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

Overview

Polygonum sphaerostachyum 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 sphaerostachyum 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 West and South China, Bhutan, Nepal, northern India and Pakistan.

About

Polygonum sphaerostachyum is a perennial knotweed that may form mats, clumps or grow tall and upright. It has deciduous, simple leaves that range from lance-shaped to heart-shaped and are often marked or take on autumn tints. Flowering occurs July to September. The species tolerates a wide range of soils, including heavy clay, and prefers moist conditions.

Wikipedia contributors, CC BY-SA 4.0 — Bistorta macrophylla

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 sphaerostachyum belongs to the Polygonaceae and is native to mountain regions of West and South China, Bhutan, Nepal, northern India and Pakistan. Forms vary from mat-forming and clump-forming types to more upright stems, and the foliage—simple, lance- to heart-shaped—is often marked and autumn-tinted. It flowers in the UK from July to September. Give it full sun to partial shade and a moist site; most specimens tolerate damp ground but dislike drought. Soil pH is not critical—acid, neutral or alkaline soils are all acceptable—and it will grow in almost any soil, including heavy clay. It is hardy in UK terms from H5 to H7, meaning it is generally hardy to very hardy and will suit most outdoor locations across Britain. Because it accepts exposed or sheltered positions and any aspect, Polygonum sphaerostachyum suits damp borders, naturalistic or meadow-style plantings and areas of heavy soil where other perennials struggle. Use clump-forming or taller forms at the back of a border and mat-forming types for ground cover. Note the seasonal sequence: summer flower spikes in July to September are followed by leaves that often colour in autumn, offering two distinct periods of interest.

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 sphaerostachyum 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: West and South China, Bhutan, Nepal, northern India, Pakistan

Care notes

Pruning and caring for Polygonum sphaerostachyum

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 sphaerostachyum?

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 sphaerostachyum?

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

What soil does Polygonum sphaerostachyum need?

Polygonum sphaerostachyum 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 sphaerostachyum?

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 sphaerostachyum hardy in the UK?

Polygonum sphaerostachyum 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 sphaerostachyum?

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

More in the Polygonum genus

Browse all Polygonum →