Rhododendrons require little maintenance, but you must follow two golden rules if you wish to grow these plants successfully: choose a suitable site and prepare the soil properly before planting.
Choosing the right site
The ideal site for planting rhododendrons is semi-shaded to sunny. It must also be sheltered from the prevailing winds, particularly in winter, as rhododendrons are highly prone to wind damage.
Most large-leaf evergreen rhododendrons prefer semi-shaded sites. Too much direct sunlight, especially in winter, should be avoided, as it can dry out the plants. Their leaves may turn brown and drop. You may wish to plant them in the dappled shade of deep-rooted trees like pines and oaks. A west-, east- or north-facing bed against a foundation wall is an ideal site, provided that it is sheltered from the wind.
While they like well-aerated sites, rhododendrons, particularly large-leaf evergreen species, will not tolerate constant wind exposure. In winter, they must be protected in particular from cold, drying west winds. A conifer hedge makes a good windbreak and will help to accumulate a layer of protective snow in winter.
Preparing the soil
Like all Ericaceae, rhododendrons are very demanding when it comes to soil quality. Their root system consists of very fine shallow roots, sensitive to both drought and overwatering. They require well-aerated and drained soil that is high in organic matter. The ideal soil mix is 50% organic matter and 50% slightly sandy topsoil. Rhododendrons are acid-loving plants, so it’s best to check your soil pH and adjust it to the 4.5 to 5.5 range if necessary. You can add sulphur or ferrous sulphate (not aluminum sulphate) to acidify the soil.
Depending on the type of soil in your garden, you may need to amend it to make it suitable for growing rhododendrons.
Overly sandy soil
Dig in organic matter, peat moss and an acidifier such as sulphur to a depth of 60 cm, blending them well.
Overly clayey soil
Treat as for overly sandy soil, with the addition of coarse sand. To promote drainage, raise the bed by 20 cm; you can surround it with a low wall or add some companion plants on the slope to protect it from erosion.
Extremely clayey soil
Remove the top 60 cm of soil and add a suitable soil mixture, raising the bed by at least 20 cm. It is also a good idea to add an inexpensive, easy-to-install drainage pipe to promote good drainage.
Spring is the ideal time to plant or transplant rhododendrons. Before planting, make sure that the roots are not overly compacted and that the rootball is sufficiently moist.
Position the plant so that the collar (where the main stem meets the roots) is level with the ground. Compact the soil lightly as you fill in the hole and mound it up slightly in a dish shape that will hold water.
It is a good idea to protect freshly planted shrubs in the first winter.