Common Name: Small-Leaved Knotweed

General Information

Polygonum arenastrum is an annual plant with prostrate to more or less decumbent (occasionally erect), much-branched stems that can form dense mats of growth. The stems become more or less woody near the base, growing up to 70cm long[].
The plant is harvested from the wild for local use as a food, medicine and source of materials.
Polygonum arenastrum is a plant of the temperate zone that has spread widely as a weed iand is now also found in many areas of the tropics[].

Known Hazards

Although no specific mention has been made for this species, there have been reports that some members of this genus can cause photosensitivity in susceptible people.
Many species also contain oxalic acid (the distinctive lemony flavour of sorrel) - whilst not toxic this substance can bind up other minerals making them unavailable to the body and leading to mineral deficiency. Having said that, a number of common foods such as sorrel and rhubarb contain oxalic acid and the leaves of most members of this genus are nutritious and beneficial to eat in moderate quantities. Cooking the leaves will reduce their content of oxalic acid. People with a tendency to rheumatism, arthritis, gout, kidney stones or hyperacidity should take especial caution if including this plant in their diet since it can aggravate their condition[].

Botanical References

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Range

Throughout Europe and northern Asia to the Russian Far East; Southwest Asia - Turkey to to the Himalayas; N. Africa - Macaronesia to Tunisia

Habitat

Waste places and roadsides, common throughout Britain[].

Properties

Weed Potential	Yes
Edibility Rating
Medicinal Rating
Habit	Annual
Height	0.30 m
Pollinators	Insects, Self
Self-fertile	Yes
Cultivation Status	Wild

Cultivation Details

Polygonum arenastrum is a very cold-hardy plant, being able to tolerate temperatures down to at least -25°c when fully dormant.
Succeeds in an ordinary garden soil[] but prefers a moisture retentive not too fertile soil in sun or part shade[]. Repays generous treatment, in good soils the plant will cover an area up to a metre in diameter[, ]. Prefers an acid soil[]. Dislikes shade.
Knotweed is a common and invasive weed of cultivated ground[]. It is an important food plant for the caterpillars of many species of butterflies[]. It also produces an abundance of seeds and these are a favourite food for many species of birds[].
Plants seem to be immune to the predations of rabbits[].
The flowers have little or no scent or honey and are rarely visited by pollinating insects. Self-fertilization is the usual method of reproduction, though cross-fertilization by insects does sometimes occur[]. The plant also produces cleistogomous flowers - these never open and therefore are always self-fertilized[].
The plant is very variable according to habitat and is seen by most botanists as part of an aggregate species of 4 very variable species, viz. - Polygonum aviculare. L.; Polygonum boreale. (Lange) Small.; Polygonum rurivacum. Jord. ex Boreau; and Polygonum arenastrum Boreau[].

Edible Uses

Young leaves and plants - raw or cooked[, ]. Used as a potherb[], they are very rich in zinc[]. A nutritional analysis is available[].

Seed - raw or cooked. Rather small and fiddly to utilize, they can be used in all the ways that buckwheat (Fagopyrum esculentum) is used, either whole or dried and ground into a powder for use in pancakes, biscuits and piñole[, , , , ].

The leaves are a tea substitute[].

Medicinal

Knotweed is a safe and effective astringent and diuretic herb that is used mainly in the treatment of complaints such as dysentery and haemorrhoids. It is also taken in the treatment of pulmonary complaints because the silicic acid it contains strengthens connective tissue in the lungs[].

The whole plant is anthelmintic, antiphlogistic, astringent, cardiotonic, cholagogue, diuretic, febrifuge, haemostatic, lithotriptic and vulnerary[, , , , , , , , , ]. It was formerly widely used as an astringent both internally and externally in the treatment of wounds, bleeding, piles and diarrhoea[]. Its diuretic properties make it useful in removing stones[]. An alcohol-based preparation has been used with success to treat varicose veins of recent origin[].
The plant is harvested in the summer and early autumn and is dried for later use[].

The leaves are anthelmintic, diuretic and emollient[].

The juice of the plant is weakly diuretic, expectorant and vasoconstrictor[]. Applied externally, it is an excellent remedy to stay bleeding of the nose and to treat sores[].

The seeds are emetic and purgative[, ].

Recent research has shown that the plant is a useful medicine for bacterial dysentery. Of 108 people with this disease, 104 recovered within 5 days when treated internally with a paste of knotweed[].

Other Uses

Yields a blue dye that is not much inferior to indigo[]. The part used is not specified, but it is likely to be the leaves.
Yellow and green dyes are obtained from the whole plant[].

The roots contain tannins, but the quantity was not given[].

Propagation

Seed - sow spring in a cold frame. Germination is usually free and easy. When they are large enough to handle, prick the seedlings out into individual pots and plant them out in the summer if they have reached sufficient size. If not, overwinter them in a cold frame and plant them out the following spring after the last expected frosts.
Division in spring or autumn. Very easy, larger divisions can be planted out direct into their permanent positions. We have found that it is better to pot up the smaller divisions and grow them on in light shade in a cold frame until they are well established before planting them out in late spring or early summer.