The genus Rhododendron which includes rhododendrons and azaleas contains many beautiful, colourful, long lived, spring flowering shrubs. With a few exceptions rhododendrons are evergreen species while azaleas are deciduous species.
SITE AND SOIL
Rhododendrons prefer a lightly shaded environment that provides sufficient sun for bud formation and compact growth while providing shade during the hottest part of the day in early spring and summer when sun can cause scorching or burning of leaves. An easterly or northerly exposure is best for planting rhododendrons. Avoid a southern exposure as this exposes plants to the winter sun making the sap flow and causing serious damage if there is a sudden drop in temperature. Planting sites should have good air drainage to reduce the risk of late spring frosts and early fall frosts. Protection from prevailing winds which can cause desiccation of the plants is also necessary.
Rhododendrons prefer light to medium, acid (pH 4.5-5.5) soil that is high in organic matter. They require well drained soil. In a poorly drained soil, planting in a raised bed above the existing grade will allow for proper drainage.
Plant rhododendrons in the spring as this allows the formation of new roots before the warm weather begins. Place the plant in a hole wider than the root ball and plant at the same depth originally planted. Firm the soil around the fine hair-like roots carefully to avoid air spaces around the roots. Do not fertilize newly set plants.
Well mulched rhododendrons rarely require supplemental watering. During prolonged dry periods or if growing in light sandy soils they should be watered thoroughly about once a week. Newly planted rhododendrons should be watered so they are always moist but never soggy.
Winter protection is usually required for up to 3 years while the new rhododendrons become well established in their location. A teepee-like structure constructed with 3 or 4 evergreen trees or branches with their points forced into the ground and tips tied together provides adequate protection or use a burlap covering around individual plants. Snow fencing alone or with a polyethylene plastic sheeting attached to it also gives effective wind protection. Cover the plants just before freeze up in the fall and uncover after all the frost is out of the ground in the spring.
Mulch is used around rhododendrons to protect the roots from sudden temperature changes, and to absorb and retain moisture. The mulch should be heavy enough to hold in the wind and attractive so it will enhance the planting. Peat is not a good mulch because it dries out and blows away exposing the fine rhododendron roots to the elements. Pine bark, woodchips, or pine needles, all provide adequate protection for rhododendron roots.
Rhododendrons have low nutrient requirements so most soils can supply adequate nutrients to the plants. If the soil is poor, nitrogen is most often deficient, this is indicated by poor growth or pale green or yellow leaves. To correct this situation spread ammonium sulfate or urea around the plant area covered by the branches at a rate of 28g ammonium sulfate, or 14 g urea per bush per year until the shrub matures. Mature bushes should not be fed more than 230g ammonium sulfate or 110 g of urea per year.
Do not fertilize after late June as this stimulates late season growth that may be damaged in the winter.
PRUNING AND SHAPING
Rhododendrons should be pruned in early spring to shape the shrub. After flowering is complete the flowers should be removed carefully to avoid damaging buds. The vegetative buds should also be pinched to increase branching. When removing a branch cut as close as possible to a whorl of shoots or leaves so no stub is left.