Rhododendrons and azaleas usually bear their spectacular, large, often scented flowers in spring – but do you know the difference between the two?
Well, both are in the genus Rhododendron – but azaleas can be distinguished by having five stamens per flower (one per lobe), whereas rhododendrons have 10 or more (two per lobe). Azaleas can be deciduous or evergreen but rhododendrons are all evergreen. Finally there is a difference in size. Azaleas are small to medium shrubs – rhododendrons range from prostrate shrubs to huge trees.
Within this guide you will find information about some of the different varieties of rhododendron and azaleas, as well as information about how to successfully plant and maintain your own.
There are more than 28,000 rhododendron or azalea hybrids, as the plants readily cross breed. However, here are the most popular varieties for gardeners:
- Hybrids or hardy hybrids: What gardeners would consider a ‘traditional’ rhododendron with large flower trusses, some scented, blooming anywhere between January until July – often growing very large.
- Dwarf rhododendrons: Mainly alpine varieties, ranging from 20cm-80cm, flowering in April.
- Yakushimanum: Mound-shaped plants reaching 80-100cm, often with unusual leaves. Many brilliant flower colours available.
- Williamsianum: Unusual rounded leaves, 80-100cm.
- Deciduous azaleas: Sun lovers, growing up to 150cm, often scented, (especially R. luteum) with rich autumn foliage. They can be divided into Species (Sciadorhodion, Pentanthera, Rhodora, Sinensi or Brachycalyx); Ghent (the oldest hybrid group with multiple small flowers); Knaphill (large flowered hybrids, some of which are scented); Mollis (earlier flowering, more compact and spreading); Rustica (small double flowers, some scented). Other deciduous azaleas include Pratt hybrids, viscosum, occidentale and the Northern Lights series.
- Evergreen azaleas: Small, slow-growers, suitable for pots, up to 75cm, some are prostrate in habit, with autumn colour. Flowering season is usually May.
Choose a sheltered site with dappled shade, however, dwarf alpine species will tolerate full sun. Avoid sites exposed to early morning sun in spring, as this may damage frosted flowers.
The most important factor deciding whether rhododendrons or azaleas will grow well in your garden is the pH of your soil. They must have moist but well-drained, acid soil between pH 5.0 and 6.0 that is rich in organic matter. Reducing soil pH is not easy.
October or March/April are the best times for planting – dig in acidic organic matter before you begin (leafmould, rotting pine needles or composted bracken will work well). Don’t just concentrate on the planting hole – mix well around the surrounding soil. Don’t plant deeply, as rhododendrons are surface rooters. Apply a loose 8cm mulch of acidic organic material and water well to finish.
To keep your plants performing at their best, in spring, apply slow-release ericaceous fertiliser and renew the acidic mulch and keep well watered.
Rhododendrons grow best in areas of high rainfall, which is naturally slightly acidic. Using tap water, especially in hard water districts, is not good for the plant as it contains too much calcium which reduces the acidity around the plant’s roots. However, if rain water runs out, tap water is OK to use for a month or so in summer.
Rhododendrons don’t require pruning apart from removing dead wood and deadheading if practical. If the plant outgrows its space, cutting back is tolerated best from deciduous azaleas and rough-barked rhododendrons. After cutting back, mulch, feed and keep well watered.
Growing in Containers
If you have alkaline soil, grow rhododendrons and azaleas as container plants. Use ericaceous loam-based compost and repot every other year into fresh compost in spring. When not fully repotting, top dress the top 5cm of growing medium with fresh compost.
Most problems with rhododendrons and azaleas stem from the soil being too alkaline, drought or other extreme weather conditions. Here are signs to look out for:
- Non-flowering and bud drop: Flower buds actually start forming in late summer – dry conditions at this time can lead to a total bud formation failure or a partial formation, causing buds to dry up and dropping unopened in spring. Mulch and water thoroughly and regularly during dry periods in summer.
- Leaf drop: Older leaves droop and roll, then drop off, following extreme moisture conditions – drought or waterlogging. Newer leaves show browning at the leaf tip or edge. However, it is normal for the shrub to shed some older leaves in spring and summer.
- Leaf droop: Usually a response to severe cold, but they usually recover.
- Leaf scorch/flower damage: Often caused by windy, cold or wet weather.
- Yellowing foliage: Caused by nutrient deficiency known as chlorosis, an iron deficiency caused by high alkalinity in the soil.
- Pests: Vine weevil can be a nuisance for container-grown plants, as can rhododendron leafhopper and scale insects.
- Diseases: Relatively uncommon are bud blast, azalea gall and honey fungus.