Polygonum affine is a perennial from the Himalayas that can form low mats, clumps or produce taller upright flowering stems. Leaves are deciduous, lance-shaped to heart-shaped and are often marked or take on autumn tints. It flowers July to September. It prefers full sun to partial shade and suits moist soils, including heavy clay, but dislikes drought.

Himalayan bistort (Polygonum affine) is a mat-forming perennial that flowers July to September and suits moist soils, borders and rock gardens.

https://en.wikipedia.org/wiki/Bistorta_affinis

The knotweeds are vigorous, long-flowering plants carrying dense narrow spikes or loose sprays of tiny flowers, usually in pink, crimson or greenish white, above simple lance-shaped or heart-shaped foliage that frequently colours well in autumn. Habit varies enormously, from low creeping mats that make dense weed-suppressing cover to tall, architectural clump-formers. Several are extremely vigorous, and the genus has been extensively reclassified, with many garden plants now placed in Persicaria.

Polygonum affine

Himalayan bistort, fleece flower | Polygonum affine

Polygonum affine 'Superbum' is a perennial knotweed notable for its variable habit, forming mats, clumps or sometimes growing more upright. Leaves are deciduous, simple and range from lance-shaped to heart-shaped, often showing markings or an autumn tint. It flowers from July to September and prefers full sun to partial shade. The plant tolerates almost any soil including heavy clay, favours moist conditions and dislikes drought.

Polygonum affine 'Superbum' (Knotweed) suits moist sites in full sun to partial shade and flowers July to September.

Polygonum affine 'Superbum'

Knotweed | Polygonum affine 'Superbum'

Polygonum albinum is a perennial in the knotweed family that can grow as a mat-forming ground cover, a clump, or as tall upright stems. It has deciduous, simple leaves that range from lance-shaped to heart-shaped and are often marked or take an autumn tint. Flowering occurs from July to September. Plants tolerate a wide range of soils, including heavy clay, and are suited to full sun or partial shade.

Knotweed (Polygonum albinum) is a perennial Polygonaceae species flowering July to September that tolerates full sun to partial shade and most soils.

Polygonum albinum

Knotweed | Polygonum albinum

Polygonum amplexicaule is a perennial in the buckwheat family that tolerates moist, even damp, ground and grows to about 1.2 metres. Forms range from mat-forming or clump-forming to taller, upright plants. Leaves are deciduous, simple and lance- to heart-shaped, often marked or taking on autumn tints. It flowers in the UK from July to September.

Red bistort (Polygonum amplexicaule) thrives in moist soil and flowers from July to September in UK gardens.

https://en.wikipedia.org/wiki/Bistorta_amplexicaulis

Polygonum amplexicaule

Red bistort, mountain fleece | Polygonum amplexicaule

Polygonum amplexicaule 'Arun Gem' is a moisture-preferring perennial knotweed that can form mats, clumps or grow more upright to about 1.2 metres. The leaves are deciduous, lance- to heart-shaped and often show markings or autumn tint. It flowers from July to September, giving a summer display. Plants tolerate a wide range of soils, including heavy clay, but dislike drought.

Polygonum amplexicaule 'Arun Gem' is a 1.2m perennial knotweed that flowers July to September and prefers moist soil in sun or part shade.

Polygonum amplexicaule 'Arun Gem'

Red Bistort | Polygonum amplexicaule 'Arun Gem'

Polygonum amplexicaule 'Atrosanguineum' is a perennial from the buckwheat family that tolerates damp soil and reaches about 1.2 metres. It can grow as a mat, a clump or a taller upright plant and has deciduous, lance- to heart-shaped leaves that often show marking or autumn tints. Flowering occurs from July to September on narrow spikes carried above the foliage. It suits a range of soil types, including heavy clay, and favours moist conditions over drought.

Red Bistort (Polygonum amplexicaule 'Atrosanguineum') is a moisture-loving perennial that flowers from July to September and suits borders and damp areas.

Polygonum amplexicaule 'Atrosanguineum'

Red Bistort | Polygonum amplexicaule 'Atrosanguineum'

Polygonum amplexicaule 'Firetail' is a perennial knotweed that can form mats, clumps or grow tall and upright to about 1.2 metres. Its deciduous leaves are simple, lance- to heart-shaped and are often marked or take on autumn tint. It prefers moist conditions and tolerates a very wide range of soils, including heavy clay, but dislikes drought. Flowering occurs July to September.

Polygonum amplexicaule 'Firetail' is a damp-tolerant perennial to 1.2 metres that flowers July to September and suits most soils, including heavy clay.

Polygonum amplexicaule 'Firetail'

Knotweed | Polygonum amplexicaule 'Firetail'

Polygonum amplexicaule 'Inverleith' is a perennial in the Polygonaceae that can grow as mat-forming or clump-forming plants or as a taller, upright specimen, reaching about 1.2 metres. Leaves are deciduous, simple and range from lance-shaped to heart-shaped, sometimes showing markings or autumn colour. It favours moist conditions and will tolerate heavy clay while disliking drought. Flowering occurs July to September.

Knotweed (Polygonum amplexicaule 'Inverleith') is a deciduous perennial to 1.2 metres that flowers July to September and suits borders and cottage gardens.

Polygonum amplexicaule 'Inverleith'

Knotweed | Polygonum amplexicaule 'Inverleith'

Polygonum amplexicaule oxyphyllum of gardens is a moisture-preferring perennial in the Polygonaceae that can grow as a mat, a clump or a tall, upright plant. Leaves are simple, lance-shaped to heart-shaped, deciduous and often marked or autumn-tinted. It reaches about 1.2 metres and carries summer flower spikes from July to September. Plants suit full sun to partial shade and tolerate almost any soil, including heavy clay, provided the site is not dry.

Knotweed (Polygonum amplexicaule oxyphyllum of gardens) is a moisture-loving perennial from China and the Himalayas that flowers July to September.

Polygonum amplexicaule oxyphyllum of gardens

Knotweed | Polygonum amplexicaule oxyphyllum of gardens

Polygonum bistorta 'Superbum' is a perennial in the family Polygonaceae that flowers from July to September. It reaches about 0.5 m and can form mats, clumps or produce taller, upright flowering stems. Leaves are deciduous, simple and range from lance-shaped to heart-shaped and may take autumn tints. It suits full sun to partial shade and tolerates almost any soil, including heavy clay, provided it is not allowed to dry out.

Knotweed (Polygonum bistorta 'Superbum') flowers July to September and suits moist soils in full sun to partial shade.

https://en.wikipedia.org/wiki/Bistorta_officinalis

Polygonum bistorta 'Superbum'

Knotweed | Polygonum bistorta 'Superbum'

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.

Knotweed (Polygonum campanulatum) is a deciduous perennial from China and the Himalaya that flowers July to September and tolerates damp, clay soils.

https://en.wikipedia.org/wiki/Koenigia_campanulata

Polygonum campanulatum

Knotweed | Polygonum campanulatum

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.

Knotweed (Polygonum carneum) flowers from July to September and suits moist, any-soil sites in full sun or partial shade.

Polygonum carneum

Knotweed | Polygonum carneum

Polygonum cuspidatum is a perennial in the Polygonaceae with a variable habit, from mat-forming or clump-forming to tall and upright growth. Leaves are deciduous, simple and range from lance-shaped to heart-shaped, often marked or taking on autumn tints. It tolerates almost any soil, including heavy clay, and favours moist conditions over drought. Flowering occurs July to September.

Japanese knotweed (Polygonum cuspidatum) is a perennial from East Asia that flowers July to September and tolerates most soils, including heavy clay.

https://en.wikipedia.org/wiki/Reynoutria_japonica

Polygonum cuspidatum

Japanese knotweed | Polygonum cuspidatum

Polygonum cuspidatum compactum of gardens is a perennial in the knotweed family with a variable habit that can form mats, clumps or grow more erect. It has deciduous, simple leaves that range from lance-shaped to heart-shaped and are often marked or take on autumn tints. Plants suit full sun to partial shade and prefer reliably moist soil, tolerating heavy clay and a wide pH range. Flowering occurs in the UK from July to September.

Knotweed (Polygonum cuspidatum compactum of gardens) is a deciduous perennial with lance- to heart-shaped leaves and a variable habit, flowering July to.

Polygonum cuspidatum compactum of gardens

Knotweed | Polygonum cuspidatum compactum of gardens

Polygonum equisetiforme is a perennial knotweed that can form mats, clumps or grow upright to about 1.0 m. 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. It suits a wide range of sites, including heavy clay and damp ground.

Knotweed (Polygonum equisetiforme) is a perennial from Portugal that reaches about 1.0 m and flowers July to September.

Polygonum equisetiforme

Knotweed | Polygonum equisetiforme

Polygonum milletii is a perennial member of the buckwheat family (Polygonaceae) that can grow as a mat-forming ground cover, clump-forming plant or as a taller upright perennial. The leaves are deciduous, simple and range from lance-shaped to heart-shaped, often showing markings or autumn tint. It flowers from July to September. It tolerates a wide range of soils, including heavy clay, and prefers moist conditions.

Polygonum milletii is a perennial knotweed that flowers July to September and suits moist soil in full sun to partial shade.

Polygonum milletii

Knotweed | Polygonum milletii

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.

Knotweed (Polygonum paniculatum) is a perennial that flowers from July to September and tolerates moist soil and heavy clay.

Polygonum paniculatum

Knotweed | Polygonum paniculatum

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.

Himalayan knotweed (Polygonum polystachyum) is a Himalayan perennial to about 1.8 m that prefers moist soil in full sun to partial shade.

https://en.wikipedia.org/wiki/Koenigia_polystachya

Polygonum polystachyum

Himalayan knotweed, Cultivated knotweed | Polygonum polystachyum

Polygonum regelianum is a perennial knotweed that shows a notably variable habit, from mat-forming and clump-forming to taller, upright growth. Its deciduous leaves are simple, lance-shaped to heart-shaped and are often marked or take on autumn tints before falling. Flowering occurs July to September. It tolerates a wide range of soils, including heavy clay, and prefers reliably moist conditions over drought.

Knotweed (Polygonum regelianum) is a perennial with variable habit that flowers July to September and suits moist soils in full sun to partial shade.

Polygonum regelianum

Knotweed | Polygonum regelianum

Polygonum rude is a perennial in the knotweed family with a variable habit that can be mat-forming, clump-forming or tall and upright. The foliage is deciduous, simple and ranges from lance-shaped to heart-shaped, often marked or taking on autumn tints. It flowers from July to September. Plants tolerate almost any soil, including heavy clay, and prefer moist conditions in full sun to partial shade.

Knotweed (Polygonum rude) is a deciduous perennial that flowers July to September and thrives in moist soils, tolerating heavy clay in sun or part shade.

Polygonum rude

Knotweed | Polygonum rude

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.

Knotweed (Polygonum sphaerostachyum) is a perennial that flowers July to September and tolerates heavy clay and damp soils.

https://en.wikipedia.org/wiki/Bistorta_macrophylla

Polygonum sphaerostachyum

Knotweed | Polygonum sphaerostachyum

Polygonum ussuriense is a perennial knotweed that can be mat-forming, clump-forming or grow tall and upright. Its deciduous leaves are simple, lance- to heart-shaped and are often marked or take on autumn tint. Flowering in the UK occurs from July to September. It tolerates full sun to partial shade and will grow in almost any soil, including heavy clay, provided the ground stays reasonably moist.

Knotweed (Polygonum ussuriense) is a deciduous perennial from northeast Asia that flowers July to September and tolerates heavy clay and moist ground.