Understand Japanese knotweed better, Land owners and managers urged

Despite increasing legislative pressures, many land owners and managers across Ireland are failing to get on top of one of the country’s most damaging and invasive weeds through lack of understanding, warns land management specialist, John Geraghty.

Lecturing in invasive pest management at the Waterford Institute of Technology, Mr Geraghty stresses that too many people are not matching their treatment of Japanese knotweed to the weed’s biology as well as they need to be.  As a result, they are at serious risk of offending under legislation obliging them to take proper measures to prevent its spread.


 "Contractors, landscapers, developers and farmers really need to be aware of their obligations to contain the development and spread of Japanese knotweed under Sections 49 and 50 of the Birds & Habitats Directive, or face the possibility of legal proceedings,” he warned.

“Even though it hasn’t been known to set seed in Ireland, the weed is such a serious threat because a piece of stem or root tissue as small as 0.5 grams is able to develop into a new plant with a dense root system capable of causing major structural damage.  This makes it all too easy for infestations to be spread in contaminated soil, along river catchments, by road or railway verge trimming, through scrub clearance or even in mowing grass in the vicinity of knotweed colonies. And it is the main reason local authorities are so insistent that cutting is avoided at all costs/

 “In my experience, though,you can get on top of even substantial stands of Japanese knotweed within two or three seasons with sufficient determination and the right herbicide programme. So it really need not be as problematic as many have been finding.”

Glyphosate is John Geraghty’s herbicide of choice to contain Japanese knotweed. It enables a greater than 90% reduction in biomass to be achieved in a single year and complete control in two or three.  But he is adamant this is only possible with the right treatment programme. He advocates glyphosate because most infestations are close to or associated with culverts, ditches or water courses so require treatment with herbicides carrying aquatic approval. And he considers foliar spraying by far the best approach.

Even if adequately managed and controlled, he sees the alternative of hollow stem treatment falling foul of most local authorities’ insistence on avoiding any form of cutting. Like the stem injection approach favoured by some UK specialists too, he considers it largely impractical for anything other than the smallest knotweed colonies. This is mainly because established mature stands present too much of a physical barrier as well as containing many canes that won’t be sufficiently developed for injecting to work at any stage.

“I have no doubt that foliar spraying is most effective way to deal with Japanese knotweed approach,” he said. “But you have to get it right. 

“Not all glyphosate products have aquatic approval so it’s important you read the label and make the right choice here.  Treatment timing is equally critical.  Many operators fall down by following the long-standing general glyphosate approach of spraying when weeds are actively growing. So they might treat in May and June while the plants are still at a reasonable height, get a good above-ground kill and feel they’ve done the job.

“This isn’t surprising given the strong upward movement of sap in the plant at these early growth stages.  It does, however, mean too little glyphosate is carried down to the very substantial underground root structure that is the key to the weed’s survival.  This also has the unfortunate tendency of causing stunted, less active re-growth which is less responsive to subsequent glyphosate applications.

“Holding-off on spraying until early post-flowering in late summer, in contrast, takes advantage of the natural flow of sap back down into the rhizomes for winter storage to carry the glyphosate to where it really matters for long-term control. This is so much more effective.

“Canes can grow up to 3m high, so extending hand lances are likely to be needed here. This does mean extra time and effort but it’s well worth it for the degree of control you get,” he insisted.

Given the location of most Japanese knotweed infestations, weed management specialist, Danny Cooper of glyphosate authority, Monsanto considers it essential to use a modern formulation approved for aquatic and forestry applications and in areas open to the public and animals.

“Of the formulations available with these approvals, Roundup Biactive XL fits the bill best,” he suggested. “We always recommend it for the long-term control of difficult perennial weeds because its unique biactivator adjuvant system gets the most glyphosate into the plant with the greatest reliability. What’s more, it has a completely hazard-free label for the greatest operator, public, animal and environmental safety.

““I’d employ the 6 litre/ha rate in 200 litres water through standard nozzles When spraying near river banks and water courses and 10litre rate in forestry and nursery situations.
For knapsack sprayers this works out a 30ml/litre or a 1:33 dilution and 50ml/litre or 1:20 dilution respectively.  Control will not be achieved from a single spray. For the most effective action, you need to cover both sides of the leaves with spray droplets but not to run-off. Spraying should occur in late summer into early autumn but before the plants dieback for winter.

Stem injection is a great system but best focused on small patches or isolated stems amongst desirable vegetation or where overall spray application is not possible.

“Do the same again the following year and you will find little survival into Year 3,” he added. “To guard against any recurrence of the infestation you should, however, monitor the site for at least three years and be prepared to spot treat any survivors wherever they appear.”