Polygonum campanulatum (Knotweed) — Mat-forming, clump-forming or tall and upright

Overview

Polygonum campanulatum 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 campanulatum 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 China, Bhutan, north Myanmar, Nepal and Sikkim.

About

Polygonum campanulatum is a perennial knotweed with a variable habit that can form mats, clumps or grow tall and upright. Leaves are deciduous, simple and range from lance-shaped to heart-shaped, often showing markings or an autumn tint. It tolerates full sun to partial shade and will grow in almost any soil, including heavy clay, provided the ground stays moist. Expect flowering from July to September.

Wikipedia contributors, CC BY-SA 4.0 — Koenigia campanulata

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 campanulatum is a perennial member of the Polygonaceae native to China, Bhutan, northern Myanmar, Nepal and Sikkim. Plants show a range of forms — from low, mat-forming patches and clumps to taller, erect stems — with simple, often marked or autumn-tinted leaves that drop back in winter. Plant where it receives full sun or partial shade and choose any soil type; this species will establish on heavy clay and tolerates acid, neutral or alkaline pH. It prefers consistently moist conditions and most plants tolerate damp ground but do not like drought. Aspect and exposure are flexible (any aspect, exposed or sheltered), and its hardiness is given as H5–H7, meaning it is hardy to fully hardy across UK conditions. Use Knotweed in moist mixed borders, alongside ponds or in heavy-soil beds where other perennials struggle. The mat-forming clones work as a ground cover, while the taller forms give vertical presence at the back of a border. Flowers appear from July to September, providing a clear late-summer display tied to the plant's growth habit.

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 campanulatum 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: China, Bhutan, north Myanmar, Nepal, Sikkim

Care notes

Pruning and caring for Polygonum campanulatum

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 campanulatum?

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 campanulatum?

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

What soil does Polygonum campanulatum need?

Polygonum campanulatum 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 campanulatum?

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 campanulatum hardy in the UK?

Polygonum campanulatum 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 campanulatum?

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

More in the Polygonum genus

Browse all Polygonum →