Many individuals who would dearly love to grow rhododendrons have steered away from them because of their clay soil. Don’t let having clay soil stop you enjoying a rhododendron garden. If heavy clay is present then dig a small hole and run some water into it; if the water does not disappear in a few minutes then you have poor drainage. This is not a sure test but it will give you a good indication. Sometimes the top soil layer will drain well, but there will be a hardpan underneath it that will not drain well. If you have good drainage your rhododendrons will like it, if you don’t, then you can create a better environment by planting the Rhododendron on top of the clay and not in it.
To achieve the desired result use plenty of mulch, leaf matter, pea straw and pine needles. By elevating your garden and planting your rhododendrons above the clay level you will create an environment where they can thrive.
One grower tells the story of President Roosevelt in his garden that was looking decidedly terrible. In frustration and anger he ripped the Rhododendron out and tossed it up the bank. Expecting his actions to have killed the Rhododendron imagine his surprise when a few weeks later he found the plant had re-established itself in the mulch and was looking better than ever. On noticing this he went around his garden and lifted all the sad looking rhododendrons so that they were now sitting on top of the soil in the heavy layer of mulch.