Japanese knotweed also produces hybrid seed from the pollen of the closely related and common garden ornamental plant, Russian vine, which is also known as ‘mile-a-minute’ and Bukhara fleece flower. Plants could be reintroduced back onto the site after treatment from windblown seed from hybrids in the surrounding area.Pre-emergent herbicides might need to be incorporated into the treatment programme and/or.Herbicide treatments might be required over longer periods.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, potentially, reproduce from seed, which can remain dormant in the soil over successive growing seasons as a ‘seed bank’.Īlthough Japanese knotweed hybrids more commonly reproduce vegetatively from rhizome fragments, the potential risk of viable seed being present from hybrids can mean that in order to remove hybrid knotweed from a site: 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. The most common of these hybrids is that of Japanese knotweed and giant knotweed. Japanese knotweed can also hybridise with its related species. What do Japanese Knotweed Leaves Look Like? You can click on each thumbnail image to make it bigger. As new growth from seeds is very rare, it is testament to the plant’s incredible invasiveness that it has spread to most parts of the UK (and many parts of western Europe and North America, for that matter) simply through the fragmentation and translocation of rhizomes in contaminated soil.īelow are some images and descriptions of the main features of Japanese knotweed that should help you identify it. It is therefore very easy to accidentally fragment pieces of rhizome and spread them by disturbing the soil several metres from where the stems appear. The rhizomes can spread several metres outwards from the visible, aboveground stems, and to depths of more than a metre. Japanese knotweed spreads mainly from its underground rhizomes/roots which lie dormant, but alive, over the winter months. The knotweed flowers that emerge by late summer are creamy-white in colour and appear in lengthy cluster/spike formations. The leaves alternate along each side of the stem, producing an obvious knotweed zigzag pattern. These can grow up to 2cms a day, thus rapidly forming dense stands of bamboo-like stems that develop green heart- or shield-shaped leaves.īy early summer the mature Japanese knotweed stems are hollow with purple speckles and can reach up to 3 metres in height.
During spring, reddish/purple shoots appear from the ground and fat, asparagus-like ‘spears’ rapidly lengthen from bright pink ‘crown’ buds. The best time to spot Japanese knotweed is during mid-summer and early autumn. When is the Best Time to Spot Japanese Knotweed?
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