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

Overview

Polygonum paniculatum 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 paniculatum 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.

About

Polygonum paniculatum is a perennial member of the buckwheat family (Polygonaceae) with a notably variable habit: it may be mat-forming, clump-forming or grow tall and upright. Leaves are deciduous, simple and range from lance-shaped to heart-shaped, often marked or taking on autumn tint. It prefers full sun to partial shade and tolerates almost any soil, including heavy clay, as long as the ground remains reasonably moist. Flowers are produced from July to September.

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 paniculatum belongs to the genus Polygonum in the Polygonaceae and behaves as a perennial with flexible habits, from low mats to upright clumps. The foliage is deciduous, the leaves simple and often marked; many plants show an autumn tint before leaf fall. Expect flowering in the UK between July and September. Plant in full sun to partial shade on any aspect; the species tolerates exposed or sheltered sites. It will grow in almost any soil and takes heavy clay and a wide pH range (acid to alkaline), but it prefers consistently moist conditions and dislikes prolonged drought. Hardiness is H5–H7, so it ranges from hardy to fully hardy for UK gardens and will survive typical winter conditions across most regions. Use Polygonum paniculatum where damp soil is an issue: margins of damp borders, pond edges and heavy-ground corners where other perennials struggle. Its varying habit means it can form ground cover or stand taller in mixed borders; the autumn-tinted leaves provide a seasonal change after flowering. Expect florets through July to September and noticeable leaf colour change in autumn.

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 paniculatum 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

Care notes

Pruning and caring for Polygonum paniculatum

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 paniculatum?

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 paniculatum?

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

What soil does Polygonum paniculatum need?

Polygonum paniculatum 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 paniculatum?

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 paniculatum hardy in the UK?

Polygonum paniculatum 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 paniculatum?

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

More in the Polygonum genus

Browse all Polygonum →