All overEurope, along the great rivers but also along the roads, in the wastelands, in the countryside as in the city, in plain as in mountain, one meets more and more often a giant herb with broad leaves, covered with white flowers in autumn: the Japanese Knotweed ( Fallopia japonica ). Click here for japanese knotweed damage.
In 1847, the agriculture and horticulture company of the city of Utrecht, in the Netherlands, awarded it gold medal for the beauty of its foliage and its fragrant inflorescences. A century and a half later, the beautiful plant has become an invasive “odious and hateful” that we look everywhere as a plant pest to eradicate by all means. Visit this site for japanese knotweed damage.
A giant herb with large leaves and a huge rhizome
Japanese knotweed is a large grass up to 3 m tall. Its stems are hollow, erect, reddish, semi-woody with marked knots that make them look like bamboo stems (beekeepers in the northern United States make a honey called “bamboo honey”). The leaves are large (they reach 20 cm long), oval-triangular, truncated at the base, ending in a point (hence one of its species names: cuspidatum) and borne by a short red petiole. The stem is surrounded at the nodes by a brown membrane called ochréa characterizing the family of Polygonaceae.
The stems come from a huge rhizome that can reach 30cm in diameter. It weaves a dense network that colonizes underground space to the point of monopolizing water and nutrients. In summer, the Japanese Knotweed accumulates a considerable amount of reserves that will allow the stems to grow very fast the next spring (they can gain 4.6cm per day!).
Small autumn white flowers
The flowers appear in September-October, making this knotweed one of the few plants blooming at this time. They are grouped into erect pyramidal inflorescences, which are ears of cymes.