Sunday 23 October 2016

Diarmuid Gavin on how to prevent invasive species taking over your garden

How to prevent invasive species from taking over your garden

Diarmuid Gavin

Published 12/06/2016 | 02:30

Diarmuid Gavin
Diarmuid Gavin
Into the wild: Rhododendrons grow wild at the foot of Benbulben in Sligo.

I've had a varied career as a media gardener. Alongside the shows where you might expect to see me, a few odd appearances stand out. Presenting an episode of Top of the Pops was one. And then a couple of years ago I briefly became the gardener for the BBC's Newsnight.

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One of my reports for them found me in a garden centre in Folkestone, Kent talking about buddleia. There's a debate as to whether these beautiful plants, which throw out long stems which end in wands or panicles of vibrant colour beloved by gardeners and butterflies alike, are aliens which must be destroyed on sight. What a strange world we live in!

Of course, as a gardener, my job was to defend that particular alien. I have followed with interest the debates in newspapers which seem to suggest gardening is a racist hotbed because of a dislike of some plants that we have previously invited onto our shores. It seems that gardening could take a Brexit stance!

Being serious, this can be a hugely important issue for gardeners and those who care about our natural environment. Some plants which were fêted in Victorian times have begun to take over areas of the country when they find conditions that they like. This can be a cause of great grief to gardeners and custodians of the landscape. Whenever I get to meet gardeners and answer questions after talks, a good proportion will deal with queries on how to eradicate the worst of these green offenders. So, Newsnight has spurred me on to talk about some eradication, dealing with overzealous visitors who have made gardens and the wild landscape their home.

Into the wild: Rhododendrons grow wild at the foot of Benbulben in Sligo.
Into the wild: Rhododendrons grow wild at the foot of Benbulben in Sligo.

As an organic gardener I don't use chemicals. However, many people find some of these species impossible to deal with without the help of weedkillers. My own preference, digging and digging again, isn't practical in some situations.

Japanese knotweed (Fallopia japonica) was formerly prized as a beautiful garden plant. One can take an objective view and admire its beautiful and interesting foliage and cream flowers. However, it has taken over and become the bane of life for both gardeners and people concerned about the damage it can do to property prices - it grows through tarmac and concrete.

The bad news is there is no easy solution for its eradication. You can control it by cutting it in early summer or spray with a systemic weedkiller. Do this again at the end of summer. The objective here is to weaken the root system as very few plants can survive without green shoots and leaves which photosynthesise for their energy. If you've only a relatively small patch you can try digging up the knotweed, but be careful because breaking off a little bit can actually mean you are propagating the plant and thereby increasing the problem and you have to dig very deep.

Rhododendron ponticum was once widely planted in our parks and estates for its beautiful purple blooms and vigorous growth. However, it has since escaped into the wild and competes with native flowers. In parts of county Kerry it is suffocating all other woodland shrubby species. It's hard work trying to remove this shrub and requires teams of volunteers to dig out and try and control its spread.


Bindweed is another weed which has ornamental properties - its beautiful white flowers - which led to its initial introduction. The problem with bindweed is that no matter how much you dig it out, if you leave even a tiny bit of the root behind in the soil, it can replenish itself. It's a good time to use weedkiller when the plant is in active growth. A good tip is to let it grow up some bamboo canes so it's easier to spray it without damaging other plants. It's particularly attracted to strangling herbaceous plants.

To clean out a bed organically means digging the whole lot up in late autumn or winter (take advantage of this time to divide clumps of herbaceous that are becoming too big). Finding the white fleshy root can be a puzzle but it's oddly satisfying. If you are using a chemical remedy, go for a systemic weedkiller, one that when brushed on will be absorbed right down to the root killing the whole thing. Ask in your garden centre for a good brand.

And as for the poor buddleias? Well, some say they are doing harm, sprouting out of buildings, choking up our railway system. However, I give them absolution because this indomitable species I regard as a lovely sign of the power and beauty of nature. And in another month or so their beautiful flowers will remind us of why they are so enjoyed by many gardeners.

Fair Co Down...

The Specialist Garden Fair at Mount Stewart, Co Down, takes place on Saturday June 25 and Sunday June 26, from 11am to 5pm. Pick up some plants from visiting garden nurseries while enjoying all the beautiful views the Mount Stewart gardens have to offer this summer. After taking a walk around the grounds, unwind with something delicious from the food stalls. And, on Sunday, you can while away the afternoon by enjoying music on the croquet lawn.

The event is free but normal garden admission prices  (£8.25/ €10.60 for an adult) apply. For more information,  visit or phone  (048) 4278 8837.

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