As rhododendrons are forest or forest-edge dwelling plants, most species prefer semi-shade or light similar to that in a forest glade, but not complete shade. If they have too little light, they will not flower nearly as well. Too much sunlight, on the other hand, will lead to excessive evaporation and the result will often be damage through dryness, particularly if it is coupled with low humidity and high temperatures. Overexposure to sunlight will also cause the flowering time to be over too quickly.
The cooler the climate, and the higher the humidity in the air, the sunnier should be the ideal position for your rhododendrons Deciduous azaleas and Japanese azaleas, small-leaved dwarf rhododendrons and Yakushimanum hybrids usually need more sunlight than the other groups of rhododendrons.
Taller trees and shrubs will naturally provide some shade. All deep-rooted species with open crowns, like pines, oaks, ginkgo, acacia, hawthorn, laburnum, magnolia and Liquidambar spp. are suitable.
Shallow-rooting trees and those with dense foliage are not suitable as they will prevent rain from reaching the rhododendron shrubs and their shallow roots will compete with the flat-rooting rhododendrons. Among the latter types are birch, maple, beech, chestnut, fir and linden(lime) trees. Do not plant rhododendrons under any of these trees. If you have no alternative, you could always build a raised bed.