Giant Knotweed
Polygonum sachalinense

Family: Polygonaceae

What it is like

Polygonum sachalinense is a PERENNIAL growing to 3.6 m (11ft) by 3 m (9ft) at a fast rate. See above for USDA hardiness. It is hardy to UK zone 5. It is in flower from July to October, and the seeds ripen from August to October. The species is dioecious (individual flowers are either male or female, but only one sex is to be found on any one plant so both male and female plants must be grown if seed is required). . The plant is not self-fertile. Suitable for: light (sandy), medium (loamy) and heavy (clay) soils. Suitable pH: mildly acid, neutral and basic (mildly alkaline) soils. It can grow in semi-shade (light woodland) or no shade. It prefers moist soil.

Height (m): 3.6

Where it is found

Along ravines and by streams in montane areas of Sakhalin Island.

E. Asia - Japan. Occasionally naturalized in Britain.

Conservation Status:

Countries/locations it is found in

How it is used

Food

Rating: 2

Young shoots in spring - raw or cooked. They can be added to salads or cooked as an asparagus substitute. They have an acid flavour and we find that they are more like a rhubarb substitute. Older stems and shoot tips - cooked. The stems are best peeled. Tasting like a mild version of rhubarb, they have a superior quality with a hint of lemon in the flavour. Seed - cooked. The seed can be ground into a powder and used as a thickener and flavouring in soups etc, or as an extender in flour. It is rather small and fiddly to utilize.

Seed: includes nuts, cereals, peas and beans.

Stem: this often intergrades into leaves.

Medicine

Rating: 0

Other

Rating: 3

A potential source of biomass. Plants are very vigorous and could be grown as a ground cover.

Biomass: Provides a large quantity of plant material that can be converted into fuel etc.

Ground cover: Ground Cover

How it is grown

Succeeds in ordinary garden soil in sun or shade. Hardy to about -25°c. An extremely invasive plant, capable of sending up new shoots at a considerable distance from the main clump. Considered a pest in many areas, if grown in the garden it should be planted within a barrier to contain its roots. Plants seem to be immune to the predations of rabbits. Dioecious, male and female plants must be grown if seed is required.

Propagating it: 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.

Best place to grow: Woodland Garden Dappled Shade; Shady Edge; Ground Cover;

Habit: Perennial

Hardiness: 4-8

Growth: Fast

Soil: Light (sandy), medium, heavy (clay)

Shade: Semi-shade, no shade

Moisture: Moist

Things to keep in mind

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.

Its other names

Local names

Synonyms

Reynoutria sachalinensis. (Freidrich.Schmidt.&Petrop.)Nakai.