Japanese knotweed has bamboo like stems and heart-shaped leaves that are around 14cm long. Its botanical name is Reynoutria japonica, also known as Fallopia japonica. This rapidly spreading weed was originally brought to Britain in the 19th century as an ornamental garden plant. It’s now classified as an invasive species. The Wildlife and Countryside Act 1981 states: ‘it is an offence to plant or cause this species to grow in the wild’.
Japanese knotweed is often confused with other plants. Here’s a few of the most commonly mistaken plants, plus our tips on how to tell the difference between them and Japanese knotweed.
Russian vine
This non-native vigorous climber can reach 10m in height, and spread 5-8m. Although it is an ornamental plant, grown for its white flowers, and it isn’t invasive, it does have the potential to become a problem due to its vigorous growing habit. Russian vine is native to Asia and a close relative of Japanese knotweed.
Similarities: it has similar shaped leaves and white flowers.
Differences: the leaves are slightly more arrow shaped and it doesn’t have tall stiff stems, as it’s a climbing vine.
Himalayan balsam (Impatiens glandulifera)
Himalayan balsam is a non-native, invasive annual that used to be grown as an ornamental plant in Britain. It’s now considered a weed and often found along waterways.
Similarities: Himalayan balsam grows to similar heights as Japanese knotweed, up to 2.5m. The stems are also hollow with red tints.
How to tell them apart: Unlike Japanese knotweed, the flowers of Himalayan balsam are pink and purple. Its leaves are long and thin, instead of heart shaped, and arranged opposite each other.
It is an offence to plant or otherwise cause to grow these species in the wild. You should stop them spreading from your garden and avoid purchasing or accepting gifts of these species.
Bindweed (Convulvulus arvensis)
Bindweed is a perennial weed but also a native wildflower. Its flowers are beneficial to wildlife but its twining stems can strangle other plants and it can quickly spread through a garden. It’s difficult to get rid of due to its ability to grow back from small pieces of root.
Similarities: large heart-shaped leaves that are arranged alternately on the stem. It can also grow back from tiny pieces of root.
Differences: bindweed climbs and does not have stiff stems. The flowers are funnel-shaped instead of held on long spires.
Houttuynia (Houttuynia cordata)
Houttuynia cordata is a perennial that’s native to countries in south-east Asia, including Japan, where it grows in shady, moist areas. It isn’t invasive but can get out of control in gardens as it spreads through underground rhizomes. Grow it in containers to restrict its growth.
Similarities: it has heart-shaped leaves and and white flowers.
Differences: the flowers are larger than those of Japanese knotweed, with yellow flowers spikes and white bracts that look like petals, but the main difference is in height. Houttuynia cordata only grows to 30cm, whereas Japanese knotweed reaches heights of 2.5m.
Himalayan honeysuckle (Leycesteria formosa)
Leycesteria formosa is native to parts of Asia including China and Tibet and although vigorous, isn’t considered invasive in the UK. It’s an easy to grow shrub that will thrive in a range of soils, in either sun or partial shade.
Similarities: similar looking, hollow stems that grow to 2m or more, and leaves that are alternately placed.
Differences: the leaves themselves are narrower than those of Japanese knotweed with pendulous purple and white flowers.
Lesser knotweed (Persicaria campanulata)
Persicaria campanulata, also known as Koenigia campanulata, is a perennial that’s native to the Himalaya region. It has pink flowers July to September and is a good plant for suppressing weeds in the border. It’s not invasive, but does spread through nodes on the plant’s stem, which put down roots when they touch the ground. It’s easy to pull out if it gets too big for its spot.
Similarities: it has similar stems and leaf arrangement.
Differences: the leaves are long and thin with visible veins and the flowers are larger than Japanese knotweed, and bell shaped.
Broad leafed dock (Rumex obtusifolius)
Broad leafed dock is from the same family as Japanese knotweed, but although it spreads easily, it does so by seed dispersal rather than rhizomes. It’s native to Britain and considered a problematic weed, but it is more of a problem on farm land than in gardens.
Similarities: in winter the dead stems look similar to the winter stems of Japanese knotweed. The plant has similar flower spikes, although broad leafed dock has green and red flowers.
Differences: the stems of broad leafed dock are not hollow and the leaves are larger, forming in rosettes near the ground as well as up the stem.
Other plants that can be mistaken for Japanese knotweed include:
- Horsetail (Equisetum arvense)
- Ground elder (Aegopodium podagraria)
- Giant fleece flower (Persicaria polymorpha)
- Redshank (Persicaria maculosa)
- Dogwood (Cornus sanguinea)
- Lilac (Syringa vulgaris)
