Bamboo-like, ten foot tall, frothy white flowers – this exotic couldn’t fail to appeal to Victorian gardeners. Oh dear … today that same plant, Japanese knotweed (Fallopia japonica), is the most invasive plant in Britain.
Only female plants were ever introduced so replication by seed isn’t a problem. It’s the almost indestructible roots that cause trouble. Unlike, Himalayan balsam, Japanese knotweed (JK) is long-lived and develops a vigorous root system that spreads some 20 feet in all directions and burrows 10 feet down. Huge enveloping stands develop. Bad news for native plants, they’re soon shaded out. Bad news for the multitude of dependent animals. Bad news even for the construction industry as JK roots can penetrate tarmac and concrete.
Pulling JK is hopeless. In fact it’s dangerous as any discarded plant fragment can start a new colony. Control usually means herbicide. Glyphosate regularly applied can eventually stop the invader but at a price. The cost of merely controlling the situation in the UK is put at £150 million annually.
River banks have proved great habitats for JK. When garden waste is dumped over fences, there’s flowing water at hand to spread plant fragments. Regrettably, the Quaggy hasn’t escaped invasion. To eradicate, herbicide treatment has to be carried out by authorised bodies such as the Environment Agency – it’s already dealt with a number of local JK colonies.
Thames21, the charity that takes on the cleaning up of the Thames and its tributaries, also has the necessary equipment and is licensed to treat the plant. On a recent River Quaggy clean up (10th October 2010), just upstream of Weigall Road, a substantial JK colony was encountered and given a first injection. Thames 21 have recently ear-marked the Quaggy from Lee to Sutcliffe Park as a river corridor for intensive clean up – great news for the local community.
The total cost of eradicating JK in the UK is currently put at a staggering £1.6 billion. But help could be at hand. JK isn’t a problem at home in East Asia – it’s kept in check by at least 20 species of insect and 6 species of fungi. On 9th March 2010, one species, Aphalara itadori, a jumping louse believed to predate only JK, was released at trial sites in the UK. First step – to confirm there’s no adverse effect on native wildlife. Then a general release. No quick fix though. Aphalara itadori doesn’t eat JK; it weakens it by sucking sap. But this assault on growth should reduce spread, and immensely help other methods of control.