Japanese knotweed, the weed that grows along the Arno

One of one hundred most invasive species of the world has begun to colonize the Arno. The Japanese polygon it is a weed that grows in many Italian regions monopolizing the banks of waterways and accelerating their erosion. In Tuscany it first targeted the minor tributaries and has also landed for some time on the banks of the Arno. The colonies of this plant are still isolated nucleuses but are now present along about fifty kilometers of the river divided between theUpper Valdarno and the from Pistoia as specified by Anbi (National Association of Consortia for the Management and Protection of the Territory and Irrigated Waters).

The Japanese knotweed is not a gregarious species. It forms bushes so thick as to prevent the native flora from developing, impoverishing the biodiversity of these environments. But that’s not all: to eliminate any possible competitor releases a toxic substance into the soil similar to a herbicide.

This plant that loves scorched earth around itself is an old acquaintance. Originally from East Asia, it was introduced into Dutch nurseries in the 19th century for the European garden market and as forage grass. Japanese knotweed has strong reproductive abilities and breaking out of the walls of captivity is child’s play. A little over fifty years after the first cultivations, the knotweed has already become naturalized in half of Europe. It adapts easily and grows at a rate of one foot a week also because it has found the road paved. Here it has no antagonists while in the areas of origin there are about forty insects and fungi that limit its uncontrolled expansion.

In the wild Japanese knotweed (Reynoutria japonica) is a perennial and dioecious plant, that is divided into males and females like animals. But in our latitudes there are almost only clones of female specimens or hybrids between different species. Despite this demographic conundrum, the knotweed can reproduce even in the absence of the male counterpart. How? By dispersing fragments of a few grams of the rhizome in the environment, the underground part of the stem which in this species can reach a depth of four metres. By altering the balance of river ecosystems, the plant also weakens their hydraulic stability. In winter, during the vegetative rest phase, the knotweed dries up, depriving the banks of the stability ensured by rhizomes and roots. The banks, in other words, remain short of natural protections against the phenomenon of erosion.

Japanese knotweed is included in both the IUCN (Union of Conservation of Nature) list of the 100 worst non-native species and the European and Mediterranean Organization for the Protection of Plants (EPO) list of alien plants. In the United Kingdomas well as in the United Statesplanting it in the garden to make the landscape wild is a crime. Despite the bad reputation this plant has not yet been included in the community invasive species which include water hyacinth and nutria.

As often happens, invasive plants also have some benefits. As in the case of buddleia (Buddleja Davidi), another weed species of Asian origin that has colonized the banks of many Italian rivers from gardens, also the flowers of the Japanese knotweed are a magnet for pollinating insects and in the past their virtues were celebrated for the production of honey . The river consortia. To limit the invasion of this plant, there are specific interventions that should be performed under the supervision of an expert. But the simplest and most effective strategy remains not to grow it in the garden.