How letting invasive plants jump the garden fence could land you in court

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Charles Quest-Ritson takes a look at invasive plants and what to look out for (or look for) in our gardens.

There are three naughty and overly exuberant plants that I really like: first, the common purple Rhododendron ponticum; second, the Himalayan balm Glandular impatiens; and third, the stinking cabbage Lysichiton American.

Sadly, all three are referred to as “invasive non-native plants”. It is an offense to allow them to escape into the wild. You can grow them in your yard, but letting them jump the fence will get you to court.

There is a list of these foreign thugs in the UK’s Wildlife and Countryside Act (1981). When the legislation was first mentioned, the draft text extended to any hybrid descendant of one of these foreigners.

The RHS and nurserymen have pointed out that hundreds of well-bred old rhododendron hybrids dating back to the 19th century had Rhododendron ponticum as one of their ancestors. It was only then that lawmakers limited their condemnation to the species, and not to its descendants.

I think R. ponticum is one of the most beautiful flowering shrubs. It was the dominant species in my childhood garden (mainly because it was used as a rootstock for more exciting cultivars, and sometimes it got the better of its grafted scion). And how I loved the view from my Irish grandmother’s dining room over the Le Morne mountains surrounded by purple splendor in June and July.

No less exciting are the many miles of ‘Ponticum’ that line the estate’s roads at Cragside in Northumberland. I once climbed the Georgian Military Highway to see wildlife blooming along with the fragrant yellow azalea R. luteum, another good maker. I never stop shivering at the beauty of the two.

Yellow azalea ‘Rhododendron luteum’.

Glandular impatiens is so easy to get rid of that I welcome its appearance – and it’s only an annual. In a planter’s garden, it’s not at all invasive, just a good plant.

However, I aspire to grow up American lysichiton. It has giant yellow arum-like husks (from the same family), which are so large and bold that they dwarf all other spring flowers. When we lived in France, one of my neighboring farmers managed to grow it in a field ditch, nearby Gunnera manicata, sometimes referred to as giant rhubarb, although it is not related to rhubarb at all.

Lysichitons need moist soil with a low pH, but now I am gardening on a warm, south-facing hill in the chalk south of Hampshire, with no ability to grow them. Go to the RHS Garden at Harlow Carr to see them at their best.

“I fought Japanese knotweed for years – and won … I sprayed the stems with glycophosphate for 10 years without success, then tried a lawn weed killer called Weedol and Japanese knotweed finally succumbed “

The danger with rogue alien species is that they alter the balance of our native ecosystems – usually for the worse. They outperform native species. But we have, of course, double standards.

I understand the need to rule out pests and diseases that can have a detrimental effect on plants of economic or environmental importance, but sometimes the toll of foreign plants shows a net benefit. Our fields and woods have been greatly improved by the addition of exotics.

Snowdrops were introduced to Britain around 1520 and were probably first cultivated in a London garden. Who now complains about their uncontrolled invasion of our forests? And most of us like to see speedwell (Veronique filiformis) grow in our lawns. It’s pretty, but was first introduced to Britain from the Caucasus around 200 years ago and is now ineradicable.

Galanthus elwesii var monostictus cinderdine white flowers green markings flower bulbs spring flowering snowdrops

Galanthus elwesii var monostictus cinderdine white flowers green markings flower bulbs spring flowering snowdrops

I fought for years against Japanese knotweed (Fallopia japonica) – and won. Experts say glyphosate will get rid of it in your garden, but I have sprayed the stems in the spring and fall for 10 years with no success.

Then, in one of the most unknown in the EU, Luddism became the hard-to-find glyphosate. So I tried a lawn weed killer called Weedol and the Japanese knotweed finally died. It is not illegal to have it in your garden, but do not let it escape. There is a very pretty variegated form in French gardens that is not so vigorous.

Enemy # 1 in my Hampshire garden is an Earthly Elder (Aegopodium podagraria). It was brought to Britain by the Romans. Forget that it is edible (Japanese knotweed and Himalayan balsam too) and watch it spread throughout your herbaceous plantations, under your hedges and in your lawns. It’s a fast move.

The leaf is quite pretty and there is a nice variegated form which is best grown in pots or troughs from which it cannot escape, but, in the soil, the species is a major nuisance.

Of course, the worst thugs in the garden are cats, dogs, muntjacs, gray squirrels, children and other animals. But this is another story.


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