Because rhododendrons have very shallow roots, mulch offers effective protection in a number of respects: by insulating the soil, it reduces temperature swings and by slowing evaporation, it keeps the soil moist. Mulch is also a good way to prevent weeds and to avoid damaging the roots, since you won’t have to cultivate the soil as often.
Evergreen needles are an excellent mulch, but you can also apply a mixture of equal parts dead leaves and sawdust in fall, in which case it is a good idea to add nitrogen fertilizer in spring.
Provided that your rhododendrons are suitably located and the soil is properly prepared, there is little need for fertilization. It is mostly a matter of applying organic fertilizer in mid-November in the form of a layer of dead leaves and sheep manure. This will also serve as mulch for your plants. Nitrogen fertilizer in early spring will stimulate growth.
A rhododendron requires a lot of water in June and July, its active growth period. In early August, the plant will become lignified and start forming floral buds for the next year. Cut back on watering to encourage the buds to harden and discourage new growth before the first frosts.
Like most Ericaceae, rhododendrons do not require any special pruning. Any branches that are growing where you don’t want them or that are unattractive or damaged can be removed. For evergreen species, some of the branches at the base of the plant should be removed, however. Keep only enough to replace the oldest ones. Large-leaf rhododendrons should also be deadheaded.