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

Overview

Polygonum carneum 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 carneum 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 carneum is a perennial in the buckwheat family (Polygonaceae) that can grow as a mat-forming ground cover, clump, or a taller upright plant. It has deciduous, simple leaves that range from lance-shaped to heart-shaped and may be marked or take on autumn tints. Flowering occurs from July to September. It suits a wide range of positions and soils, and prefers reliably moist conditions.

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 carneum sits in the Polygonum genus of the Polygonaceae and presents a variable habit: you may find it spreading as a mat, forming discrete clumps or producing taller, upright stems. Leaves are deciduous and typically lance- to heart-shaped; they are often marked and can show autumn colour. Flowering runs from July to September. Grow Polygonum carneum in full sun to partial shade on almost any soil, including heavy clay. Soil pH is not critical — it tolerates acid, neutral or alkaline ground — but it performs best with consistent moisture and dislikes drought. It will tolerate exposed sites as well as sheltered ones, and can be planted on any aspect. Rated H5–H7, it is hardy to very hardy and fully hardy in many parts of the UK, so it survives typical winter conditions across most regions. Use the mat-forming forms as a ground cover on damp borders or alongside ponds, and place clump-forming or upright plants where their summer flowers can be seen between July and September. It copes where heavier soils or fluctuating exposure would challenge less adaptable perennials. Expect the deciduous foliage to change character into marked patterns or autumn tints as the season progresses.

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 carneum 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 carneum

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 carneum?

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 carneum?

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

What soil does Polygonum carneum need?

Polygonum carneum 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 carneum?

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 carneum hardy in the UK?

Polygonum carneum 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 carneum?

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

More in the Polygonum genus

Browse all Polygonum →