Polygonum cuspidatum (Japanese knotweed) — Mat-forming, clump-forming or tall and upright

Overview

Polygonum cuspidatum is a mat-forming, clump-forming or tall and upright perennial in the genus Polygonum, part of the Polygonaceae family, with deciduous; simple, lance-shaped to heart-shaped, often marked or autumn-tinted foliage. Polygonum cuspidatum grows best in full sun to partial shade, and tolerates almost any soil and including heavy clay soils and acid and neutral or alkaline pH. Its UK hardiness is rated H5-H7. It is native to East Asia.

About

Polygonum cuspidatum is a perennial in the Polygonaceae with a variable habit, from mat-forming or clump-forming to tall and upright growth. Leaves are deciduous, simple and range from lance-shaped to heart-shaped, often marked or taking on autumn tints. It tolerates almost any soil, including heavy clay, and favours moist conditions over drought. Flowering occurs July to September.

Wikipedia contributors, CC BY-SA 4.0 — Reynoutria japonica

About the genus

Polygonum is a genus of about 130 species of flowering plants in the buckwheat and knotweed family Polygonaceae. Common names include knotweed and knotgrass. In the Middle English glossary of herbs Alphita, it was known as ars-smerte.

Wikipedia contributors, "Polygonum", CC BY-SA 4.0 — Polygonum

In the garden

Polygonum cuspidatum (Japanese knotweed) is a perennial native to East Asia, placed in the Polygonaceae. It can establish as a low, mat-forming cover, form clumps or grow as tall, upright stems, and its leaves are simple and often show marked or autumn-tinted colouring. Light and site are straightforward: it will grow in full sun through to partial shade and accepts any aspect, whether exposed or sheltered. Soil type is not critical — it tolerates almost any soil, including heavy clay, and will grow on acid, neutral or alkaline ground. Moisture is important: most plants tolerate damp conditions but dislike prolonged drought, so a spot that remains reliably moist suits it best. Hardiness runs H5–H7, meaning it is hardy to very hardy across most of the UK (H5 hardy; H6 very hardy; H7 fully hardy). Use positions where soil remains moist and compaction or heavier soils make other plants struggle; it will also tolerate sites with variable exposure. It flowers in late summer, from July to September, so expect its bloom period to fall in that window when assessing seasonal interest and management needs.

Growing conditions

Best soil and position for Polygonum

Sunlight: Full sun to partial shade
Soil type: Almost any soil, including heavy clay
Soil pH: Acid, neutral or alkaline
Soil moisture: Moist; most tolerate damp ground and dislike drought
Aspect: Any
Exposure: Exposed or sheltered
UK hardiness: H5-H7

Plant details

Polygonum cuspidatum size, habit and characteristics

Plant type: Perennial
Habit: Mat-forming, clump-forming or tall and upright
Foliage: Deciduous; simple, lance-shaped to heart-shaped, often marked or autumn-tinted
Native to: East Asia

Care notes

Pruning and caring for Polygonum cuspidatum

Cultivation: Undemanding to the point of needing restraint rather than encouragement. Give reliable moisture and they flower for months. Vigour is the main consideration: check the spread of any given species before planting, since the running kinds colonise fast and are difficult to remove once established, while the clump-forming ones are well behaved. Note that most garden knotweeds are now classified under Persicaria.
Pruning: Cut back to the base in late autumn, or leave the dried spikes standing for winter interest and clear in early spring. Shear mat-forming kinds after the first flush to tidy them.
Propagation: Divide in spring or autumn - most divide very easily. Basal cuttings root readily in spring.
Pest resistance: Slugs graze new growth; otherwise notably trouble-free.
Disease resistance: Healthy and robust. Powdery mildew occurs on plants that dry out.

Frequently asked questions

How do I grow Polygonum cuspidatum?

Undemanding to the point of needing restraint rather than encouragement. Give reliable moisture and they flower for months. Vigour is the main consideration: check the spread of any given species before planting, since the running kinds colonise fast and are difficult to remove once established, while the clump-forming ones are well behaved. Note that most garden knotweeds are now classified under Persicaria.

Where is the best place to plant Polygonum cuspidatum?

Polygonum cuspidatum prefers full sun to partial shade. It suits exposed or sheltered positions.

What soil does Polygonum cuspidatum need?

Polygonum cuspidatum grows best in almost any soil and including heavy clay soil. It prefers acid and neutral or alkaline soil. Aim for soil that stays moist; most tolerate damp ground and dislike drought.

How do I prune Polygonum cuspidatum?

Cut back to the base in late autumn, or leave the dried spikes standing for winter interest and clear in early spring. Shear mat-forming kinds after the first flush to tidy them.

Is Polygonum cuspidatum hardy in the UK?

Polygonum cuspidatum has an RHS hardiness rating of H5, meaning it is hardy in most of the UK, tolerating -15 to -10°C.

How do I propagate Polygonum cuspidatum?

Divide in spring or autumn - most divide very easily. Basal cuttings root readily in spring.

More in the Polygonum genus

Browse all Polygonum →