Rhododendrons like an acidic soil, mulched with a shredded bark. The site should be partially shaded, especially from harsh winter sun, and away from strong winds. They are shallow rooted plants, thus it is necessary to mulch them, and avoid planting in a high traffic area. Though rhododendrons like a moist, well-drained soil, they dislike a wet soil which can lead to root rot. When digging to plant rhodendrons, dig up the area, not just the hole, and amend with moistened peat moss. After planting, mulch and water in well. This will provide the best environment for root development. It will reward you with stunning blossoms and shiny full green foliage.
The variety of rhododendron pictured above is a PJM Rhododendron. It grows 3-6 feet tall and wide but does so slowly. It may take 10 years for a rhododendron to reach 4 feet (1.2 m). It has large clusters of violet-pink blossoms early to mid May (zones 3/4) and the flowerbuds are hardy to -40C (-40F). Rhododendrons bloom for two to four weeks in late spring in our area. Mine was two years old in my garden when it bloomed for the first time. It was a scattering of blossoms; but this year, one year later, it is covered in blooms!
The leaves take on a mahogony tone in late fall and will curl in the cold, which is the plant's natural means of retaining moisture. When temperatures are above zero again, the leaves will unfurl and revert to the deep glossy green loved by rhododendron enthusiasts. An application of an anti-dessicant spray before temperatures reach 0C (32F) is recommended. An anti-dessicant spray is a foliar spray applied to the plant to help prevent loss of moisture. As the rhododendron retains its leaves all year round, it is highly beneficial to retain the moisture therein through winter.
My PJM rhododendron is planted near my deck, out of drying winds, facing west. It receives shade late in the afternoon from the neighboring trees. Full summer sun would be ideal, in Alberta, as long as there is shade or protection from winter sun. I must emphasize the need for mulching with a finely shredded pine or fir mulch as it is essential for retaining an even soil moisture level and, over time, contributes to a more acidic soil.
If necessary, prune your rhododendron immediately after flowering, just like any spring flowering shrub or tree. Sometimes you will be lucky and get a second flush of blossoms, though more sporadic, late in summer. I have not had it happen personally, but I have seen it in the greenhouse. Of course, it is warmer in the greenhouse. (Yes, the greenhouse I am referring to has a ventilated enclosed tree, shrub and perennial area that is separated by a courtyard from the other greenhouses catering to annuals and vegetable plants. But that is a story for another post.)
Fertilize your rhododendron once a month with an evergreen fertilizer, like 30-10-10. In Alberta, begin fertilizing in May and do your last application at the end of July.
Snow cover in winter, particularly in areas where cold winters are the norm, is essential. If possible, shovel a light snow onto your rhododendron through the winter months to insulate the plant. It is also recommended to plant this near a house where it will benefit from the warmth of the foundation, but don't plant under an overhang where it will not receive rain.
In their natural habitat, rhododendrons prefer to grow in an acidic soil, sometimes among evergreen and deciduous trees that provide a dappled shade and protection from wind. It is not uncommon, however, to find Pacific Rhododendrons growing in sand dunes near the ocean, though this is not a preferable site. What this says is it prefers a shallow well-draining soil. In a somewhat shaded site, the rhododendron will become somewhat leggy, more tree-like, while in full sun, the plant grows in a condensed compact manner.
With over 100 new varieties of rhododendron introduced every year, it is becoming easier for the home gardener to find a variety suited to their gardening zone. Catawba rhododendrons are hardy to zone 3 and, according to Lois Hole's Favorite Trees and Shrubs, PJM rhododendron is also hardy to zone 3. Note the hardiness of the flower bud, which is a great indicator of the hardy nature of the PJM rhododendron.