The best time to spot Japanese knotweed is during mid-summer and early autumn. During spring, reddish/purple shoots appear from the ground. These can grow up to 2cms a day, thus rapidly forming into dense stands of bamboo-like stems that develop green heart- or shield-shaped leaves.
By early summer the mature Japanese knotweed stems are hollow with purple speckles and can reach up to 3 metres in height. The leaf growth alternates on each side of the stem producing an obvious knotweed zigzag pattern.
The flowers that emerge by late summer are creamy-white in colour, and appear in lengthy cluster/spike formations. Japanese knotweed spreads mainly from its underground rhizomes/roots which lie dormant, but alive, over the winter months. Knotweed rhizomes (roots) can spread 7 metres outward and 3 metres deep from the visible, aboveground stems.
Japanese knotweed (Fallopia japonica) can also hybridise with its related species. The most common of these hybrids is that of Japanese knotweed and giant knotweed (Fallopia x bohemica). The hybrid knotweed then has the ability to spread by seed, which Japanese knotweed was lacking during its early introduction due to the absence of any male plants in the United Kingdom. This is a very important distinction because Japanese knotweed plants usually only reproduce from cut pieces of stem and rhizome, whereas hybrid knotweed plants can also reproduce from seed, which can remain dormant in the soil over successive growing seasons as a "seed bank". This can mean that in order to remove hybrid knotweed from a site:
Japanese knotweed also produces hybrid seed from the pollen of the closely related and common garden ornamental plant, Russian vine (Fallopia baldschuanica), which is also known as "mile-a-minute". Luckily, this seed rarely develops into viable plants, which is probably due to them being sensitive to the relatively mild and wet winters we experience in the UK.
Giant knotweed (Fallopia sachalinensis) is native to the island of Sakahlin which is just north of Japan but is actually part of Russia. As with Japanese knotweed it was brought to Europe to be grown in botanical gardens. It has also escaped into the wild where it is spreading. It is similar to Japanese knotweed in many respects but is larger, growing over 4m high and having leaves around 20-40cm long. Giant knotweed also has flowers that are greener in colour and leaves that are more rounded at the base than Japanese knotweed. The leaves also have scattered hairs on the undersides.
The hybrid plant (Fallopia x bohemica) grows slightly larger than Japanese knotweed and has slightly larger leaves but is smaller than giant knotweed. The leaves also have a pointed tip and a slightly rounded base, which makes the leaves appear more heart-shaped than its Japanese knotweed parent. Other hybrids from various back crosses with Japanese knotweed and hybrid knotweed generally appear similar to Fallopia x bohemica.
The table below details some of the more obvious leaf features that can be used as a rough guide to identify whether a plant is hybrid knotweed or Japanese knotweed.
|Japanese knotweed leaf||Hybrid knotweed leaf|
Other forms of knotweed that are found in some areas of the UK include a dwarf variety (Fallopia japonica var. ‘compacta’ and Himalayan knotweed (Polygonum polystachyum). These plants can be differentiated from Japanese knotweed by dwarf knotweed only growing to approximately 1m in height and Himalayan knotweed having long, slender leaves and can grow up to 2m in height.
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