Polygonaceae; Knotgrasses and Knotweeds: Part two!

Last week we talked about the more prominent genera and species of the Polygonaceae, this week about the other group, which has one famous non native weed: the Japanese knotweed!

As we have all heard about this infamous one I would like to explore a little bit about its positive site, if there is any, and if there is anything we can do about its spread.

Japanese knotweed and native bindweed looking beautiful and innocent near my allotment boundary!
Japanese knotweed and native bindweed looking beautiful and innocent near my allotment boundary!

Japanese knotweed (Reynoutria japonica)

is a fast-growing and strong clump-forming perennial, with tall, dense annual stems. Stem growth is renewed each year from the stout, deeply-penetrating rhizomes (creeping underground stems).

It is a very costly affair to have it removed see more here:

Yes; the Japanese Knotweed was introduced in the Victorian age as an attractive foliage and flowering, ornamental plant and there is still one, but a much smaller growing variety grown at present for gardens called Fallopia japonica var. compacta:

But all weeds may have a ‘higher purpose’  as mostly they will improve their local environment for the immediate soil as well as their surrounding life forms such as insects and soil organisms?

I do realize the Japanese Knotweed is too greedy and may not give enough back to earn its keep. But what if the soil is infertile or useless for anything better and there are no buildings it can interfere with?

What they have done in an experiment to control Japanese Knotweed in the Netherlands is to let  a rare breed of pigs loose on it which consume the tops as well as dig out and eat the rhizomes! This sort of control makes good sense to me!

Maybe we will find other good uses for this plant in future. It grows all along the outside of my allotment and is probably a useful food provider for our bees!


As a source for honey:

Japanese knotweed flowers are valued by some beekeepers as an important source of nectar for honeybees, at a time of year when little else is flowering. Japanese knotweed yields a monofloral honey, usually called bamboo honey by northeastern U.S. beekeepers, like a mild-flavoured version of buckwheat honey (a related plant also in the Polygonaceae).

The young stems are edible as a spring vegetable, with a flavour similar to rhubarb. In some locations, semi-cultivating Japanese knotweed for food has been used as a means of controlling knotweed populations that invade sensitive wetland areas and drive out the native vegetation. It is eaten in Japan as sansai or wild foraged vegetable.

It is used in traditional Chinese and Japanese medicine to treat various disorders through the actions of resveratrol, although there is no high-quality evidence from clinical research for any medical efficacy.

Ground-feeding songbirds also eat the seeds.

Bee lovers have found another use for Japanese knotweed. The hollow stems are often cut into lengths and bundled for use as native bee habitat. The stem diameters vary just enough to provide suitable housing for a wide range of tunnel-nesting bees, including mason bees and leafcutters.’

Enough about Japanese Knotweed; what about our own other native Polygonaceae such as the Knotgrasses?

There are 9 Polygonum spp. mentioned in Stace but only 6 are native or archaeophyte. There are 19 species of Persicaria and only 8 are true natives. Some of those can become a ‘weed’ but mostly rather harmless. The following are covered in the Medicinal Flora of J. Barker:

Polygonum aviculare (Knotgrass), Persicaria hydropiper (Water-pepper or Smartweed), Persicaria maculosa (Redshank) and Persicaria lapathifolium (Pale Persicaria). Also the Bistort, Persicaria bistorta which is sometimes grown in gardens as an ornamental is mentioned as well as Buckwheat (Fagopyrum esculentum) formerly cultivated crop, grown for its starchy ‘ grains’, which are the fruit & seeds.

Polygonum aviculare or Knotgrass

I have listed all their uses from pfaf website below in bullet points

Edible Uses:

  • Young leaves and plants – raw or cooked.
  •  Used as a potherb, they are very rich in zinc.
  •  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:

  •  astringent and diuretic 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.
  •  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.
  • 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 knotgrass.

Other Uses:

  • Yields a blue dye that is not much inferior to indigo
  •  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.
Persicaria hydropiper or Water pepper

Persicaria hydropiper  or Water-pepper

 Edible uses:

  • Leaves and stems – raw or cooked. They can also be made into an acid peppery condiment. 
  •  Seed – raw or cooked. It is rather small and fiddly to utilize. The seed is used as a condiment – a pepper substitute
  • The sprouted seeds or young seedlings can be used as a garnish or added to salads, they are commonly sold in Japanese markets. They are very hot.

Medicinal uses:

  • Water-pepper (or Smartweed) has a long history of herbal use, both in Eastern and Western herbalism.  The leaves are anti-inflammatory, astringent, carminative, diaphoretic, diuretic, emmenagogue, stimulant, stomachic, styptic. They contain rutin, which helps strengthen fragile capillaries and thus helps prevent bleeding. Use with caution.
  • The seed is carminative, diuretic and stimulant. The whole plant, either on its own or mixed with other herbs, is decocted and used in the treatment of a wide range of ailments including diarrhoea, dyspepsia, itching skin, excessive menstrual bleeding and haemorrhoids
  •  A poultice of the plant is used in treating swollen and inflamed areas.
  • A homeopathic remedy is made from the leaves. It is used in the treatment of piles, menstrual pains and other menstrual complaints.

Other Uses:

  • A yellow-gold dye is obtained from the stalks.                                                                                                                                                 
two toned Persicaria lapathifolia is attractive enough as a border plant?

Persicaria maculata and  P. lapathifolia are of similar use as  P. hydropiper but are acting a lot weaker, which makes it more appropriate for the skin and for the kidney and urinary passages.  P. lapathifolium has greater amounts of Vitamin C. too.

Bistorta officinalis or Bistort

The Latin name bistorta refers to the twisted appearance of the root.

Edible uses:

  • Bistort has been cultivated as a vegetable, its roots, leaves and young shoots being steamed or boiled. Many of its occurrences in the wild may be as a garden escape.
  • In Northern England the plant was used to make a bitter pudding in Lent from a combination of the leaves, oatmeal, egg and other herbs. It is the principal ingredient of dock pudding or Easter-Ledge pudding.

Other uses:

This species is grown as an ornamental garden plant, especially the form ‘Superba’ which has larger, more showy flowers, and has gained the Royal Horticultural Society’s Award of Garden Merit. It is suitable for use as a marginal or in bog gardens.

Medicinal uses:

  • J. Barker finds that it works remarkably well with other astringents in the treatment of peptic ulcers, diverticulosis and other irritable or inflammatory conditions of the bowel providing always that chronic constipation is not present. 

Fagopyrum esculentum or Buckwheat

This is native of Asia but found here as an escape from cultivation but rarely grown now due to considerable difficulties  with its harvest on a field scale.  I list its uses below: All info from the pfaf website, see link above.

Edible uses:

  • Leaves – raw or cooked like spinach. Not that wonderful raw, they improve somewhat with cooking [says Ken Fern [K] of PFAF]. The leaves are rich in rutin (see below for more details) and so are a very healthy addition to the diet[K].
  • Seed – raw or cooked. A nutty flavour, though it has a somewhat gritty texture[K]. The seed can be soaked overnight in warm water then sprouted for a few days and added to salads
  •  It can also be ground into a powder and used as a cereal when it can be made into pancakes, noodles, breads etc. or be used as a thickening agent in soups etc. Rich in vitamin B6.
  • An excellent beer can be brewed from the grain.

Medicinal uses: Some caution should be exercised in the use of this herb because it has been known to cause light-sensitive dermatitis. Buckwheat is a bitter but pleasant tasting herb that is frequently used medicinally because the leaves are a good source of rutin. Rutin is useful in the treatment of a wide range of circulatory problems:

  • it dilates the blood vessels, reduces capillary permeability and lowers blood pressure
  •  The leaves and shoots of flowering plants are acrid, astringent and vasodilator
  •  It is used internally in the treatment of high blood pressure, gout, varicose veins, chilblains, radiation damage, etc.
  • It is best used in conjunction with vitamin C since this aids absorption.
  • Often combined with lime flowers (Tilia species), it is a specific treatment for haemorrhage into the retina.
  •  A poultice made from the seeds has been used for restoring the flow of milk in nursing mothers.
  • An infusion of the herb has been used in the treatment of erysipelas (an acute infectious skin disease).
  • A homeopathic remedy has been made from the leaves. It is used in the treatment of eczema and liver disorders.

Other uses:

  • A very good green manure plant, it can be used to reclaim badly degraded soils and subsoils.
  • A blue dye is obtained from the stems. A brown dye is obtained from the flowers.
  • A very easily grown plant, it prefers dry sandy soils but succeeds in most conditions including poor, heavy or acid soils and even sub-soils. Prefers a cool moist climate, but it also succeeds in dry and arid regions.
  • Plants have poor frost resistance but they are disease and insect resistant. They inhibit the growth of winter wheat.
  • The flowers have a pleasant sweet honey scent and are extremely attractive to bees and hoverflies.
The Amphibious bistort is an attractive floating plant! (picture by I, Svdmolen, Wikipedia https://commons.wikimedia.org/w/index.php?curid=2242889)

P. amphibia or Amphibious bistort

The last Persicaria I’d like to mention is the Amphibious bistort or P. amphibia. This has two different growth forms; a floating, amphibious form which has mostly the flowers and a landform which is very similar looking to P. maculosa or P. lapathifolia.


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