Herbs & Plants

Small-Leaved Knotweed

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Botanical Name : Polygonum arenastrum – Bor.
Family :                Polygonaceae
Genus  : Polygonum
Synonyms: Polygonum calcatum Lindman, Polygonum aequale Lindin, Polygonum aviculare subsp. depressum (Meissner) Arcang., Polygonum aviculare subsp. aequale (Lindman) A. et. Gr., Polygonum aviculare subsp. calcatum (Lindman) Thell.
Common Names :  mat grass, oval-leaf knotweed, stone grass, wiregrass, and door weed, as well as many others.

Habitat :Throughout Europe, including Britain. Waste places and roadsides, common throughout Britain .

Small-Leaved Knotweed is an annual herb growing to 0.3m.with prostrate or ascending bluish-green slender, terete stems, not sharply angled, 1-12 dm. long; lvs. lanceolate to almost oblong, 5-20 mm. long, blue-green, scattered to approximate, not much reduced upward, joined with the ocreae and 1-nerved, mostly 2 or more in the axils, short pedicelled; branch-lvs. much smaller than stem lvs.; stipule-sheaths silvery, soon torn; fls. 1-5 in axillary clusters; calyx 2-3 mm. long, greenish with pinkish to purplish margins, persistent, divided almost to base; stamens 8, rarely 5; aks. dull or slightly shiny, 2-2.5 mm. long, somewhat roughened.
It is hardy to zone 5. It is in flower from June to October, and the seeds ripen from August to October. The flowers are hermaphrodite (have both male and female organs) and are pollinated by Insects. The plant is self-fertile. It is noted for attracting wildlife.

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The plant prefers light (sandy), medium (loamy) and heavy (clay) soils. The plant prefers acid, neutral and basic (alkaline) soils and can grow in very acid soil. It can grow in semi-shade (light woodland) or no shade. It requires moist soil. The plant can tolerate maritime exposure.

Cultivation :

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. – P. aviculare. L.; P. boreale. (Lange.)Small.; P. rurivacum. Jord. ex Box.; and P. arenastrum. Bor.

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.

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 Uses:

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, astringent, cardiotonic, cholagogue, diuretic, febrifuge, haemostatic, lithontripic 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 whole plant is anthelmintic, antiphlogistic and diuretic. 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.

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.

The information presented herein is intended for educational purposes only. Individual results may vary, and before using any supplements, it is always advisable to consult with your own health care provider


POLYGONUM ARENASTRUM Boreau – truskavec obecný / stavikrv pobrežný

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