Reynoutria japonica

RHS Plant Profile
Japanese knotweed

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Japanese knotweed visionspictures.com
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Other common names

giant knotweed

Synonyms

Fallopia japonica, Polygonum cuspidatum, Polygonum reynoutria

Herbaceous Perennial

This plant is listed as an invasive non-native species in the UK – see Cultivation section for details 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

Position

Full sun, Partial shade

Soil Types

Chalk, Clay, Loam, Sand

Max Height

1.5-2.5 metres

Max Spread

2.5-4 metres

Size

Time to Maturity
1 year
Max Spread
2.5-4 metres
Max Height
1.5-2.5 metres

Growing Conditions

Chalk Clay Loam Sand
Moisture
Moist but well–drained or Well–drained or Poorly–drained
pH
Acid or Alkaline or Neutral

Position

Full sun Partial shade
Aspect
East–facing or North–facing or South–facing or West–facing
Exposure
Exposed or Sheltered
Hardiness

Hardiness Ratings

All ratings refer to the UK growing conditions unless otherwise stated. Minimum temperature ranges (in degrees C) are shown in brackets

  • H1A: under glass all year (>15C)
  • H1B: can be grown outside in the summer (10 - 15)
  • H1C: can be grown outside in the summer (5 - 10)
  • H2: tolerant of low temperatures, but not surviving being frozen (1 to 5)
  • H3: hardy in coastal and relatively mild parts of the UK (-5 to 1)
  • H4: hardy through most of the UK (-10 to -5)
  • H5: hardy in most places throughout the UK even in severe winters (-15 to -10)
  • H6: hardy in all of UK and northern Europe (-20 to -15)
  • H7: hardy in the severest European continental climates (< -20)
H7

Colour & Scent

Season Stem Flower Foliage Fruit
Spring
Summer
Autumn
Winter

Botanical Details

Family
Polygonaceae
Native to GB/Ireland
No
Foliage
Deciduous
Habit
Columnar upright
Genus
Name Status
Correct

How to Grow

Cultivation

This plant is listed on Schedule 9 of the Wildlife & Countryside Act (1981), the Wildlife (Northern Ireland) Order 1985 and the EU list of Invasive Alien Species of Union Concern (applies in NI). ?It?should?not be planted or caused to grow in the wild?in?the?UK.?It is also banned from sale, and gardeners who already have it should take steps to control it. See RHS advice on Japanese knotweed and invasive non-native species for more information

Propagation

See cultivation notes

Pests

See cultivation notes

Diseases

See cultivation notes