Fallopia japonica is a broadleaved perennial shrub 3-4m tall. The stems are hollow with swollen nodes that give it a bamboo-like appearance.
It can be easily identified by its broad heart-shaped leaves and bamboo-like stems.
Fallopia japonica Classification
- Domain: Eukaryota
- Kingdom: Plantae
- Phylum: Spermatophyta
- Subphylum: Angiospermae
- Class: Dicotyledonae
- Order: Polygonales
- Family: Polygonaceae
- Genus: Fallopia
- Species: Fallopia japonica
- Varieties: Fallopia japonica var. japonica, Fallopia japonica var. compacta, Fallopia japonica var. variegata, and Fallopia japonica var. rubra.
- Common Names: Asiatic knotweed, Donkey rhubarb, Gypsy rhubarb, and Japanese bamboo.
Nativity and Distribution
Japanese knotweed is native to Japan, China, Korea, and Taiwan. Today it can also be found in:
- Europe
- North America
- Australia
- Part of South America
Physical Characteristics
- Leaves: Alternate, broadly ovate leaves with an entire margin.
- Fruits: Triangular achene, glossy, and dark brown in colour.
- Stems: Semi-woody. It can grow to 3-4m tall.
- Flowers: 3-5 mm wide small creamy white borne in dense clusters
- Roots: Fibrous root system.
Fallopia japonica is a perennial shrub that grows 3-4 tall. It is easily recognized by its broad heart-shaped leaves and stems with distinct raised nodes that give it a bamboo-like appearance.
Stems are hollow, mostly smooth with swollen reddish grooves and light green in colour. The leaves are alternate, broadly ovate with an entire margin.
New leaves are typically red, then turn green with red veins, and when mature, turn to flat, heart-shaped, truncated at the base about 5-8 wide and 5-12 cm long.
Knotgrass produces small creamy white flowers in dense clusters about 10 cm long on plume-like panicles. Flowering occurs between August and September.
The seeds are light brown and somewhat shiny, flattened, and oval in shape about 2.5mm long.
Reproduction, Dispersal, and Life Cycle
- Life Cycle: Perennial
- Seeds: Can produce between 50,000 to 150,000 seeds
- Climate: Temperate climate with moderate temperature and moisture levels
- Dispersal: Wind, water, and animals
Fallopia japonica behaves as a perennial weed and although it’s a prolific seed producer, the weed primarily relies on rhizomes for propagation.
The rhizomes grow horizontally beneath the soil surface, producing nodes where new shoots emerge to form dense colonies. In summer, the rhizomes produce vertical shoots reaching 3-4m in height.
The winged seeds are dispersed by wind and water — where flooding moves the plants or fragments to new areas, hooved animals, and human activities like flail mowing and using contaminated soil for farming activities. The viable seeds mainly help start new growth in recently disturbed areas.
Similar Plants
- Fallopia sachalinensis
- Polygonum polystachyum
- Bambusa vulgaris
- Impatiens glandulifera
Uses
- It is used as traditional medicine in Japan and China.
- The root has high levels of resveratrol used to formulate anti-cancer drugs and nutritional supplements.
- The shoots are used as animal fodder.
- The plant’s young shoots are a common source of food in Japan.
- It is also used as an ornamental and in soil stabilization, but these uses are highly discouraged due to its invasive nature.
Impact on Farms and Environment
F. japonica is listed among the most invasive plants globally, bringing with it significant negative effects on agricultural and natural environments. It is listed as a noxious weed in many states in the U.S. and on many invasive weed lists worldwide.
On farmlands, knotgrass competes for important resources including water and nutrients, reducing crop yield. The weed also creates dense thickets that smother plants, leading to huge economic losses. Due to its extensive growth, it adds more control and eradication effort expenses for the farmers.
Additionally, the Japanese knotweed root system exploits structural weaknesses to create bigger cracks and holes, causing damage to farmland property. This further increases damage remediation costs.
Structural damage can also lead to property devaluation, creating big economic losses for landowners. For instance, mortgage lenders in the United Kingdom have been reported to decline financing for properties infested by F. japonica.
Japanese knotweed can damage the environment by increasing the flood risk. During floods, whole plants or fragments often impede water flow and block storm drainages, intensifying flooding. Also, the dead plants can cause blockages on river banks, leading to streambank erosion.
Its early emergence, aggressive growth, and towering height often hinder the growth and regeneration of native species, negatively impacting biodiversity.
Riparian habitats infested by F. japonica have been shown, for instance, to have a lower abundance of native plant species. The displacement impacts local fauna dependent on native plants for food and habitat.
Control
- Cultural control: Grazing
- Mechanical control: Cutting and mowing
- Chemical control: Herbicides like Glyphosate, Imazapyr, 2,4-D amine, and Triclopyr
- Biological control: Aphalara itadori (Knotweed psyllid)
The best way to prevent a large F. japonica infestation is to remove the small colonies as soon as they are discovered. You can use these natural removal methods.
If prevention is no longer possible, repeated mowing or cutting throughout the growing season can help limit growth.
Consider herbicide treatments after cutting for more effective control. Stem injections, for instance, can help achieve significant weed dieback in smaller areas if correctly done in the first year.
Chemical control can provide better Japanese knotweed control but it depends on the area of infestation and the chemical control regiment laws in different countries.
Herbicides like Triclopyr and Imazapyr are applied on young plants in grasslands while 2,4-D amine can be applied in amenity areas, forests, and grasslands. Glyphosate and 2,4-D amine are allowed for use near water bodies.
When using any herbicide, read the label and follow the manufacturer’s instructions. Consult a local professional for detailed information on the best herbicide for your farm and property.
Aphalara itadori, the knotweed psyllid insect, has been approved as a biological agent in over five states including Oregon, North Carolina, and New York, and the National Park Service (NPS) for control of Japanese knotweed.
Lead Editor for Insight Weeds.