Rhododendron growing guide
How to grow rhododendrons and azaleas
Rhododendrons are beautiful and majestic plants. Most species are native to Asia, but they can be found in the wild throughout the northern hemisphere. Rhododendrons have been a part of our gardens for centuries thanks to their beautiful foliage and large flowers that bloom in spring in striking shades of pink, purple, red and white. This shade-loving plant is not difficult to grow: all it needs to thrive is a suitable semi-shady site with plenty of moisture and acidic, well-drained, nutrient-rich soil.
There are thousands of native rhododendron species and varieties that come in different sizes, shapes, colors and properties. With dwarf, creeping, and medium-sized plants up to those the size of a smaller tree, and a variety of flower shapes, from globe-shaped to rounded and structured, rhododendrons can find a place in any garden.
Growers often refer to rhododendrons as azaleas and vice versa without knowing the difference between the two. Find out more about the difference between azaleas and rhododendrons. However, whether we call them rhododendrons or azaleas, they all have similar growing and care requirements:
How to grow rhododendrons and azaleas
Basic rules for growing rhododendrons:
1. Grow rhododendrons in semi-shaded location
2. Provide rhododendrons with acidic, well-drained soil and fertilize to resupply nutrients
3. Water abundantly, but avoid permanently waterlogged soil
4. Spread a layer of mulch around the plant
Soil for rhododendrons
The composition and permeability of the soil is very important for rhododendrons. They prefer acidic soil with a pH between 4.5 and 6. Yellowing leaves with green veins indicate that the soil isn't acidic enough. If such symptoms appear, the soil can be acidified with, for example, iron sulphide acidifiers (avoid aluminium sulphides, which are toxic to rhododendrons). Because of their soil pH needs, neither rhododendrons nor azaleas should be planted near concrete walls or pavements, as concrete increases the alkalinity of the soil over time.
The best soil for rhododendrons should be at least half organic matter (immature compost, peat, dry leaves, pine bark) and be well-drained, as rhododendrons and azaleas need oxygen for healthy root growth. Decomposing pine bark is a good choice for aerating the soil, which also prevents fungal root diseases. You can also use specialized rhododendron substrates with high organic content and a low pH, which are available in every garden center, or you can make your own 1:1 mix of peat moss and compost amended with pieces of bark, needles, dry leaves or other organic material.
You can find more details on rhododendron planting here: How to Plant Rhododendrons and Azaleas
Fertilizer for rhododendrons
Rhododendrons and azaleas aren't very demanding when it comes to fertilization. If they have nutritious soil, they can do without fertilizer altogether. In less nutritious soils, fertilizer can be applied in the pre-spring period, but only in very limited quantities as compared to how you may feed other plants, as over-fertilization could lead to root and leaf damage.
Rhododendrons also have great variability in their resistance to low temperatures. Most rhododendrons can be grown in USDA climate zones 4 - 8, but their cold tolerance varies between species. In general, evergreen rhododendrons are less hardy than deciduous species. Less hardy varieties can often be maintained by planting them in a well-sheltered location as well as adding a layer of mulch around the plant.
Rhododendrons and azaleas are not difficult to care for, but they do tend to need frequent watering, especially during periods of warm sunny days, as they have shallow roots that make them more susceptible to prolonged periods of drought. It is therefore advisable to mulch around the plant (5-10 cm thick, but not touching the trunk) to keep it moist and cool. Organic mulch such as pine bark, pine needles or sawdust is ideal as it adds nutrients to the soil as it decomposes. Dry flowers can be carefully removed to encourage the rhododendron to grow instead of directing its energy into seed production.
How to prune rhododendrons
The best time to prune rhododendrons and azaleas is immediately after flowering. This allows the rhododendrons to set flower buds for the following year and avoids making the plant susceptible to frost damage, which can happen if azaleas or rhododendrons are cut back in autumn.
Pests and diseases
Rhododendrons are generally very resistant to pests and diseases, especially if they're grown in their ideal conditions (well-drained, acidic soil, partial shade and plenty of moisture). The most common rhododendron disease is root rot caused by the phytophthora or armillaria fungi in poorly-drained soil. In addition to these diseases, rhododendron can be attacked by various bacterial leaf diseases (leaf spot, rust, black rust). The most effective way to deal with these diseases is to remove the affected parts of the plant and burn them.
The most significant pest of rhododendrons and azaleas in mild climates is a small, black beetle (Otiorrhynchus sulcatus) that consumes leaf edges, while its larvae attack the roots and trunk. Biological control, such as parasitic nematodes that attack the beetle larvae, is effective against the this insect. In addition to these beetles, rhododendrons can be attacked by whiteflies (Aleyrodidae), aphids (Aphidoidea), parasitic root-knot nematode (Meloidogyne), moths (Arichanna), and the strawberry mite (Tarsonemus fragariae). Chemical spraying or biological control in the form of parasites of these pests can be used.
1. Rhododendron planting guide: How to plant a rhododendrons