Japanese knotweed
Reynoutria japonica · Polygonaceae
The gardener's calendar
Ideal time to plant japanese knotweed: set it out when the soil is neither frozen nor waterlogged, and water generously as it establishes.
ID card
- Foliage
- Deciduous
- Flowering
- August to October, cream-white flowers in panicles
- Adult size
- 1.5 to 3 m in height
- Growth
- Very fast and vigorous
- Hardiness
- Excellent, down to -20 °C
- Origin
- Japan and China
- Soil type
- Undemanding, tolerating most soils
- Character
- Invasive, highly proliferating
Care
Watering
Japanese knotweed is very tolerant of drought once established. Water regularly at the start of growth and during prolonged dry spells. An adult plant requires minimal watering. Avoid waterlogging the soil, which can promote rhizome rot.
Exposure
Plant the knotweed in full sun to partial shade for optimal growth. It tolerates partial shade well but will be more vigorous in a sunny location. It adapts to all environments, even disturbed or degraded areas.
Pruning and control
Annual pruning is essential: cut the plant back to ground level in late autumn or early spring. Make regular cuts throughout the season to limit its invasive spread. Warning: this species is highly proliferating and difficult to control; prioritize pulling up young shoots and shallow rhizomes.
Soil
Knotweed adapts to various soils: well-drained, chalky, acidic or neutral. It tolerates poor, compacted or disturbed soils. No soil improvement is necessary; in fact, overly rich soil stimulates even more vigorous growth.
Fertilizer
Fertilizer is not recommended: knotweed has extremely vigorous growth without any input. Soil enrichment amplifies its invasive development. Let the plant develop without additional nutrition.
Diseases and pests
Japanese knotweed is rarely affected by diseases or pests, as its natural vigor allows it to resist them. However, watch for leaf spots appearing in humid conditions. The main problem remains its invasive nature: control depends above all on regular removal of aerial and root parts.
Did you know?
Introduced from Asia to Europe in the 19th century as an ornamental and fodder plant, Japanese knotweed has become one of the continent's most feared invasive species. Its underground rhizomes can spread over several meters and resist many eradication attempts: sustainable management requires patience and vigilance. In France, it is listed in the register of invasive exotic species and its deliberate planting is discouraged.
Where to buy it
Plant in spring or early autumn, avoiding spells of intense heat. Pick a plant with healthy foliage and roots that are not exposed. Best time: March, April, May, September.
Add this plant to my garden →Plants from the same family
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Tips generated and checked by Viridia — adapt them to your local climate.