Reynoutria japonica

RHS Plant Profile

Japanese knotweed

Other common names

Japanese knotweed

Synonyms

Polygonum reynoutria
Fallopia japonica
Polygonum cuspidatum

Herbaceous Perennial

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

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

Japanese knotweed can grow in a wide range of situations spreading rapidly, even from small fragments, through vigorous, fast-growing rhizomes. 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 it to grow in the wild IN ENGLAND, WALES, NORTHERN IRELAND and the REPUBLIC OF IRELAND. Gardeners possessing it should undertake measures to control it. See Japanese knotweed, and RHS advice on invasive non-native species, for further information

Propagation

Please see cultivation notes

Pests

May be susceptible to Aphalara itadori psyllids but generally pest free

Diseases

May be susceptible to leafspot but generally disease-free