Rules for rhodos
Dick observes that with nearly 1,000 species and close to 30,000 cultivars, there is a rhododendron suited to just about any locale, if the home gardener is willing to do a little planning and preparation.
In Central Canada, where summer and winter temperatures are more extreme than on the coasts, plant rhododendrons in light shade to help them cope with summer heat. Protection from searingly cold winter winds is essential, as many rhododendrons are evergreen and will become desiccated. Snow cover will protect dwarf varieties, but if you're in a low-snow area, build a protective, burlap-wrapped frame around the plants, making sure the frames aren't in contact with the limbs.
Rhododendrons demand excellent drainage, which points to their origins as hillside plants. Poor drainage can quickly kill a rhododendron or make it more susceptible to attack from insects or disease.
All members of the Ericaceae plant family prefer an acidic soil, ideally with a pH level between 5 and 5.5. Amend alkaline soil with ammonium sulfate, available at garden centres, or with slowly acidifying materials such as peat moss, pine needles, aged manure or woodland litter. Additional organic matter also helps to improve soil drainage around your rhododendrons, and mulching will protect the roots from temperature extremes. Dick is emphatic that growers not use aluminum sulfate, as the aluminum ions build up in the soil and become toxic to rhododendrons, gradually killing them.
Rhododendrons don't specifically need pruning, but deadheading after flowering is recommended. This helps the plant concentrate energy on developing future flower buds rather than seeds (unless, of course, you're interested in creating your own hybrids).
Although rhododendrons have many enemies, ranging from botrytis blight and powdery mildew to rhododendron midges and plant girdling from woodpecker damage, good cultivation practices can prevent many problems. Dick's biggest difficulty at his plant farm in Bayport is damage from deer browsing on leaves and buds, despite the fact that rhododendrons are mildly toxic.