Invasive Species



Himalayan Balsam


One of the greatest threats to our native ecosystem and domestic gardens is the arrival of pervasive and invasive species. Anyone who has taken a walk along local riverbanks, which are frequently clothed in Himalayan balsam, can see how these can become a real pest.  Himalayan balsam, or Impatiens gladulifera, is a relation of the popular summer bedding plant, busy lizzie. This has colonised huge tracts of river banks, suffocating native plants which our native wildlife depends on for food and nesting. The summertime is when it is most difficult to get in amongst garden plants to weed and sneaky species can get a foothold, so autumn, when you are clearing and cutting back is a good time to look for evidence of unwanted species and dig them out before they become a problem. There are a three categories of invasive plants; the first are the common weeds we all know and battle against year in year out like dandelion, daisy and buttercups. The second category are garden plants like Japanese anemones and Echium that have self-seeded to such an extent that they have become a nuisance, but cause no real damage.  The third category are species which were introduced by horticulturalists as curiosities but quickly naturalized and spread throughout the countryside.  The latter category causes real ecological and structural damage by escaping from our gardens and into the countryside beyond. A classic example of such a pest is Horsetail or Equisetum arvense. This plant has creeping rhizomes which can go down 2m below the surface, making them almost impossible to eradicate by digging. Even when using a pesticide persistent applications are necessary, you may need to call in specialist help. Japanese Knotweed, or Himalayan Balsam, was introduced from Japan in the nineteenth century and since its introduction it has spread nationwide. It can destroy the foundations of houses as it can grow through concrete and tarmac. It also blocks wildlife corridors and clogs up watercourses.

Removal and control of invasive species causes millions of euro a year. Other pests include Bamboos – Philostachys species which form impenetrable thickets, block out native plants and spreads rapidly, again popping up through hard surfaces with ease.



Japanese knotweed

Obviously the best way to fight invasive species is not to plant them in the first place so here are some tips to keep these thugs out:

  • Check a plants habit before you buy it and see if there is a non-invasive alternative.
  • If you have been walking along river banks or areas where invasive non-native species have colonized always wash the soles of your shoes thoroughly before wearing them in your garden. Seeds are commonly brought in on the soles of boots or on animal’s coats.
  • Never add any part of an invasive plant to the compost heap.
  • Invasive aquatic plants have caused considerable problems to our waterways and it is important to stop their spread. Avoid swapping cuttings or fragments from friend’s ponds. Otherwise you may end up with an invasive species in your pond.
  • Wash the roots of plants you buy too – I have a dreadful duck weed problem in my pond which came in on a plant bought from a very reputable garden centre.
  • Learn to recognise seedlings of invasive plants. Dig out seedlings as soon as you see them in spring and before they establish.
  • When weeding out mature plants make sure to remove all of the roots. Plants with tap roots like dandelion species will generate from every little bit of root left in the soil.
  • Rhizomatous weeds such as couch grass, bindweed and ground elder respond well to glyphosate if treated twice over a two-week period. It is very important to avoid drift so apply on a very still day. To treat bindweed, spray the weed killer onto rubber gloves and run your hands along the plant.
  • To eradicate ivy on trees and old walls cut it back to the ground and dig out the woody stump.



Weed Identifier

I just came across this – a very handy weed identifier

Particularly useful at this time of year as it shows what the seedlings look like so you can grub the blighters up before they get a chance to put down roots.


Wild Garlic – caveat emptor!

Allium ursinum

This morning I spent 3 hours at my friend Mrs Wilson’s garden trying to dig out a bed which was infested with wild garlic, Allium ursinum. This plant is a beguiling and beautiful creature when seen wending its way through woodlands and along roadside banks, it has also become madly fashionable with the craze for food foraging as it makes a very nice addition to soups and salads and has edible flowers. However a word of warning, if you have a small garden and do not own acres of wild woodland, do not, under any circumstances be tempted to introduce it into your patch. Allium ursinum is a thug of the highest degree and will spread like the clappers once it has taken root. It seeds prolifically, each little grass-like seedling is attached to a little bulbule and these are practically impossible to eradicate once established. Today I cleared a patch approx 2m x 3m, laboriously teasing the leaves and flower heads from underneath plants and stones and between rocks – when you pull the plant the leaves come away in your hand leaving the bulb behind, when you try to dig underneath the bulb it just seems to make the problem worse, spreading bulbules around even further. Clearing is a Pyrrhic victory as as sure as eggs is eggs, next year the infestation will be just as bad. My only advice is do remove all stalks and flower heads, this will weaken the bulbs and stop the things seeding and pull out any seedlings. Never, ever add to a compost heap – either burn or bag and bin.

Gatecrashers: watch out for unwanted guests.


Plants are like people … Well, so much for my waxing lyrical about the joyous party that is the garden; I forgot to mention the gate crashers and other tricky customers that inevitably show up to make a nuisance of themselves – weeds. My particular bugbear is that absolute pest, oxalis, an odious little pernicious uninvited guest. Similarly, hairy bittercress has that “it’s only me!” characteristic of a bore, popping up on top of the rhizomes of irises, in pots, and anywhere you really don’t want it. If you leave it too late, it will have developed seed pods and the second you touch it, they burst, pinging thousands of their irritating offspring into the surrounding soil. This year’s stop/start weather seems to suit them just down to the ground – a few days of heavy rain, then sunshine, allows them to germinate, then a few more days of rain keeps us out of the garden and gives them an opportunity to put on growth. Infuriating. Some of these party poopers were actually invited into the garden in the first place, plants like Japanese anemones, which I initially loved but now hate with a passion. It is like a really annoying ex-boyfriend, who every time you think you have got rid of him turns up to stalk you at the most inopportune moment, insinuating himself right into the middle of an otherwise lovely gathering. While in this anthropomorphic vein, what about delphiniums? They are like extremely beautiful, refined, but ultimately incredibly tiresome women; high maintenance, needing rich compost (as opposed to men), staking and constant protection from slugs and snails. As friends, you put up with them for years until finally a eureka moment happens and you are able to say “no more!” and cut them out of your lives. I am replacing mine with steadier, ever-reliable and easy alternatives, Aconitum napellus ‘Arendsii’ and a really good campanula persicifolia or glomerata ‘Superba’. Both of these plants will provide the height and deep blues of delphinium without the constant care and attendance the latter need. July can be a tricky month. The garden is often neglected for long periods of time due to holidays and away days, the early flowering stars are over, and the great late summer/early autumn stalwarts – dahlias, heleniums, crocosmias and rudbeckias – are not in flower yet, so fill up with long-flowering and easy standbys. These are the old dependable friends, ones we often take for granted and don’t nurture as much as we should – acanthus, achillea, anthemis (all the As), knautia and lychnis – all easy and long-flowering, drought-tolerant and can manage without staking or much faffing around, and slugs hate them. Happy gardening, and fingers crossed the sun will shine on us.

Digging out horseradish

Digging out horseradish

A really great book for gardeners

Weeds, Weeding (& Darwin) – The Gardener’s Guide by William Edmonds
Frances Lincoln

This is a really fab book and exactly what I have been looking for for years, a comprehensive guide to weeds and wild flowers which grow in the wrong how place. It is full of pictures for identification and ways to get rid of these pesky inaders.
Weeds are infuriating to gardeners, but the fact is that weeds are just another plant. The ones that are most infuriating are native wild flowers and exotics that come from a similar climate. They love the conditions in our gardens, are adept at self-propagating and are vigorous so tend to run amok. William Edmonds, the author of this book sees Charles Darwin as his mentor. Darwin was intrigued by the nature of variation in plants and how this related to which plants thrived and survived. Informed by Darwin’s insights, and by over thirty years of gardening experience, Edmonds describes and illustrates one hundred significant garden weeds, arranged in the order in which they have evolved. For each there is a what to Do and a further chapter sets out the pros and cons of twenty tried and tested approaches to weeding. Learning to recognise, understand and deal with each weed will take you well on the way to coping in a relaxed – even enjoyable – tussle with these devilish despoilers. Weeds, Weeding (& Darwin) is an enlightening guidebook for every gardener.