RED VALERIAN REALLY CAUGHT MY ATTENTION THIS VISIT TO KILKENNY [SECOND WEEK IN JUNE 2025]
I may have noticed this plant on previous visits and failed to pay any attention but when I explored the old grave yard on the Old Dublin road a few days ago I was really surprised by how much the site was dominated by the plant and I found many other examples at various locations throughout the city.
The Red Menace on Our Walls? Unpacking the Truth About Red Valerian
You've seen it, haven't you? Those vibrant splashes of red, pink, or even creamy white adorning old stone walls, railway embankments, and forgotten corners of our towns and cities. In Kilkenny, as in many parts of Ireland, Red Valerian, scientifically known as Centranthus ruber (though sometimes listed as Valeriana rubra), is a familiar sight. But beneath its charming facade lies a tale of Mediterranean origins, surprising resilience, and growing ecological concern.
A Closer Look: Beauty and Brawn
Let's start with what we know about this striking plant. Red Valerian is a perennial, typically reaching heights of up to 75cm, sometimes stretching to 80cm. Its fragrant flowers, usually 8-10mm long, are clustered in dense panicles, each with a slender tube and spur at the base. The leaves, a distinctive grey-green, are untoothed and ovate, growing in opposite pairs. You'll notice the lower leaves have stalks, while the upper ones cling directly to the stem. Its flowering season is impressively long, from June to September, and in milder conditions, it can even extend into autumn.
From Mediterranean Gardens to Irish Wilds
Originally hailing from the sun-drenched Mediterranean region – think Portugal, Spain, France, Italy, and North Africa – Red Valerian was introduced to Britain and Ireland as a garden ornamental around the late 16th century. Since then, it's made itself very much at home, becoming widely naturalised across the Emerald Isle, particularly thriving in our warmer, often coastal or southern sites.
It’s a plant that loves a challenge, flourishing in dry, disturbed, or unstable habitats. Give it low-nutrient, high-pH, calcareous conditions, and it’s in its element. This explains its penchant for maritime cliffs, shingle beaches, rocky outcrops, old stone walls (where it expertly roots into lime mortar), railway embankments, and even urban wasteland. So, your observations in Kilkenny – spotting it on walls, wasteland, and even graveyards – are spot on and perfectly illustrate its preferred stomping grounds.
The Elephant in the Room: Is Red Valerian a Problem?
While the National Biodiversity Data Centre currently classifies Red Valerian as having a "Low risk of Impact" in Ireland, with an invasive score of 11, there's a growing consensus that its vigorous nature warrants closer attention. It’s undeniably a non-native invasive species that can, and does, cause problems.
One of its key superpowers lies in its seeds. Equipped with tufts, much like a dandelion's, they are readily dispersed by wind, allowing Red Valerian to self-seed prolifically and spread rapidly. It's thought that even the wind gusts from passing trains along railway lines have played a part in its widespread colonisation.
More Than Just a Pretty Face: The Environmental Impact
When Red Valerian takes root in old stone walls, its vigorous, rhizomatous growth and branched, woody rootstock can be destructive. It can loosen stones or bricks, potentially compromising the structural integrity of these historic structures.
Globally, its invasive tendencies are far more pronounced in some regions. In parts of South Africa and Auckland, New Zealand, for instance, it’s a significant invasive species, actively controlled due to its ability to form dense populations that outcompete and displace native vegetation. Here, it can prevent native seeds from germinating and even compete with threatened short-stature plant species.
Closer to home, an ecological adviser in Bray, Ireland, has strongly advocated for placing Red Valerian on a prohibited list due to its rampant growth at the expense of native plants. Volunteers have already put in considerable effort to uproot large quantities along the coastline.
"Be Plant Wise": Our Responsibility as Gardeners
The spread of invasive species from our gardens is a significant and often overlooked issue. The message is clear: "Be Plant Wise." Gardeners are encouraged to consider alternatives to known invasive plants. If you do have Red Valerian in your garden, a simple yet effective measure to prevent its self-seeding is to cut back the stems after the first flowering in July.
Not All Valerians Are Created Equal
It's crucial to distinguish Red Valerian (Centranthus ruber or Valeriana rubra) from its namesake, Common Valerian (Valeriana officinalis). The latter is a native Irish plant and is the herb traditionally used in herbal medicine for its purported sleep-inducing properties. They are entirely different plants.
A Beautiful Intruder with a Cost
In conclusion, an initial assessment of Red Valerian as a potentially serious problem in the future is well-founded. While its ornamental qualities and resilience are undeniable, its prolific seeding and ability to establish itself in disturbed and rocky areas – including our cherished built structures – mean it can outcompete native flora and cause structural damage. It serves as a stark reminder of how a seemingly innocuous and popular garden escape can evolve into a significant ecological concern.