Japanese knotweed (Reynoutria japonica) is a notoriously problematic invasive weed that can devastate gardens and commercial properties. It grows at a staggering rate, spreading rapidly and quickly forming dense thickets. It thrives in riverbanks, roadsides and moist areas. As such, it can be extremely difficult to remove and is likely to return once removed. For this reason, it is important to be able to identify knotweed early on to ensure that the correct treatment method can be used.
Knotweed roots are known as rhizomes identification japanese knotweed roots and they are one of the most distinctive features of this species of plant. The rhizomes are hollow and resemble bamboo stems, with the potential to grow up to 3 metres in height. In the spring, shoots rise from a network of underground rhizomes that have been in place since late winter. The resulting shoots are green with purple speckles and are a similar colour to asparagus shoots. This makes them an easy identification point for gardeners.
In the summer, Japanese knotweed flowers. These elongated clusters of creamy white flowers are another key feature that help to distinguish it from other plants. These flowers can appear as early as August, but most commonly in September and October. The elongated flowering spikes can be up to 15cm long and they produce sterile seeds that can continue the growth of the knotweed.
As the year progresses, the leaves on the Japanese knotweed begin to turn yellow and then brown. This is a good indicator that the plant is preparing for its annual autumnal regrowth. If a gardener notices any of these signs, they should take action immediately to prevent the spread of this invasive weed.
While having an invasive weed on your own property may not break the law, if it spreads to neighbouring land or public spaces then it is considered illegal. This is why it is so important to know how to identify Japanese knotweed, especially when it is dormant over the winter months.
When the weather starts to warm up in the spring, Japanese knotweed shoots are an easy and visible way to spot this invasive species. They emerge from the ground in large numbers, resembling bamboo and appearing to be like asparagus spears. The leaves on these shoots are rolled up in the early stages and then they begin to unroll as the stems grow.
It is important to remember that despite this invasive weed’s bamboo-like appearance, it is actually a herbaceous perennial and not a woody shrub. As a result, it can be misidentified by gardeners who are more familiar with other types of herbaceous plants such as lilac, azaleas and poplars. However, the distinguishing feature is that Japanese knotweed leaves are shovel-shaped with a point at the tip and are staggered on the stem (one leaf per node) which creates a characteristic zig-zag pattern of growth. If you are unsure of what you’re seeing, you can use Cornell University’s Turf and Landscape weed identification app to help you identify it.