Fallopia japonica Seeds | Japanese Knotweed
LATIN NAME: Fallopia japonica
COMMON NAME: Japanese Knotweed
Plant Profile
Anyone reading around this plant meets three genus names for one species. Older medical and agricultural literature calls it Polygonum cuspidatum, the knotweed genus being drawn very widely at the time. Botanical work then separated the genus Reynoutria, and later treatments placed the species in Fallopia, alongside the climbing bindweeds.
In spring the shoots come up fast and thick, red-tinged and fleshy enough to be eaten, and they extend into canes reaching two or three metres in a single season. The stem is hollow and jointed like a bamboo, speckled with purple-red, and the leaf is a broad shield with a squared-off base, up to fifteen centimetres long, held alternately along the stem in a slight zig-zag. In late summer the canes carry drooping sprays of small creamy-white flowers.
The rhizome runs deep and wide, carrying a reserve of starch that lets the plant push up new canes after the top growth has been cut. Digging a stand, moving soil from it, or strimming it and scattering the pieces spreads it, which is why official advice is to leave cut material on site. Even small sections of rhizome can renew a stand, so a digger bucket is a far more effective dispersal agent than any seed.
Repeated cutting through the season exhausts the rhizome slowly; stem injection with a systemic herbicide delivers the chemical into the root without spray drift; and a root barrier or a sheet membrane stops lateral spread. On a garden scale, cut the canes before they set any flower, lay the material on plastic to dry out completely, and only then compost it. Never put fresh cane in a green-waste bin, and never rototill ground where rhizome is present.
Because it grows so fast on poor ground, the species has also been trialled as a biomass crop, but a crop that spreads by rhizome across a boundary is not something to plant beside a neighbour’s land.
Science & Biotechnology
It spreads as it does because of a quirk of introduction. The plant that escaped across Europe is essentially a single female clone, an octoploid with 88 chromosomes, and with no male nearby it sets almost no viable seed. It does not need to. Growth comes from a rhizome, and every fragment of that rhizome can start a new stand, which is why the whole infestation is in effect one plant copied thousands of times. The North American introduction was made for the same reasons.
Uses & Benefits
Japanese knotweed arrived in European nurseries in the middle of the nineteenth century as an ornamental, brought out of Japan by collectors working through the Dutch trade, and by the 1840s it was being shown and praised for its bold foliage and its late sprays of creamy flower. Within a hundred years the same plant was being blasted out of railway embankments.
The root and rhizome carry stilbenes including resveratrol and its glucoside polydatin, together with the anthraquinones emodin and physcion, and they are the commercial source of much of the resveratrol sold as a supplement, listed in medicine as a purgative knotweed. Those anthraquinones are purgative at low doses, so the plant is not safe forage and should be kept away from children and livestock.
Beekeepers value the plant for exactly the trait that makes it a problem: it flowers late, in bulk, and yields a strong nectar flow when little else is open. Knotweed honey is pale with a distinctive flavour, and stands are sometimes left uncut within a control programme.
Growing from Seed
The fruit is a small three-angled achene inside a papery perianth, and where seed does form, sow it on the surface of a moist compost in a deep pot that makes root disturbance easier at potting time. Where the plant is female and alone there is no seed, as in most gardens. Either way, keep the pot on hard standing, never on open ground.
If you want it for the late flowers, the medicinal root or the curiosity, give it a container or a pit lined on all sides and at the base with a root barrier, and check that barrier twice a season. Check your local rules before you plant it: in several countries it is an offence to let it spread onto neighbouring land, and lenders in the United Kingdom require a treatment plan when it is found on a property.
Our Take
I am honest with people about this one: it is a fascinating plant and a poor neighbour. If you want the family look without the risk, the related Polygonum multiflorum is grown in Chinese medicine for its root, and buckwheat shows the same family habit in an annual that dies at the end of the season, both far easier to control. If you garden on damp ground where garden sorrel already self-sows, that is the sort of soil this plant loves. Grow it in a lined pit, cut it hard, and check your boundary every spring.
Related Reading
More species in the same orbit as Japanese Knotweed: