The not-so-fab five: Outlaw #3: Rhododendron ponticum

Published on February 23rd 2019
The spectacular sight of a Welsh or Scottish mountainside awash in rich swathes of purple blooms may seem like a dream, but the reality is more of a conservation nightmare. Smothering our native flora, carrying disease and poisoning our already-suffering honeybee, the common rhododendron may not get the same level of press as giant hogweed and Japanese knotweed, but it is an equally potent threat to our delicate ecology.

A bullish beauty

The name rhododendron derives from the Greek rhodon (rose) and dendron (tree). Ranging from huge, tree-like giants to tiny bonsais, deciduous fragrant shrubs to tender indoor azaleas, their appeal is easy to appreciate.
Introduced during the late 1700s from Spain or Portugal, R. ponticum became a favourite among gardeners and game-keepers alike
The common rhododendron combines two propagation methods which make it super invasive: mass seed dispersal and suckers (underground stems which develop into new bushes).
The masses of purple flowers we see blanketing hillsides produce trillions of tiny seeds which are so small they are carried on the wind to form new colonies. The dense roots and suckers swamp native plants, suffocating them and starving them at the same time.

Death by rhododendron

The plant is a carrier of sudden oak death, a fungus that has devastated oak populations in the United States and has been recorded in the UK since 2003. What’s more, the plant is toxic to livestock, meaning it cannot be controlled by grazing cattle or sheep in its favourite hillside habitats.
Mad honey causes nausea, drowsiness, weakness, tongue numbness and seizures.
The most famous reports of rhododendron toxicity, however, come from the ‘mad honey’ produced by honeybees that have been visiting the flowers. Interestingly, this was actually the first ever recorded instance of chemical weaponry in ancient Turkey, when the honey was used to stupefy enemies ahead of marching into battle.
Recent research has also found that the nectar can kill honeybees, though bumblebees and honeybees resident in the rhododendron’s native habitat were unaffected.
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Removing rampant rhodos

As with all invasive plants control is not easy and removing rhododendron, as with Japanese knotweed, is a matter of persistence. Thankfully, unlike the latter, rhododendrons have a shallow root system and are easier to dig up, which may be possible with a single plant or smaller clump.
Another popular method is to cut down plants and treat stumps with a weedkiller, usually glyphosate. You can also drill stems and inject the chemical directly.
In the Highlands measures are being taken to eradicate the weed, though it is a laborious project.

Do your bit to help

Avoid growing Rhododendron ponticum and its varieties if you ever see it for sale. Also, be cautious and remove any suckers that come up from your own plants if they have been grafted.
Over the last fifty years, the locals of the island of Brownsea in Dorset have successfully removed the common rhododendron from their shores and, in the process, have seen the red squirrel return to a healthy population – the story inspires hope that whatever the weed, if we work together we can save our native species from these opportunistic invaders.

Next month we'll be looking at Indian balsam.

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