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Herbaceous Perennial

Reynoutria japonica

giant knotweed

A tall, vigorous herbaceous perennial to 2.1m high, with cane-like stems flecked with purple, and shovel-shaped leaves borne alternately (in a zig-zag pattern) along the stems. Produces tassels of white flowers in late summer and early autumn. This plant is considered invasive, and legal requirements apply to its control.

Other common names
Japanese knotweed
Synonyms
Polygonum reynoutria
Fallopia japonica
see morePolygonum cuspidatum
Size
Ultimate height
1.5–2.5 metres
Time to ultimate height
1 year
Ultimate spread
1.5–2.5 metres
Growing conditions
Chalk
Clay
Loam
Sand
Moisture
Moist but well–drained, Well–drained, Poorly–drained
pH
Acid, Alkaline, Neutral
Colour & scent
StemFlowerFoliageFruit
Spring Green
Summer White Cream Green
Autumn White Cream Green
Winter
Position
  • Full sun
  • Partial shade
Aspect

East–facing or North–facing or South–facing or West–facing

Exposure
Exposed or Sheltered
Hardiness
H7
Botanical details
Family
Polygonaceae
Native to the UK
No
Foliage
Deciduous
Habit
Columnar upright
Name status

Correct

How to grow

Cultivation

This plant is listed on Schedule 9 of the UK Wildlife & Countryside Act as an invasive non-native species. Although not banned from sale, it is an offence to plant or cause these to grow in the wild IN ENGLAND, WALES, NORTHERN IRELAND and the REPUBLIC OF IRELAND. Gardeners possessing them should undertake measures to control them. See RHS advice on invasive non-native species for further information

Propagation

Please see cultivation notes

Suggested planting locations and garden types
Pruning

Cutting and disposal should be handled by specialist contractors; plant material should not be moved off-site or disposed of with garden waste

Pests

May be susceptible to Aphalara itadori psyllids but generally pest free

Diseases

May be susceptible to leafspot but generally disease-free

Get involved

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