Rhododendron a cool customer
Rhododendrons are a very diverse genus of over 800 species of evergreen and some deciduous shrubs widely distributed across the temperate to cool regions of the Northern Hemisphere. The genus takes in plants not only known as rhododendrons but also evergreen and deciduous azaleas, the Vireya section from tropical southeast Asia as far south as northern Australia, with New Guinea being home to more than 200 species.
The headquarters of the rhododendron is China which has by far the most species.
Rhododendrons are very variable in stature with some being ground hugging dwarfs adapted to exposed mountainous conditions, while others could correctly be classified as small trees usually as part of the understory in mountainous forests.
Some grow on the ground but quite a few grow on rocks or even as epiphytes in trees where they send their roots into the moss and detritus which collects on the branches or crotches of trees. The common factor in their habitat is moisture combined with excellent drainage.
Rhododendrons will never survive a poorly-drained soggy soil and getting the soil right is a key to success in their cultivation. They all prefer a soil with an acidic reaction between pH 4.5 to 6, high in organic matter for water retention.
A cool root run is also essential for most rhododendrons and this is best achieved by organic mulches which have the effect of conserving moisture and doing away with cultivation around the plants which could damage their delicate root system which is usually near the soil surface.
Pine needles make an excellent acidic mulch for rhododendrons which lets the water through readily. A light dressing of artificial fertiliser for acidic loving plants is beneficial to give plants a boost - I successfully used a light dressing of one part of sulphate of ammonia to two of superphosphate, as recommended by Peter Cox, the famous British rhododendron breeder.
Many rhododendrons are ideally suited to planting under deciduous trees which give them light in the winter months and shade in summer. In general the larger the leaves the more shade the plant will tolerate and remember that too much shade will also decrease the number of flowers on many rhodos. Many of he newer hybrids have been bred to with- stand open garden conditions so there is literally a rhododendron for all situations. While most prefer a sheltered site away from strong winds and frosts, many will also stand these conditions.
To get the best out of your rhododendron’s flowering, deadheading of the spent flowers before they develop seed capsules is beneficial. You will find you get more terminal shoots if these are removed, resulting in many more flowers next season.
An epiphytic Vireya rhododendron on Mt Kinabalu, Borneo.