Rhododendrons and Azaleas (both members of the same plant family) can be troublesome, if you don’t follow some basic rules. Once you realise that these plants need certain conditions met to flourish, you will be able to enjoy them at their best.
What Compost and Fertiliser?
The most important factor in the health of rhododendrons and azaleas is that they are acid-loving plants, thriving in a soil with a pH of between pH 5.0 and 6.0. If you don’t know the pH of your soil, use a home test kit. If your soil is alkaline (more than 7.0), choose compact varieties and grow them in large containers in ericaceous compost.
The soil needs to be well-drained, as the plants are shallow rooted but rich in acidic organic matter – dig in composted tree bark, leafmould, decomposing conifer needles, or composted bracken. Apply an 8cm-deep loose mulch of chipped conifer bark or other acidic organic matter. Renew the mulch each spring.
Tap water, especially in hard water districts, reduces acidity around rhododendrons’ roots.
Use rainwater for watering but if your water butts run dry, tap water is better than nothing for a month or so in summer.
Using the wrong fertiliser can lower the soil’s acidity; don’t use lime or other alkaline-based additives. Keep the soil’s pH level to about 5.5. Try to be organic – fish or seaweed fertiliser is ideal.
How to Prune?
If you’ve chosen the right size plant for the space, rhododendrons don’t need much pruning other than the removal of dead, damaged and diseased wood and deadheading.
If some shaping is required, they are classed as early flowering evergreen shrubs (pruning group 8) and need to be cut back lightly immediately after flowering.
However, if your plant has outgrown its allotted space, many rhododendrons respond well to being cut back hard, especially deciduous azaleas and rough-barked rhododendrons. After cutting back, mulch, feed and keep well-watered.
Sun or Shade?
Most rhododendrons and azaleas are natural forest dwellers on the edge of clearings and prefer a sheltered site with dappled shade. Avoid deep shade beneath trees.
Dwarf alpine species will cope with full sun as long as the soil does not dry out. Other varieties’ leaves may be scorched by too much sun.
Avoid frost pockets and sites exposed to early morning sun, which will damage flower buds.
What to do if Leaves Turn Brown?
All rhododendrons are evergreen but there are both evergreen azaleas (Japanese azaleas) and deciduous azaleas, which lose their leaves in the autumn – check which type you have.
- Crown and root rot: If leaves turn yellow first, the plant could be waterlogged. Rots flourish in soils with too much moisture and too little drainage. Foliage will wilt, turn yellow and eventually fall – by this time, it is too late to save the plant. Always plant in a location with well-draining soil and never plant deeper than the soil depth at which it grew in its container.
- Dieback: Caused in rhododendrons by Botryosphaeria dothidea and in Azaleas usually by Phomopsis. While leaves die, they may remain brown and crispy on branches until they fall off in summer. Plant it in partial shade and avoid drought stress and unnecessary wounds. Infected branches should be cut below the infection site and never put on the compost heap. Disinfect pruning shears between cuts.
- Leaf scorch: This happens when leaves dry out, become crispy at the edges and may fall off the plant. It can happen during drought or when rainy weather is followed by a dry, windy spell. Apply a 5cm thick layer of acidic mulch around an affected rhododendron, but don’t let it touch the plant’s stems. Water until the soil beneath is evenly moist.
- Winter burn: Drying winter winds can sap moisture from leaves, leading to a condition like leaf scorch. To prevent this, plant in a sheltered position or use shields to protect from prevailing winds. A thick mulch will also help to protect the plant.
Mandy Watson is a freelance journalist who runs www.mandycanudigit.com.
A plantaholic with roots firmly planted in working-class NE England, she aims to make gardening more accessible to the often excluded – the less able, the hard-up or beginners.
Advocate of gardening for better mental health.