With its large, showy purple flowers in late May and June Rhododendron ponticum is certainly easy to spot in some of our local woods and estate ground.
Unfortunately this plant is a thug and an environmental disaster. It was first introduced in 1763 from its native Spain and Portugal and in Victorian times was extensively planted in country estates for its ornamental value and as cover in woods for game birds.
Rhododendron ponticum requires an acid soil and likes plenty rain. It is prevalent in many areas in the west of Scotland and Wales where it finds the climate ideal and has spread over fields and hillsides to the exclusion of all other plants. It makes an impenetrable thicket which neither man nor beast can enter. Here in the east it is not so rampant but never-the-less can spread and get out of control given half a chance.
As well as growing wild in Spain and Portugal it is also found wild in parts of Turkey and the plants now growing wild here have genes in their makeup from all three populations. It has also hybridised with a North American species which has made it more tolerant of conditions here.
The main problem with this plant is it will out-compete all other plants. With its thick evergreen canopy of leaves, no light reaches the ground where smaller plants would grow. It produces a toxin, which, when released into the soil suppresses the germination and establishment of native plants seeds and it is also unpalatable to most grazing animals.
It can spread by tiny seeds which are distributed far and wide in the wind and it has been estimated that a large plant can produce a million seeds in a season! Its stems will root where ever they make contact with the ground so it spreads out horizontally and where it is beside a river or loch its branches can extend well out over the water.
Control is not easy once it gets established. On a small scale cutting down and burning the branches is one remedy as long as you remain vigilant for several years, looking out for the seedlings and suckers which are bound to reappear. Where it covers whole hillsides its physical removal is a herculean task especially as it likes high rainfall areas favoured by midges.
Most of the other rhododendrons we cultivate in our gardens pose no threat to the environment as they do not sucker or seed themselves, but be warned - sometimes ponticum is used as a rootstock for grafted varieties and given half a chance it will sucker and overtake your nice selected plant and you will end up with a purple ponticum.