No, it's not, though you could be forgiven for thinking it is - it's even appeared in some gardening literature. Most rhododendron species leaves and buds are toxic though - which means that animals do not graze on them, so that this natural check on a plant's spread isn't there, thus helping plants like R. ponticum to take over vasts swathes of land. And its certainly true that the amount of shade they cause kills off other plants in the area - there are also some effects in the soil on mycorrhizal function and type which affect other flora negatively. None of the studies thus far undertaken have proven any allellopathic activity (that's the name for biotoxins killing other plants nearby), although it is supposed that, even if there is allelopathy occurring, it's at an insignificant level and wouldn't account for the spread of the plant.

Thank you! I had seen a programme on TV a while ago about people clearing a woodland of rhodo's as they were taking over and one of the people clearing it had seemed to say they were....only caught part of the conversation so wasn't sure! Have a few rhodo's in the garden so was beginning to worry!

You will hear many more people say it I'm sure - word of mouth has it to be true - but its not.

and allelleopathic toxins tend to be on seed germination [inhibition of germination- remember doing research ay uni on thi]

If the foliage is very dense it will stop light and rain from reaching the ground beneath. This will inhibit the growth of other plants and is one of the problems with R. ponticum, the one being cleared from many woodlands.

Including the couch grass that tried to invade my garden from next door! Perhaps rhododendrons should be advertised as organic weedkillers?

Thank you everyone! :-)