This is part one of a two-part article. Read the second part here: 'Mechanical methods to contain and eradicate Japanese knotweed'.
Japanese knotweed is an invasive, non-native plant with a remarkable capacity to spread rapidly and a tenacious resistance to control. Introduced into Ireland in the late 1800s, it is causing major problems for engineers across a wide range of disciplines, particularly for those involved in the construction sector and State networks (road, rail and waterways)
An invasive, non-native species (INNS) is any introduced animal or plant that has the ability to spread causing damage to the environment, the economy, our health and the way we live. In Europe, approximately ten new species become established each year. In 2010, it was estimated that the total annual cost of Japanese knotweed to the British economy runs at £166 million; the total loss to the world economy as a result of INNS has been estimated at 5% of annual production. Globally, INNS have contributed to 40% of the animal extinctions that have occurred in the last 400 years and there is evidence that invasive species are a more significant cause of biodiversity loss than climate change .
In 2011, EU legislation was introduced in an attempt to curb the spread of the most damaging INNS and the legislation was enacted in Ireland in 2015. Of the plant invasives, Japanese knotweed and its relatives are perhaps the most serious . Regulation 49, EC Birds and Habitats Directive, SI 477, 2011 states that it is not against the law to own land with Japanese knotweed present (or other regulated INNS) but it is an offence to knowingly allow their dispersal or escape .
Invasive species’ capacity for dispersal is the main reason for their success and preventing dispersal is a major challenge in eradication programmes. Japanese knotweed is an opportunist: it can adapt to a wide range of environmental conditions and it has evolved a brilliant mechanism of dispersal. The rhizomes and shoots are brittle and break easily, and even tiny fragments (700mg) have the capacity to regenerate into new plants.
Knotweed’s capacity to grow from fragments has important implications for control, with movement of knotweed-infested soil being one of the main reasons for its rapid spread across Ireland. Indeed, a number-one rule when faced with an infestation is: do not disturb it until a plan of action is devised and implemented. Ignoring this rule may lead to infestations that are impossible to eradicate.
The consequences of not understanding Japanese knotweed’s growth habit can be illustrated by a site in a residential area of Dublin. A small knotweed infestation was identified in the corner of a garden. The infestation was treated with herbicide, but it was assumed in error that the plant would be killed immediately. Ground levelling proceeded, which included the movement of soil from the knotweed-infested area across the garden. The result was massive re-growth with thousands of new plants densely infesting the entire area. Remediation was challenging and very expensive.