Japanese knotweed

How to recognise it

Habit

Perennial, in dense stands 2–3 m tall; dies back above ground every winter and shoots up again in April.

Stem

Hollow, hairless and bamboo-like, with clear nodes; green with red to purple speckles, slightly zigzag. Young shoots are red and look like asparagus.

Leaf

Broadly oval to almost triangular, 10–15 cm, with a straight-cut base and a short point; alternate on the zigzag stem.

Flower

Small creamy white flowers in upright sprays from the leaf axils, August to October.

Roots

Thick, woody rhizomes, orange-yellow inside, that run deep and several metres underground.

Confusion with

Giant knotweed is even larger, with leaves up to 30–40 cm and a heart-shaped base. Bohemian knotweed is the hybrid of the two and sits in between; all three are invasive. Native knotweeds and docks stay much lower and have no hollow, bamboo-like stems.

Flowering: August–October · Pruning: April–October

What you need to know

• It is one of the most harmful invasive plants in Europe: recognising it early saves a great deal of work and cost.
• A small, young stand can still be tackled; an old stand needs years of persistent work.
• It shows how a prized ornamental can get out of hand: in the 19th century it was promoted as a garden plant and fodder.
• Its flowers give bees nectar in late summer, but that does not outweigh the damage: native plants such as hemp-agrimony do the same without spreading.
• Almost nothing grows under its dense canopy, and in winter riverbanks lie bare and erode.

Japanese knotweed: where does it grow best?

• In the wild: river and canal banks, railway embankments, road verges, waste ground, woodland edges and neglected gardens.
• Grows on almost any soil, from dry to wet, best on fertile, moist soil in sun or partial shade.
• Spreads mainly with moved soil, garden waste, mowing machinery and fragments carried downstream by rivers.
• In the garden it belongs nowhere: not in a pot, not in a closed container and not in a wild corner.

Plant these instead

• Hemp-agrimony: A native alternative for a tall waterside plant: pink heads in late summer, a magnet for butterflies.
• Meadowsweet: Tall, fragrant plumes on moist soil, without running wild.
• Elder: Fast-growing native shrub that quickly covers a bare patch after control.
• Hazel: Dense native shrub for screening, with nuts for squirrels and mice.
• Purple loosestrife: Purple spikes along banks and ditches, loved by bees.

How to remove it

1. Never plant Japanese knotweed and never give pieces away: it is on the Belgian list of invasive species.
2. Report a stand in a public place to your local council; for a large stand in your garden, ask your council or a specialist firm for advice.
3. Tackle a small, young stand at once: dig the plant out with all its rhizomes, as deep and wide as needed, and search the soil for fragments left behind.
4. For a larger stand, mow or cut the stems to the ground every 2–3 weeks from April to October, year after year. This exhausts the rhizomes.
5. Collect all stems and roots in sealed bags, leave nothing behind and dispose of everything with the general (residual) waste or as your council instructs. Never on the compost heap or in the green waste bin.
6. Do not move soil from a place where knotweed grows, and clean spades, tyres and shoes before you leave.

Control

Exhausting by cutting

Mow or cut all stems every 2–3 weeks from April to October. After a few years the shoots become thinner and shorter; do not stop too soon.

Digging out

Works for small, young stands. Remove every rhizome; any fragment left behind can sprout again.

Covering

Mow the stand, cover it with strong, root-proof sheeting that extends well beyond the edges and leave it for years. Check edges and seams regularly for shoots.

Aftercare

After control, sow or plant native species so the soil is covered. Keep checking the spot for new shoots for years.

Waste

Stems and roots in sealed bags with the general waste. No shredding, no composting and never dump them in the countryside.

Tip: Never chop or rotavate a knotweed stand: every scrap of rhizome becomes a new plant. Cut persistently instead and dispose of everything with the general waste.

Pruning and mowing

Pruning months: April–October.

When: From April to October, every 2–3 weeks: not to look after the plant, but to exhaust it.

How:

• Cut or mow the stems as low as possible as soon as they reach 20–30 cm.
• Pick up every piece: even a stem fragment with a node can root on moist soil.
• Do not use a flail mower or shredder: they scatter small fragments over a large area.
• Clean the mower, shoes and tyres before you leave the site.

Caution: Occasional cutting does not help and actually triggers extra shoots: only frequent cutting, kept up for years, wears it out.

Pruning: when and how · Pruning calendar and basic rules

How it spreads

Rhizomes

The main route: a small piece of rhizome grows into a new plant. It travels with moved soil, diggers and garden waste.

Stem pieces

Fresh stem pieces with a node root on moist soil or in water; along rivers they drift and establish downstream.

Seed

In Europe seed plays almost no role: the plants are female and largely form a single clone. Hybrids with giant knotweed can, however, produce viable seed.

Japanese knotweed: how does it get through the winter?

Hardy to −30 °C.

• It dies back above ground after the first frost; the hollow, reddish-brown stems stay standing until spring.
• The rhizomes survive hard frost with ease and sprout again in April.
• Under the dead stems the soil is bare: on riverbanks it washes away in winter.
• A good time to map the size of a stand and to clear and dispose of the dead stems.

Diseases and pests

• Smothering: Its dense canopy takes all the light: native plants and the animals that depend on them disappear.
• Damage to paving: Rhizomes and shoots push through joints in paving, walls and tarmac and widen cracks. Deal with shoots at once.
• Bank erosion: In winter riverbanks lie bare and erode faster. Report stands along watercourses to your council or the water authority.
• Regrowth after digging: Shoots keep appearing for years after digging out. Pull them up each time, roots and all, and keep checking the spot.
• Contaminated soil: Imported soil can contain rhizome fragments. Ask where topsoil comes from and check the spot during the first years.

Through the year

• March: Inspect the site: the first red buds appear at the base of the old stems.
• April: Red, asparagus-like shoots appear. Start cutting or pulling straight away.
• May: Cut every 2–3 weeks. Dispose of everything with the general waste.
• June: Keep cutting. Check the edges of any covering sheet.
• July: Keep cutting. Watch for new shoots outside the stand.
• August: Flowering starts. Keep cutting, even in hot weather.
• September: Keep cutting. Note how large the stand still is.
• October: Last cut; the parts above ground die back.
• November: Clear and dispose of the dead stems.
• December: Dormant. Plan next year’s control.

Did you know?

• Philipp Franz von Siebold brought the plant from Japan to his nursery in Leiden in the 19th century. In 1847 a horticultural society in Utrecht even awarded it a gold medal as the most interesting new ornamental plant.
• In Europe the plants are almost all female and genetically nearly identical: here the species spreads as one giant clone, without seed.
• In Japan the plant is called itadori and the young shoots are eaten as a vegetable. There, insects and fungi keep it in check.