How to identify Japanese knotweed?

For japanese knotweed identification, you need to know and be able to look for some key features. We describe Japanese knotweed throughout the seasons and other types of knotweed to be aware of. Also, to help, we’ve put together Japanese knotweed pictures showing knotweed at its various stages of growth. We hope the images help you identify if you have knotweed or not.

Japanese knotweed (Fallopia japonica) is the most widespread form of knotweed. Distinctive identifiers are a growth pattern of one stem per node, which forms a zigzag stem growth pattern. The leaves are fairly smooth, mid-green in color, with a characteristic straight back edge, giving the overall leaf a shield, shovel or heart type shape.

The knotweed flowers are small creamy-white and form in loose clusters (panicles) in late summer/early autumn. All Japanese knotweed plants growing in the UK are female and therefore do not produce viable seeds. Japanese knotweed synonyms are Reynoutria japonica and Polygonum cuspidatum. Japanese knotweed in spring.

The fastest Japanese knotweed growth is during the spring. New shoots that emerge are red/purple and can look like asparagus spears. The leaves are normally rolled up and dark green or red. In late spring, canes can reach up to 3 meters (10 feet) high. The pictures below show Japanese knotweed in spring.

Japanese knotweed in summer
During the summer the knotweed leaves are green and heart/shovel-shaped and can be 20cm across. In late summer early autumn small clusters of white flowers will appear. The stems are mostly hollow, and bamboo-like and the general growth habit has a distinctive zigzag appearance. The photos below show what Japanese knotweed typically looks like in summer.

Japanese knotweed in autumn
In autumn the dense covering of leaves will remain. However, they start to turn yellow and wilt as we move into September and October. The knotweed plants are still about 2-3 meters tall, and the hollow stems start to turn brown.

Japanese knotweed in winter
During late autumn and the beginning of winter the knotweed canes die off, and the weed becomes dormant. The leaves turn yellow, then brown and fall off. The canes are hollow, dark brown and brittle and they collapse upon one another. If the area hasn’t been treated, often previous year’s decomposition can be seen underneath.

Japanese knotweed stems
Japanese knotweed stems grow to 2-3 meters tall. They’re similar to bamboo with nodes, and purple speckles and the leaves shoot out from the nodes in a zigzag pattern. The inside of the stem is hollow. At the mature stage, the stems are hollow and not woody and can be snapped easily to show their hollowness. However, in the winter the stems become brittle.