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Fallopia × bohemica (Bohemian giant knotweed)
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Perennials

Fallopia xbohemica

Bohemian giant knotweed

Polygonaceae
Hybrid origin; parents from East Asia (Japan, Sakhalin Island)

At a Glance

TypePerennial
HabitUpright
FoliageDeciduous
Height60-156 inches (150-400 cm)
Width72-240 inches (180-600 cm)
Maturity3 years

Growing Zones

USDA Hardiness Zones

4 - 9
Zone 4Zone 5Zone 6Zone 7Zone 8Zone 9
These zones indicate the coldest temperatures this plant can typically survive.
What's my zone? →
Frost Tolerancehardy

Key Features

Attracts PollinatorsDeer ResistantDrought Tolerant
Maintenancehigh

Overview

Fallopia x bohemica (syn. Reynoutria x bohemica) is the hybrid between F. japonica (Japanese knotweed) and F. sachalinensis (giant knotweed), combining traits of both parents into a vigorous, rhizomatous, herbaceous perennial reaching 5–13 feet (1.5–4 m) tall with colonies spreading 6–20 feet (1.8–6 m) or more. Stems are erect, hollow, bamboo-like, with swollen nodes. Leaves are intermediate between the parents: broadly ovate, 5–10 inches (13–25 cm) long and 3–8 inches (8–20 cm) wide, with a slightly cordate to truncate base. Leaf undersides have scattered short hairs on the veins, less dense than F. sachalinensis but present (unlike the glabrous undersides of F. japonica). Flowers are small, white to greenish-white, in axillary panicles. The hybrid frequently exhibits greater vegetative vigor than either parent, with faster rhizome extension rates. Both male and female flowers may be produced, and backcrossing with either parent species generates genetically diverse offspring. Rhizome biology mirrors F. japonica: lateral extension 15–65 feet (4.5–20 m) at depths to 10 feet (3 m), with fragments as small as 0.5 inch (1 cm) regenerating. Listed as a noxious weed in Washington State and other jurisdictions. Increasingly recognized as the most common knotweed taxon in many invaded areas, as many populations previously identified as F. japonica are actually the hybrid.

Native Range

Hybrid origin; does not occur naturally in the native ranges of the parent species. Arises where F. japonica and F. sachalinensis co-occur in North America, Europe, and elsewhere. Naturalized in riparian corridors, roadsides, disturbed urban sites, and construction waste areas throughout the Pacific Northwest, northeastern United States, and northern Europe.

Suggested Uses

Used in invasive species identification training alongside the parent species for teaching the knotweed species complex. Studied in hybridization genetics, invasion biology, and the role of polyploidy in invasive vigor. The increasing recognition that many knotweed infestations are hybrid rather than pure species is an important lesson in applied taxonomy for land managers.

How to Identify

Intermediate between the parent species. Leaves 5–10 inches (13–25 cm) long, larger than F. japonica (3–6 inches / 8–15 cm) but smaller than F. sachalinensis (8–16 inches / 20–40 cm). Leaf base is slightly cordate to truncate, intermediate between the deeply cordate base of F. sachalinensis and the flat truncate base of F. japonica. Leaf undersides have scattered short hairs on the veins (absent in F. japonica, more abundant in F. sachalinensis). Definitive identification requires genetic testing (flow cytometry or molecular markers), as morphological overlap between the hybrid and its parents is substantial.

Appearance

Size & Dimensions

Height5' - 13'
Width/Spread6' - 20'

Reaches mature size in approximately 3 years

Colors

Flower Colors

white
green

Foliage Colors

green

Fall Foliage Colors

yellow

Bloom Information

Bloom Period

~3 weeks
J
F
M
A
M
J
J
A
S
O
N
D
SummerFall
Flowers from August through September. Panicles are intermediate in size and density between the parents. Unlike F. japonica populations in North America (predominantly female), hybrid populations may produce both male and female flowers, enabling seed production and backcrossing with parent species. Pollinated by bees and flies.

Detailed Descriptions

Flower Description

White to greenish-white

Foliage Description

Dark green, broadly ovate, intermediate in size between parents; base slightly cordate to truncate; scattered short hairs on veins beneath

Growing Conditions

Sun Requirements

Full SunPartial Shade
Requires 4-12 hours of direct sunlight daily
• Full Sun: 6+ hours of direct sunlight
• Partial Shade: 3-6 hours of direct sunlight
• Full Shade: Less than 3 hours of direct sunlight

Soil Requirements

pH Range4.5 - 8.5(Neutral)
357912
Soil Types
loamclaysandsiltrocky
Drainageaverage

Water & Climate

Water Needs

Medium

Frost Tolerance

hardy

Time to Maturity

2-3 years

Drought Tolerance

Drought tolerant when established

Care & Maintenance

Care Guide

Management follows the same protocols as F. japonica and F. sachalinensis; the hybrid may exhibit greater vigor and faster recovery from cutting. Repeated cutting every 2–3 weeks throughout the growing season for 5–10 years is required. Complete excavation of rhizome-containing soil to 10 feet (3 m) depth is the only reliable non-chemical eradication method. All cut stem and rhizome material must be disposed of in landfill; do not compost. The ability to produce viable seed through backcrossing with parent species adds a sexual reproduction pathway absent in most F. japonica populations. Where all three knotweed taxa occur, management should target all plants to prevent continued hybridization and genetic diversification.

Pruning

No pruning applicable. Stems are cut at ground level every 2–3 weeks throughout the growing season. All cut material must be removed and disposed of in landfill or desiccated on impervious surfaces. The intermediate vigor and potential for seed production make management slightly more complex than for pure F. japonica populations.

Maintenance Level

high

⚠️ Toxicity Warning

Non-toxic