Could you help me figure out what has happened to my five-year-old rhododendron? The one in the forefront seems to be on its way out. But the one right behind it is still doing OK. Both plants are about 10 feet away from a black walnut tree.
Thanks for the help.
Thank you for contacting the Toronto Master Gardeners.
As you probably suspected, your Rhododendron issues may have something to do with the black walnut tree. This tree produces a toxin called juglone that is picked up by the rhododendron’s roots and may be slowly impacting the health of your plant.
This article contains information about the nature of the toxin and gives information about how to garden under the tree. It recommends placing plants, like your rhododendrons, in raised beds in order to give them clean uncontaminated soil. This approach works but all debris such as leaves, twigs, fruit or saplings from the tree, needs to be diligently & continuously removed from the raised beds, otherwise the container soil will be contaminated with juglone too.
Another approach would be to remove all juglone susceptible plants to a location much further away from the tree. Once that is done, re-plant the beds closer to the tree with plants that are not susceptible to the toxin. Some of these plants are listed in the above article but there are many more so you may have to do a little research to get ones that you find aesthetically pleasing and will fit in with your garden.
Here is a lovely article about the value of Black Walnut trees in nature and the interesting collections of native and non-native flowers that make a home under these trees. You may find it inspirational.
Also, it may be a good idea to get a soil test done, before you make any radical decisions, so you can accurately determine if it is the Black Walnut tree causing your issues and not just a simple case of the soil pH in your garden being unsuitable for Rhododendron growth, as they like acidic soil.
Hope this information is helpful