Late spring is just the time that country estates and gardens are opening their gates, to showcase their Rhododendrons. They are stunning. The flowers are short lived, and you must be quick to catch them at their best.

Rhododendrons are also on the ‘most wanted’ list of invasive aliens, as one of greatest threats to our woodland habitats and heathland.

There can be few wild plants in Britain that engender such a wide range of emotions as the Rhododendron. Are they ‘Himalayan beauties or a poisonous Mediterranean invader’? They are both.

There are over 800 species of Rhododendron, over half of which come from the Himalayas and western China. These are, in the main, non-invasive. The ‘troublemaker’ is a pink-purple flowered species - Rhododendron ponticum from the Mediterranean.

The etymology describes this plant well. Rhododendron, from the Greek rodon = rose, dendron = tree, and ponticum, describing its origin, ‘Pontus’, now part of modern-day north-east Turkey.

Our ‘troublemaker’ arrived earlier than the Victorian period of Rhododendron-fuelled fever. Arriving in 1761 it soon became widely planted on country estates, as a garden ornamental, and was also used as a shelter belt and game cover. However, having escaped the gardens of the rich it became widespread. It favours wetter, acidic conditions, which is not surprising as a member of the Heather family (Ericaceae).

Ponticum is an aggressive competitor; it shades out native species and in time can even replace woodlands. Rhododendrons, like other ericaceous species, have specialised fungi, which allows improved nutrient extraction compared to other plants. This is why members of the Heather family are also good at growing on poor soils, and out-competing their neighbours, by depriving them of essential nutrients.

Finally, yes, it is poisonous. The entire plant contains the neurotoxin, Grayanotoxin. This prevents grazing herbivores from chomping away at its leaves. Although animals generally leave it well alone, poisoning of sheep and deer can happen when they are starving, as in deep snow.

The nectar also contains the same neurotoxin, which can kill the British honeybee. Fortunately for Rhododendrons, the toxin is tolerated by Bumblebees, who are the preferred pollinator, and unknowingly are encouraging the spread of this invasive species.

Given the right conditions, alien plants can destroy an entire ecosystem by upsetting the fragile natural balance that exists between plants, animals, and fungal inhabitants. Remember, be careful what you bring back from your holiday and plant in the garden.