Anja Murray: Rhododendron — more to this beauty than meets the eye

The aesthetic appeal of rhododendron flowers in spring and early summer was what made Victorian plant hunters bring back specimens from the shores of the black sea and from other parts of Asia, especially from the slopes of the Himalayas, from China and from Malaysia. As many as a thousand different rhododendron species grow in the wild across these regions.

Victorian landed gentry, already enthusiastic about exotic plants being brought back from new colonies, were enthralled by rhododendron. A trend took off, and various varieties were grafted and planted up in the gardens and woods of stately homes across Britain and Ireland. A rhododendron walk was the ‘in’ thing to have, so ladies and gentlemen could stroll through a tunnel of rhododendron flowers in springtime and experience the joy of their bright chunky flowers and their rich sweet scent. Sometimes rhododendron was simply planted in existing woodlands on the estate — some varieties, particularly Rhododendron ponticum, thrived in partial shade and acidic soils and so naturalised easily.

Victorians had a colonial mindset, a frivolous appetite for the latest horticultural fashions, and little understanding of ecology. They were not inclined to consider the potential negative consequences of these introduced rhododendron bushes spreading through woodlands.

One of the early introductions of rhododendron here was by the Herbert Family of Muckross House Killarney, seat of the Earl of Kenmare. This estate was much celebrated, and was visited by Queen Victoria in 1861. We could call these the ‘influencers’ of the time; what was fashionable there was copied in the estates of the landed gentry across the land. Rhododendron was planted in the woodlands there because of its aesthetic appeal, but also as cover for pheasants — another ecologically problematic introduced species. Rhododendron thrived in the acidic soils of the Killarney valley and soon took over the understorey of the oak woodlands all around the lakes and slopes there.

In these woodland habitats, under natural conditions, the understorey is filled with holly, hawthorn, blackthorn, hazel, and spindle, underneath which grows a blanket of woodland wildflowers. Swathes of bluebells; stunning white flowered wood anemone; delicate wood sorrel and shimmering yellow lesser celandine — the native flora that would be in bloom now during April — all are excluded where rhododendron has taken over.

Healthy woodland flora normally support an enormous diversity of specially adapted woodland moths; specialist butterflies; bumblebees and solitary bees; colonies of ants; leaf mining bugs; shield bugs; and hundreds of other invertebrate specie. In turn, these are what sustain many of the woodland habitat birds, such as treecreepers, spotted woodpecker, woodcock, warblers, and jays. Relationships between the many plant and animal species in a woodland such as this have been evolving over millennia, fine-tuned as the many interdependent elements of functional ecosystem.

But when rhododendron invades, it bullies out all the native understorey trees and shrubs with its vigorous growth, in particular the big waxy leaves that cast a heavy shade and prevent most of the flowering plants that would otherwise thrive here from taking hold. Butterflies are deprived of the plants they need to nectar on and the native plants that their caterpillars have evolved to feed on. Moths and hoverflies are similarly impacted. Rhododendron leaves contain a toxin which helps the plant to repel insects and mammals from eating its leaves, and combined with the shade it casts, dominion becomes absolute. In this way, rhododendron is a death knell for countless species that would otherwise thrive among native woodland habitats.

Add to this the discovery by botanists in Trinity College Dublin that rhododendron nectar is toxic to some Irish bees.

The nectar contains grayanotoxins, which are chemicals naturally produced by these plants to help them avoid being eaten by insects and mammals. In experiments, honeybees died just hours after consuming rhododendron nectar. Native solitary bees became disoriented or even paralysed by the toxin. Interestingly, native bumblebee species were unaffected by the toxins in rhododendron nectar, they were found to be able to eat the nectar with no negative side effects at all.

Another impact of dense growth of rhododendron is that tree saplings are unable to establish themselves beneath its heavy shade. Most tree saplings here, such as oak, holly, hawthorn, and elm, are well adapted to the shady conditions of a woodland habitat, but the dense shade and waxy leaf litter of rhododendron is too extreme for these trees to be able to reproduce. The consequence is that rhododendron-infested woodlands are unable to regenerate themselves — there will be no next generation of native trees.

Ireland still has one of the lowest proportion of woodland cover in Europe, only around 2% of the country is covered by what is termed native or semi-natural woodland. Only a fraction of this is long established and almost none is free from overgrazing by deer or invasions of cherry laurel, and rhododendron.

The dearth of healthy woodlands here is an open ecological wound. While work is being carried out to clear rhododendron from some areas of Killarney National Park, there have been problems in the management approaches. Since the noughties, there has been massive criticism of the State’s incompetent handling of this ecological calamity. Killarney National Park is owned by the State, so there is no excuse for ongoing neglect in one of the last remaining refugia of temperate deciduous rainforest in Ireland. Other woodlands across the country are suffering the impacts of rhododendron and overgrazing too.

If you’re out and about this April and May, admiring the rhododendron and perhaps feeling overcome by the bounty of the blossoms, remember that there is more to this beauty than meets the eye.