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Useful Temperate Plants

Polygonum japonicum

Meisn.

Polygonaceae


The Temperate Database is in the process of being updated, with new records being added and old ones being checked and brought up to date where necessary. This record has not yet been checked and updated.

+ Synonyms

Common Name: Japanese Knotweed

No Image.

General Information

Polygonum japonicum is a Perennial up to 3.00 metres tall.
It has edible, medicinal and miscellaneous uses.

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[
238
Title
Encyclopaedia of Herbs and their Uses.
Publication
 
Author
Bown. D.
Publisher
Dorling Kindersley, London.
Year
1995
ISBN
0-7513-020-31
Description
A very well presented and informative book on herbs from around the globe. Plenty in it for both the casual reader and the serious student. Just one main quibble is the silly way of having two separate entries for each plant.
].

Botanical References

17
Title
Flora of the British Isles.
Publication
 
Author
Clapham, Tutin and Warburg.
Publisher
Cambridge University Press
Year
1962
ISBN
-
Description
A very comprehensive flora, the standard reference book but it has no pictures.
,
58
Title
Flora of Japan. (English translation)
Publication
 
Author
Ohwi. G.
Publisher
Smithsonian Institution
Year
1965
ISBN
-
Description
The standard work. Brilliant, but not for the casual reader.
,
200
Title
The New RHS Dictionary of Gardening. 1992.
Publication
 
Author
Huxley. A.
Publisher
MacMillan Press
Year
1992
ISBN
0-333-47494-5
Description
Excellent and very comprehensive, though it contains a number of silly mistakes. Readable yet also very detailed.

Range

E. Asia - Japan. Commonly naturalized in S. England.

Habitat

Wet grassy places in lowland all over Japan[
58
Title
Flora of Japan. (English translation)
Publication
 
Author
Ohwi. G.
Publisher
Smithsonian Institution
Year
1965
ISBN
-
Description
The standard work. Brilliant, but not for the casual reader.
].

Properties

Edibility Rating *  *  *
Medicinal Rating *  *  *
HabitPerennial
Height3.00 m
Growth RateFast
Self-fertileNo
Cultivation StatusWild

Cultivation Details

A very easily grown plant, it succeeds in ordinary garden soil in sun or shade[
1
Title
RHS Dictionary of Plants plus Supplement. 1956
Publication
 
Author
F. Chittendon.
Publisher
Oxford University Press
Year
1951
ISBN
-
Description
Comprehensive listing of species and how to grow them. Somewhat outdated, it has been replaced in 1992 by a new dictionary (see [200]).
,
111
Title
Popular Hardy Perennials.
Publication
 
Author
Sanders. T. W.
Publisher
Collingridge
Year
1926
ISBN
-
Description
A fairly wide range of perennial plants that can be grown in Britain and how to grow them.
], though it prefers a moist soil in partial shade[
187
Title
Perennials. Volumes 1 and 2.
Publication
 
Author
Phillips. R. & Rix. M.
Publisher
Pan Books
Year
1991
ISBN
0-330-30936-9
Description
Photographs of over 3,000 species and cultivars of ornamental plants together with brief cultivation notes, details of habitat etc.
].
Japanese knotweed is an extremely invasive plant capable of sending up new shoots at a considerable distance from the main clump and also of growing up through tarmac[
200
Title
The New RHS Dictionary of Gardening. 1992.
Publication
 
Author
Huxley. A.
Publisher
MacMillan Press
Year
1992
ISBN
0-333-47494-5
Description
Excellent and very comprehensive, though it contains a number of silly mistakes. Readable yet also very detailed.
]. The sub-species cuspidatum is the form that is most likely to cause problems. It is considered to be a pest in many areas, where it is forming large clumps in the wild and out-competing native species. If grown in the garden it should be planted within a strong barrier to contain its roots. The sub-species compacta is only about 70cm tall and is far less invasive, but should still not be grown in small gardens[
200
Title
The New RHS Dictionary of Gardening. 1992.
Publication
 
Author
Huxley. A.
Publisher
MacMillan Press
Year
1992
ISBN
0-333-47494-5
Description
Excellent and very comprehensive, though it contains a number of silly mistakes. Readable yet also very detailed.
].
A report on the Natural History Programme stated that Japanese knotweed is actually becoming a very valuable habitat for spiders, frogs, grass snakes and many other creatures. Its hollow stems allow a wide variety of insects and other small creatures to overwinter and find hiding places, thus a greater abundance of food is provided for insectivores such as frogs, who are themselves eaten by grass snakes. In areas of north Wales where Japanese knotweed has run rife, it is now the primary habitat for grass snakes[
215
Title
The Natural History Programme 16th September 1994.
Publication
 
Author
BBC
Publisher
BBC
Year
1994
ISBN
-
Description
A report on the Natural History radio 4 programme about the value of Japanese knot weed to British wildlife.
].
Plants seem to be immune to the predations of rabbits[
233
Title
Perennial Garden Plants
Publication
 
Author
Thomas. G. S.
Publisher
J. M. Dent & Sons, London.
Year
1990
ISBN
0 460 86048 8
Description
A concise guide to a wide range of perennials. Lots of cultivation guides, very little on plant uses.
].
Often cultivated as a dye and a medicinal plant[
46
Title
Dictionary of Economic Plants.
Publication
 
Author
Uphof. J. C. Th.
Publisher
Weinheim
Year
1959
ISBN
-
Description
An excellent and very comprehensive guide but it only gives very short descriptions of the uses without any details of how to utilize the plants. Not for the casual reader.
,
61
Title
A Dictionary of Plants Used by Man.
Publication
 
Author
Usher. G.
Publisher
Constable
Year
1974
ISBN
0094579202
Description
Forget the sexist title, this is one of the best books on the subject. Lists a very extensive range of useful plants from around the world with very brief details of the uses. Not for the casual reader.
].
Very closely related to P. conspicuum[
58
Title
Flora of Japan. (English translation)
Publication
 
Author
Ohwi. G.
Publisher
Smithsonian Institution
Year
1965
ISBN
-
Description
The standard work. Brilliant, but not for the casual reader.
].
Dioecious, male and female plants must be grown if seed is required[
58
Title
Flora of Japan. (English translation)
Publication
 
Author
Ohwi. G.
Publisher
Smithsonian Institution
Year
1965
ISBN
-
Description
The standard work. Brilliant, but not for the casual reader.
].

Edible Uses

Young shoots in spring - cooked. They can be used as an asparagus substitute[
55
Title
Eat the Weeds.
Publication
 
Author
Harris. B. C.
Publisher
Pivot Health
Year
1973
ISBN
-
Description
Interesting reading.
,
62
Title
A Field Guide to N. American Edible Wild Plants.
Publication
 
Author
Elias. T. and Dykeman. P.
Publisher
Van Nostrand Reinhold
Year
1982
ISBN
0442222009
Description
Very readable.
,
116
Title
Oriental Herbs and Vegetables, Vol 39 No. 2.
Publication
 
Author
Brooklyn Botanic Garden
Publisher
Brooklyn Botanic Garden
Year
1986
ISBN
-
Description
A small booklet packed with information.
, ,
179
Title
Famine Foods listed in the Chiu-Huang Pen-ts'ao.
Publication
 
Author
Reid. B. E.
Publisher
Southern Materials Centre; Taipei
Year
1977
ISBN
-
Description
A translation of an ancient Chinese book on edible wild foods. Fascinating.
,
183
Title
Cornucopia - A Source Book of Edible Plants.
Publication
 
Author
Facciola. S.
Publisher
Kampong Publications
Year
1990
ISBN
0-9628087-0-9
Description
Excellent. Contains a very wide range of conventional and unconventional food plants (including tropical) and where they can be obtained (mainly N. American nurseries but also research institutes and a lot of other nurseries from around the world.
]. They have an acid flavour and can also be used as a rhubarb substitute in pies, fruit soups, jams etc[
183
Title
Cornucopia - A Source Book of Edible Plants.
Publication
 
Author
Facciola. S.
Publisher
Kampong Publications
Year
1990
ISBN
0-9628087-0-9
Description
Excellent. Contains a very wide range of conventional and unconventional food plants (including tropical) and where they can be obtained (mainly N. American nurseries but also research institutes and a lot of other nurseries from around the world.
].
Older stems and shoot tips - cooked[
62
Title
A Field Guide to N. American Edible Wild Plants.
Publication
 
Author
Elias. T. and Dykeman. P.
Publisher
Van Nostrand Reinhold
Year
1982
ISBN
0442222009
Description
Very readable.
,
159
Title
Wild Food Plants of Indiana.
Publication
 
Author
McPherson. A. and S.
Publisher
Indiana University Press
Year
1977
ISBN
0-253-28925-4
Description
A nice pocket guide to this region of America.
]. They taste like a mild version of rhubarb[
K
Title
Plants for a Future
Author
Ken Fern
Description
Notes from observations, tasting etc at Plants For A Future and on field trips.
].
Seed - raw or cooked. It is rather small and fiddly to utilize. The seed can also be ground into a powder and used as a flavouring and thickener in soups etc, or can be mixed with cereals when making bread, cakes etc.
The root is sometimes eaten[
183
Title
Cornucopia - A Source Book of Edible Plants.
Publication
 
Author
Facciola. S.
Publisher
Kampong Publications
Year
1990
ISBN
0-9628087-0-9
Description
Excellent. Contains a very wide range of conventional and unconventional food plants (including tropical) and where they can be obtained (mainly N. American nurseries but also research institutes and a lot of other nurseries from around the world.
].

Medicinal

The root is antiphlogistic, bechic, depurative, diuretic, emmenagogue, emollient, febrifuge, stomachic and vulnerary[
46
Title
Dictionary of Economic Plants.
Publication
 
Author
Uphof. J. C. Th.
Publisher
Weinheim
Year
1959
ISBN
-
Description
An excellent and very comprehensive guide but it only gives very short descriptions of the uses without any details of how to utilize the plants. Not for the casual reader.
,
61
Title
A Dictionary of Plants Used by Man.
Publication
 
Author
Usher. G.
Publisher
Constable
Year
1974
ISBN
0094579202
Description
Forget the sexist title, this is one of the best books on the subject. Lists a very extensive range of useful plants from around the world with very brief details of the uses. Not for the casual reader.
,
147
Title
A Barefoot Doctors Manual.
Publication
 
Author
?
Publisher
Running Press; Philadelphica.
Year
1977
ISBN
0-914294-92-X
Description
A very readable herbal from China, combining some modern methods with traditional chinese methods.
,
178
Title
Chinese Materia Medica.
Publication
 
Author
Stuart. Rev. G. A.
Publisher
Taipei. Southern Materials Centre
Year
1911
ISBN
-
Description
A translation of an ancient Chinese herbal. Fascinating.
,
218
Title
Medicinal Plants of China
Publication
 
Author
Duke. J. A. and Ayensu. E. S.
Publisher
Reference Publications, Inc.
Year
1985
ISBN
0-917256-20-4
Description
Details of over 1,200 medicinal plants of China and brief details of their uses. Often includes an analysis, or at least a list of constituents. Heavy going if you are not into the subject.
]. It is also used in the treatment of women's complaints[
147
Title
A Barefoot Doctors Manual.
Publication
 
Author
?
Publisher
Running Press; Philadelphica.
Year
1977
ISBN
0-914294-92-X
Description
A very readable herbal from China, combining some modern methods with traditional chinese methods.
]. A decoction is used in the treatment of burn injuries, boils and abscesses, poisonous snakebites, acute hepatitis, appendicitis, traumatic injuries and menstrual irregularities[
147
Title
A Barefoot Doctors Manual.
Publication
 
Author
?
Publisher
Running Press; Philadelphica.
Year
1977
ISBN
0-914294-92-X
Description
A very readable herbal from China, combining some modern methods with traditional chinese methods.
]. The leaves can be crushed and applied externally as a poultice to abscesses, cuts etc, whilst the dried roots can be ground into a powder and applied externally[
147
Title
A Barefoot Doctors Manual.
Publication
 
Author
?
Publisher
Running Press; Philadelphica.
Year
1977
ISBN
0-914294-92-X
Description
A very readable herbal from China, combining some modern methods with traditional chinese methods.
].
Extracts of the plant have shown antitumour activity[
218
Title
Medicinal Plants of China
Publication
 
Author
Duke. J. A. and Ayensu. E. S.
Publisher
Reference Publications, Inc.
Year
1985
ISBN
0-917256-20-4
Description
Details of over 1,200 medicinal plants of China and brief details of their uses. Often includes an analysis, or at least a list of constituents. Heavy going if you are not into the subject.
].

Other Uses

A yellow dye is obtained from the root[
46
Title
Dictionary of Economic Plants.
Publication
 
Author
Uphof. J. C. Th.
Publisher
Weinheim
Year
1959
ISBN
-
Description
An excellent and very comprehensive guide but it only gives very short descriptions of the uses without any details of how to utilize the plants. Not for the casual reader.
,
61
Title
A Dictionary of Plants Used by Man.
Publication
 
Author
Usher. G.
Publisher
Constable
Year
1974
ISBN
0094579202
Description
Forget the sexist title, this is one of the best books on the subject. Lists a very extensive range of useful plants from around the world with very brief details of the uses. Not for the casual reader.
].
The plant is potentially a good source of biomass.
Plants can be grown to form a ground cover that will exclude all other growth[
208
Title
Plants for Ground Cover
Publication
 
Author
Thomas. G. S.
Publisher
J. M. Dent & Sons
Year
1990
ISBN
0-460-12609-1
Description
An excellent detailled book on the subject, very comprehensive.
]. It is best to use the sub-species compactum since this is less invasive[
208
Title
Plants for Ground Cover
Publication
 
Author
Thomas. G. S.
Publisher
J. M. Dent & Sons
Year
1990
ISBN
0-460-12609-1
Description
An excellent detailled book on the subject, very comprehensive.
].

Propagation

Seed - sow spring in a cold frame. 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.

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