Common knotgrass, birdweed, pigweed and lowgrass are all familiar names for the herb Polygonum aviculare, an annual crop and wasteland weed occasionally used in folk medicine. While not a popular plant in European, American, Chinese or Unani herbal medical traditions, all of these make some use of the plant's astringent properties. For this reason, Polygonum aviculare has historically been used as a treatment for dysentery, hemorrhoids, diarrhea, the symptoms of chronic venous insufficiency, and both internal and external bleeding. Modern herbalists consider the herb to have diuretic, emollient, stomach acid promoting, expectorant, anti-inflammatory, heart tonifying, vasoconstrictive, fever reducing, wound healing and antiparasitic properties.
Polygonum aviculare has also been used in the treatment of a variety of pulmonary conditions, as the connective tissue of the lungs may be strengthened by the high concentration of silicilic acid in the plant. This plant's diuretic effects has led to its historical use in assisting with the passage of kidney stones as well as in the treatment of high blood pressure and water retention. As an astringent treatment for chronic venous insufficiency, extracts of the plant have shown some promise in treating newly developed varicose veins. The plant has not been shown to be effective in treating varicose veins in patients where the condition is chronic. Internal use of Polygonum aviculare paste as a treatment for dysentary has the most scientific support, with the results of a number of different studies supporting this traditional application.
While all parts of the plant are believed to possess these medicinal properties, the leaves in particular are thought to be useful in the treatment of parasitic infection, water retention and skin conditions. The juice is predominantly used to treat phlegmy cough and as a vasoconstrictive agent. Seeds from Polygonum aviculare are considered to be toxic. These toxins have been exploited in the past to induce vomiting or diarrhea in patients that have ingested poisons.
Polygonum aviculare is often used as a salad herb in either raw or cooked form. While a rich source of zinc, the plant also contains a large amount of oxalic acid, which is also present in raw spinach, rhubarb and sheep sorrel. In addition to interfering with the absorption of some nutrients in the gut, oxalic acid can aggravate gout, rheumatoid arthritis and kidney stones in some patients. Cooking the plant, however, dramatically diminishes its oxalic acid content, making it relatively safe to use in moderation — even for patients with conditions that may be aggravated by it.