Polygonum polystachyum (Himalayan knotweed, Cultivated knotweed) — Mat-forming, clump-forming or tall and upright

Overview

Polygonum polystachyum 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. It typically grows around 1.8 m tall. Polygonum polystachyum 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. Suggested uses include ground cover, erosion control and ornamental. It is native to Himalayas.

About

Polygonum polystachyum is a perennial in the knotweed family that can be mat-forming, clump-forming or grow tall and upright. Plants reach about 1.8 m and carry deciduous, lance- to heart-shaped leaves that are often marked or turn autumn-tinted. It flowers from July to September. It tolerates full sun to partial shade and a wide range of soils, including heavy clay.

Wikipedia contributors, CC BY-SA 4.0 — Koenigia polystachya

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 polystachyum belongs to the family Polygonaceae and originates from the Himalayas. Forms range from low mats to clumps or upright stems, bearing simple, lance-shaped to heart-shaped leaves which frequently show markings or autumn colour. Flowering takes place from July to September. Grow it in full sun to partial shade on almost any soil, including heavy clay; it tolerates acid, neutral or alkaline pH. Plants prefer moist ground and most forms dislike drought, so plan irrigation or a reliably damp site for summer. It will accept any aspect and copes in exposed or sheltered positions. In UK hardiness terms the rating H5–H7 means it is hardy to fully hardy. Use Polygonum polystachyum as a ground cover, for erosion control or as an architectural ornamental where its variable habit suits the border. Upright selections may reach around 1.8 m and can be given space at the back of a bed, while lower forms work on banks and damp corners where they can help hold soil.

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 polystachyum 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
Height: 1.8 m
Suggested uses: Ground cover, Erosion control, Ornamental
Native to: Himalayas

Care notes

Pruning and caring for Polygonum polystachyum

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 polystachyum?

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 polystachyum?

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

What soil does Polygonum polystachyum need?

Polygonum polystachyum 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 polystachyum?

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.

How big does Polygonum polystachyum grow?

Polygonum polystachyum reaches a height of 1.8 m.

Is Polygonum polystachyum hardy in the UK?

Polygonum polystachyum 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 polystachyum?

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

More in the Polygonum genus

Browse all Polygonum →