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The author of this answer to this question mentioned that toxins in rhododendron leaves can prohibit growth in nearby plants. (This is a paraphrase, not a direct quote.)

I have six huge rhododendron bushes that have a variety of bleeding hearts, azaleas, hostas, forsythia, pussy willows, young trees, old trees, daylilies, daisies, other wildflowers, and some things I can't identify, growing under, through, and around them. Various groundcovers and low-growing plants, including vinca, catmint, lily of the valley, are all growing in abundance around the roots. It looks like a jungle out there and I love it!

Since everything is growing nicely, I assume the rhododendrons are not having a negative affect. However, if I understood the answer correctly, there may be some things I should be aware of for future plantings.

Is there anything I should not plant in the vicinity of my rhododendrons? If so, what?

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There is some evidence that Rhododendron ponticum inhibits germination and growth of seeds and seedlings nearby which may be attributed to allelopathy, but no evidence they have such a strong allelopathic effect that it kills surrounding vegetation. Large rhododendrons may cast too much shade on nearby plants, as well as taking up all the water and nutrients, so where necessary, size should be controlled, as well as leaf litter cleared away. The leaves are large, tough and fairly water resistant, and therefore take a long time to break down, so a thick layer of fallen rhododendron leaves on other plants' foliage won't do them much good.

Otherwise, as you've observed, there's no reason why you can't grow other plants with rhododendron varieties, so long as the plants aren't those that prefer alkaline soil conditions, because, if a rhododendron's growing well, then the soil will be on the acidic side of the scale.

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