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Fallopia japonica

Image of Fallopia japonica

James Burghardt

Family

Polygonaceae

Botanical Name

Fallopia japonica

Plant Common Name

Japanese Knotweed

General Description

Japanese knotweed is an immense herbaceous perennial with attractive foliage and flowers but a rampantly invasive habit. Native to East Asia and naturalized worldwide, it is listed as a noxious weed in many areas including the eastern United States. It is known by several other botanical names including Reynoutria japonica, Polygonum cuspidatum, and Polygonum japonicum.

The vigorous underground rhizomes of this "super-weed" spread rapidly to form dense nearly ineradicable colonies. Thick, purple- or red-tinged, asparagus-like shoots break ground in spring, rapidly elongating into imposing stems with conspicuously swollen leaf nodes. The nodes are sheathed by colorful papery stipules. Lush green, heart-shaped to oval leaves with pointed tips alternate along the stems. In summer, frothy clusters of creamy- or greenish-white flowers appear at the stem nodes. The small woody dark-brown seeds ripen within the papery, persistent remains of the spent flowers. The leaves turn shades of purple and yellow before dropping in fall.

Japanese knotweed thrives in a wide range of soils and exposures. Rambunctious in full sun, it tolerates all but the densest shade. Several cultivars (including 'Crimson Beauty', 'Milk Boy' and 'Variegata') have been selected for their showy leaves and flowers and their relatively restrained habit. Cultivar 'Compacta' is low-growing, pink-flowered, and a vigorous spreader. Planting of this species is banned in many regions including England and much of the United States.

Characteristics

  • AHS Heat Zone

    9 - 1

  • USDA Hardiness Zone

    4 - 9

  • Sunset Zone

    A1, A2, A3, 1a, 1b, 2a, 2b, 3a, 3b, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24

  • Plant Type

    Perennial

  • Sun Exposure

    Full Sun, Partial Sun, Partial Shade

  • Height

    4'-8' / 1.2m - 2.4m

  • Bloom Time

    Summer, Late Summer, Early Fall

  • Native To

    Eastern Asia, Japan

Growing Conditions

  • Soil pH

    Acidic, Neutral, Alkaline

  • Soil Drainage

    Average

  • Soil type

    Clay, Loam, Sand

  • Tolerances

    Pollution, Drought, Salt, Soil Compaction

  • Growth Rate

    Very Fast

  • Water Requirements

    Drought Tolerant, Average Water

  • Habit

    Thicket/Colonizing

  • Seasonal Interest

    Summer, Fall

Ornamental Features

  • Flower Interest

    Showy

  • Flower Color

    White, Pink, Light Green, Ivory

  • Fruit Color

    Tan, Brown

  • Foliage Color (Spring)

    Light Green

  • Foliage Color (Summer)

    Light Green

  • Foliage Color (Fall)

    Red, Light Yellow, Burgundy

  • Fragrant Flowers

    No

  • Fragrant Fruit

    No

  • Fragrant Foliage

    No

  • Bark or Stem Fragrant

    No

  • Repeat Bloomer

    No

  • Showy Fruit

    No

  • Edible Fruit

    No

  • Showy Foliage

    No

  • Foliage Texture

    Medium

  • Foliage Sheen

    Matte

  • Evergreen

    No

  • Showy Bark

    No

Special Characteristics

  • Usage

    Cutflower, Groundcover, Mixed Border, Screening / Wind Break

  • Sharp or Has Thorns

    No

  • Invasive

    Yes

  • Attracts

    Birds

  • Self-Sowing

    Yes