and Botanical Identity
Quality of the herbal drug is specified in the European Pharmacopoeia (Ph. Eur.), which provides standards for identification and purity to guard against substitution or adulteration with related Polygonum species.
10. Summary of Evidence Strength
- Respiratory (cough, catarrh): Traditional use with plausible biological rationale; Commission E and EMA HMPC approval for traditional use. No controlled clinical trials. Pre-clinical smooth muscle relaxation data support the traditional rationale.
- Urinary flushing / mild urinary complaints: Traditional use; EMA HMPC traditional-use recognition; no clinical trial data.
- Oral mucosa / gingivitis: Single incomplete clinical observation; pre-clinical anti-inflammatory and antibacterial support. Evidence insufficient for firm conclusions.
- Anti-obesity: In vitro pancreatic lipase inhibition and animal study data only. No human evidence.
- Wound healing: Pre-clinical (cell culture and mouse model) only.
- Antimicrobial: In vitro disc diffusion studies only.
- Hepatoprotective: Animal model only.
- Cardiovascular (vasorelaxation, anti-atherosclerosis): Pre-clinical only.
Across all areas, the absence of adequately powered, placebo-controlled randomized clinical trials means that knotgrass's clinical efficacy cannot be confirmed for any indication by current evidence standards. Its regulatory status across the EU rests on the "traditional use" framework, which acknowledges a plausible rationale and an established safety record rather than proven clinical efficacy.
References
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