By late summer Japanese knotweed is usually at its tallest. The stems have reached their full height and the leaves form thick green stands that can dominate the area where the plant is growing. Around this time the plant begins producing flowers, which are often one of the most noticeable features during the later part of the growing season.
These flowers appear once the plant is well established for the year and can remain visible into early autumn.
The appearance of knotweed flowers
The flowers of Japanese knotweed are small and creamy white. Individually they are quite delicate, but they grow in large clusters that hang from the stems.
Common features include:
- small pale flowers arranged in sprays
- clusters growing along the upper stems
- a light cream or off-white colour
- flowers that appear above the surrounding leaves
When a large stand of knotweed is in bloom the flower clusters can create a soft white haze across the top of the plants.
What the plant looks like at this stage
By the time the flowers appear the plant usually stands well above surrounding vegetation. The hollow stems are strong and upright, while the leaves spread out along the zig-zag structure of the canes.
In gardens or along boundaries the plant may form dense groups where many stems grow close together. This thick growth can shade the ground beneath it.
Changes as autumn approaches
As the season moves toward autumn the leaves begin to lose their bright green colour. They may gradually turn yellow before falling away from the stems.
The flowers fade and the plant begins to dry out. The once green stems slowly turn brown and become more brittle.
Although the plant appears to be dying back above ground, the underground rhizomes remain in the soil.
Dry stems through the colder months
After the leaves have fallen, the hollow canes often remain standing. These dry brown stems can stay in place through much of the winter.
They may look like tall clusters of brittle sticks marking the spot where the plant grew earlier in the year.
When spring arrives the cycle begins again as new shoots emerge from the rhizomes below the surface.
Why the flowering stage is easy to spot
Japanese knotweed is often most visible during the flowering period. The height of the stems and the pale flower clusters make large stands of the plant easy to notice along garden edges, waste ground or riverbanks.
Photographs taken at this stage often show the plant clearly because the leaves, stems and flowers are all visible at the same time.
Looking at the plant across the whole season
Identifying Japanese knotweed usually involves looking at several features together. The spring shoots, the hollow zig-zag stems, the broad leaves and the late-season flowers all contribute to the overall appearance of the plant.
Seeing how these features develop across the year can help confirm whether knotweed is present in a particular location.