The Guardian, in un long read di Samanth Subramanian, narra la vicenda della diffusione di una specie vegetale alloctona, pericolosamente infestante, il poligono del Giappone (Reynoutria japonica Houtt.). Un problema di grande rilievo in Gran Bretagna, un vero incubo per i giardinieri che fa crollare anche il prezzo delle proprietà i cui giardini sono infestati da questa pianta.
Knotweed has been a British plant since the 19th century, but as a species, it is still called “invasive” – a word referring not only to its origins abroad, in Japan, but also to how it has run riot through the country. Through many countries, in fact: across much of Europe and North America, knotweed has conquered woods and pasture, asphalt and urban wasteland. Unchecked, knotweed grows audaciously: it can rise by eight feet in the course of a single summer month. One biologist told me that she regularly came across doubledecker-sized colonies of knotweed during her fieldwork in northern France. Another expert, in Wales, recalled the biggest stand of knotweed he’d ever seen: a monster that straggled over 20,000 sq metres – nearly enough room to park four Boeing 747s. Some of the sites chosen for the 2012 Olympics had so much knotweed that it cost £70m to clean them up.
Il poligono del Giappone ha la sua arma segreta: il rizoma che si trova sotto terra, che può rimanere dormiente per decenni, è il vero strumento grazie al quale si diffonde con tanta facilità.
Other hardy invasives in Britain, such as the rhododendron or the Himalayan balsam, spread through seed. Their propagative apparatus – the seed pods – are plain to see, easier to assail. Knotweed produces seeds as well, but its singular weapon lies out of sight. Beneath the earth is its rhizome, a network of stems that grow laterally, sending up new shoots wherever they can. Bowes described the rhizome to me as the plant’s “battery”, a dense store of energy. Even when it has been shorn of its stalks, or suppressed by weedkiller, or trapped below concrete, a rhizome can stay dormant for as long as 20 years, waiting for better days. This is why knotweed prospers – and why it preys on the mind. There is always the fear that it is merely lying in wait, ready to burst forth again. Disturbing the rhizome in even the mildest way provokes it to grow; shredding it is like lopping off the head of a hydra. Even a thumbnail-size fragment of it, resembling raw, orange-coloured ginger, can generate a whole new plant.
Il 9 agosto 1850 è registrato negli archivi a Kew Gardens l’arrivo da Leida di un lotto di 40 piante, all’interno del quale era presente il poligono del Giappone che arrivò così in Gran Bretagna.
In the archives at Kew Gardens, there are volumes called Inwards Books: old, fat, marvellous ledgers listing the plants received by the gardens over the centuries. One of these books records the arrival, on 9 August 1850, of a set of 40 plants from a nursery in Leiden. The nursery was run by a German doctor named Philipp Franz von Siebold, who, after a stint in Japan, had brought back sheaves of exotic plants – among them knotweed, which, in an act of self-advertising, he’d named Polygonum sieboldii. In that 9 August entry, written out in spidery cursive, P. sieboldiishowsup as item 34. It is the earliest available record of knotweed’s arrival on British shores.
Persino William Robinson (1838–1935), famoso giardiniere inglese padre delle teorie sul «giardino naturale» che cambiarono la storia del giardino, raccomandava l’uso di questa pianta, riconosciuta come dannosa solo dopo il 1921.
From the outset, Britain’s relationship with knotweed has been cast by commerce – or, more specifically, by the country’s successive modes of commerce: the colonial, the industrial and the post-industrial. Colonialism’s grasping hands stretched into the botanical world, and the same acquisitive spirit that brought knotweed to Britain also brought rubber and cinchona trees to Kew, wisteria to the English garden, and rhododendrons to Scottish forests. At the time, no one had a sense of foreign plants as invasives, as agents that could overwhelm the balance of domestic ecosystems, says Keith Alcorn, a historian of gardens. Through the second half of the 19th century, knotweed vendors praised the plant’s ability to stabilise sand dunes, bear flowers suited for bouquets and feed cattle. And when, in this same period, the wild gardening movement began, its founder, William Robinson, recommended knotweed for looking “handsome in rough places in the wild garden”. Alcorn told me: “I haven’t come across any writing from before 1921 that said: ‘On no account allow this plant.’”
In Giappone il poligono viene chiamato itadori, è molto diffuso, ma ha anche qualche nemico naturale in più:
In Japan, knotweed has an altogether more poetic name: itadori. One translation of the word is “tiger cane”, perhaps referring to the way a stand of tall dead canes resembles a tiger’s striped back. Its native ecosystem imposes more limits on knotweed. Silvergrass and bamboo vie strongly with it for water and soil nutrients. A species of psyllid, an insect not much bigger than the head of a nail, feeds exclusively on knotweed sap, weakening and killing the plant. Even so, itadori grows luxuriantly, ever ready to annex neglected land.
L’invasività di questa pianta non è un problema solo inglese, ovviamente. Si sta diffondendo sempre più anche sugli argini dei fiumi italiani, indicata come pericolosa anche da noi. Ne parlava la Rivista di Agraria nel 2019, ma è segnalata la sua presenza in molte zone del nostro paese.
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