Tied up in Knotweed

Peter Dowdall amongst the Japanese Knotweed. This scourge of a plant is on the Global Invasive Species Programme's list of 100 worst invasive species. It has vigorously invaded natural habitats, hedgerows and waterways all over Ireland.

Peter Dowdall on an EU-wide plan to tackle invasive plants which are becoming a major nuisance here too.

There are aliens among us. A statement more often associated with bad Hollywood movies but I’m talking about horticultural horror stories and not the type with three legs and eyes out on stalks.

There are currently 59 plants listed on the National Biodiversity Centre’s website as invasive and care should be given when choosing any plant for the garden, but particularly if it is a new species or one that you don’t know. Try and find out as much information on its eventual height and spread as you can. You could be introducing a problem plant.

For example Impatiens walleriana are a welcome addition to the garden every summer. Better known as Busy Lizzies, they grow as an annual summer bedding plant offering great colours. So if I suggest incorporating Impatiens glandulifera into your garden, and I show you a close up picture of its pretty pink flowers, you may indeed think about planting it.

If I went on to explain that it was a fully hardy perennial plant that would withstand even the coldest of winters and would come up from ground level each year and produce pink flowers in abundance each summer, you may have already decided that this needs to get a position in your herbaceous border.

However, you would have just welcomed one of the Invasive Species of Ireland (ISI) ‘Most Unwanted’ plants into your garden. Referred to as Himalayan Balsam, this absolute thug of a plant will spread in front of your eyes pushing your more desirable plants out of its way as it continues its colonization.

Biodiversityireland.ie the website of The National Biodiversity Data Centre is an extremely useful resource. In particular, look out for plants listed as ‘invasive’ as make sure they are never, ever allowed to even see your garden, let alone be made to feel welcome.

Not every plant listed is a huge problem but it is good advice to go and look to see which plants are problematic. For example Phormium tenax (New Zealand Flax) and Lysichiton americanus (Yellow Skunk Cabbage) are both ‘Problematic Plants’ but often available to buy in garden centres.

Meanwhile Prunus laurocerasus (Laurel) which has been the hedge of choice for many over the last few years, as several others have succumbed to the extremely harsh winters of 2010-11 and disease, is on the ISI Amber List.

Introduced into the UK from Holland as an ornamental plant, Fallopia japonica has since colonised all areas of the UK and much of Ireland. Better known as Japanese Knotweed, this scourge of a plant is on the Global Invasive Species Programme’s list of 100 worst invasive species.

It has vigorously invaded natural habitats, hedgerows and waterways all over Ireland and pushed out our own native species and led to huge areas of monoculture where no other species can survive, and in which biodiversity is greatly reduced.

It has been identified as one of the highest risk, invasive, non-native species in Ireland. The problem is that it is extremely difficult to eradicate once it has become established. Glyphosate, which is the active ingredient in several weedkillers including Round Up, can be effective, but it will need several applications over a period of up to two years.

I am speaking from experience here as I have managed to eradicate a stand of Japanese Knotweed in this way. But you need to be alert and diligent if this is your approach, because if you spray it once and then forget about it, then you just make it stronger, a bit similar to only half treating an infection with an antibiotic.

Non-chemical approaches are not only pointless but may well worsen the problem. Cut the knotweed back and leave the stems on the ground and you are simply creating a worse problem, as these stems will take root and form new plants. A piece of stem as small as a fingernail can reproduce so if you do find Japanese Knotweed on your property please do seek professional advice.

Apart from the environmental havoc that it creates, Japanese knotweed will also cause problems to hard surfaces like tarmac and paving, interfere with septic tanks, plumbing and even the foundations of a house. Insurance companies and mortgage providers in the UK will not look favourably at your property if it is present.

Recognising how serious invasive alien species like these have become, the The European Council adopted a regulation on the prevention and management of the introduction and spread of invasive alien species. The regulation, which is binding, will come into force from January 1, 2015. It lays down rules on the prevention and the minimisation of the adverse impact of the introduction and spread, both intentional and unintentional, of invasive alien species on biodiversity and the ecosystems, as well human health and the economy.

The Cork Branch of the Wildlife Trust is running a project funded by Cork City Council to survey the extent of the problem in the city and county.

IWT Cork Knotweed Project ( http://corkknotweed.org ) aims to map the distribution of invasive species in Cork, including Japanese Knotweed,Bohemian Knotweed, and Winter Heliotrope.


Last week saw the first frost of this, the second half of the year and although it wasn’t severe, I did need to get the hot water to the windscreen before starting the car.

This blast of cold should act as your clarion call to bring in your frost-tender perennials.

Bedding and patio plants like geraniums, tender fuchsias, some salvias, thunbergias amongst many more, need to be brought in to ensure that they will survive until next year — and develop as even stronger plants.


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