Rhododendrons are a varied group of plants. Whether large or small, evergreen or deciduous, native or imported, rhododendrons are popular shrubs throughout their growing range. Rhododendrons grow best in dappled sunlight in rich soil with excellent drainage and a pH of 4.5 to 5.5. A loose, organic mulch will protect their fragile surface roots. Rhododendrons are reliable bloomers, so if yours aren't blooming, rule out these cultural problems.
Small rhododendrons may not bloom for several years simply because they're too young. One reason rhododendrons are expensive plants is that most nurseries don't sell these shrubs until they're mature enough to bloom.
Too much shade
Rhododendrons are understory woodland plants, but they don't bloom well in heavy shade. Their natural habitat is under deciduous trees, so they get filtered sunlight all day in summer and more sunshine from late autumn until early spring. They'll flower best if you replicate that environment. Morning sun and afternoon shade will also suit them. Rhododendrons that get insufficient light become spindly and don't bloom well.
After rhododendron petals fall, snap off the spent blossoms. This neatens up the plant's appearance, and directs the shrub's energy into making more flower buds for next year's display.
If pruning is needed on your rhododendron, do so immediately after flowering so the plant has time to put out new growth and make flower buds before midsummer. However, diseased or broken branches should be removed whenever found, regardless of season.
Flower buds are damaged by excessive cold, but you can lessen winter injury. Plant rhododendrons where they're sheltered from harsh winter winds. To ensure the shrubs are sufficiently hydrated before winter, give them a thorough watering in late November if autumn rainfall was light. Buds that have broken dormancy are particularly vulnerable to cold damage, so if a late freeze is forecast for your area, cover the shrubs with a tarpaulin or sheet before sunset to protect the buds. Most importantly, always verify the variety you buy is hardy in your area.
Use fertiliser specially formulated for acid-loving plants like azaleas and rhododendrons, which will have an NPK rating around 6-10-4. The fertiliser helps to keep the soil pH low. Also check that lawn fertiliser is not leaching into your rhododendron area, as grass food is high in nitrogen and encourages leaf growth at the expense of flowers. Rhododendrons are not heavy feeders, so fertilise them lightly in May or June, never later. The tender growth fostered by late feedings doesn't have enough time to harden off and is easily damaged by winter cold.
Sometimes it's normal
Some rhododendrons naturally flower in two-year cycles, with heavy bloom one year and light bloom the next. Just continue to care for your shrub as always and know that the display will be beautiful again next year.