How to grow rhododendrons

Rhododendrons are often grown alongside azaleas. Once categorised as different species, rhododendrons and azaleas are now placed together in the Rhododendron genus. They share many characteristics but there are some differences:

• Rhododendrons are almost always evergreen, while azaleas are usually deciduous
• Rhododendron leaves are thick and leathery, while azalea leaves are thin and often coated in fine hairs
• Rhododendron flowers typically have 10 or more stamens, while azalea flowers have five to six stamens
• Rhododendron flowers grow together in large 'trusses', while azalea clusters number around three, at the end of a stem
• Rhododendrons have fewer, but more stout, stems than azaleas.

How to grow rhododendrons

Rhododendrons are perfect for growing at the edge of a woodland border or shady spot. Plant them in humus-rich acidic soil in a sheltered spot in partial shade or full sun. Mulch rhododendrons annually and water well with rainwater.

While unlikely, if ingested by pets, all parts of azaleas and rhododendrons cause nausea, vomiting, depression, difficulty breathing and even coma. They can be fatal to dogs if eaten in large enough quantities.

Growing rhododendrons: jump links

• Where to grow rhododendrons
• How to care for rhododendrons
• How to propagate rhododendrons
• Rhododendron problem-solving
• Where to buy rhododendrons
• Types of rhododendron to grow

More on growing rhododendrons:

• Rhododendron leaf spot
• 10 plants for acid soils

Where to plant rhododendrons

Grow rhododendrons only if you have neutral to acidic soil – use a test kit to measure the pH of your soil and check neighbouring gardens to see if rhododendrons or other acid-loving plants grow well in your area. Then, chose a sheltered spot in partial shade to full sun. Many rhododendrons suit planting at the edge of a woodland border or shady area. Dwarf alpine rhododendrons work well in rock gardens. Smaller varieties look best at the front of a border, or in pots.

How to plant rhododendrons

All rhododendrons require humus-rich, neutral to acidic soil that's moist but well-drained. Rhododendrons are shallow-rooted plants, so make sure the roots are just below the soil's surface. Dig a hole that's wider than it's deep and back fill with leaf mould and peat-free ericaceous compost. Water well.

How to plant rhododendrons in pots

Use peat-free ericaceous compost for rhododendron in pots. Allow plenty of space between the rootball and the edge of the pot, around 15cm is ideal. Plant as you would in the ground, with the top of the rootball in line with the pot of the compost. Firm well and water thoroughly, ensuring the pot drains well. Feed with an ericaceous fertiliser throughout the growing season. Re-pot into a larger pot every two to three years.

Rhododendron care

Mulch rhododendrons in borders annually with an acidic mix of leaf mould, peat-free ericaceous compost or conifer bark chippings. For container-grown rhododendrons, replace the top layer of compost with fresh peat-free ericaceous compost, or repot completely in spring and feed with an ericaceous fertiliser. Water with rainwater as much as possible – tap water might contain too much calcium.

How to prune rhododendrons

There should be little or no need to prune your rhododendron, but if you need to remove dead or damaged wood, do so in late winter, when the plant is dormant. Deadhead spent blooms after flowering to improve the appearance of your shrub.

How to propagate rhododenrons

Species rhododendrons can be grown from seed, but cultivars should be propagated from cuttings and layering. Take heel cuttings of rhododendrons in late summer and autumn. Layering rhododendrons involves taking a low-growing branch and pegging it to the ground so it can take root. This is best done in late-summer.

Pests and problem solving

There are no pests affecting rhododendrons. However, the fungal disease rhododendron leaf spot can affect the leaves, causing them to drop.

Rhododenron ponticum is on the invasive plant list, so should not be planted in gardens and should be removed where possible.

Sometimes rhododendrons stop flowering. Find out why, and how to resolve the problem, in our Quick Tips video:

Rhododendrons to grow

• Rhododendron ‘Pleasant White’ - a dwarf, evergreen rhododendron with white flowers. It will suit a small garden, in a semi-shady border, or can be grown in containers
• Rhododendron ‘Scintillation’ - a large shrub, growing to over 2m. The pink and apricot blooms are very exotic and it will brighten up any area in part shade. A very hardy cultivar, it's a good for more exposed and northerly areas
• Rhododendron ‘Nancy Evans' - a gorgeous, golden-yellow compact rhododendron with leaves that are bronze when young but mature to a deep green. It looks good in a border and in containers
• Rhododendron ‘Vulcan’ -a compact variety, with long leaves and large, scarlet blooms that will suit containers or a mixed border. It needs a sheltered spot in partial shade, and protection from winter winds
• Rhododendron ‘Orakel’ - a hybrid with purple blooms with a crimson-red blotch. Hardy and compact it's ideal for a mixed border or container display. Protect from harsh winter winds
• Rhododendron yakushimanum 'Kochiro Wada' - one of a group of compact hybrids, this variety has white flowers, with silvery young leaves that mature to dark green. The Royal Horticultural Society has given it the prestigious Award of Garden merit (AGM)

Frequently asked questions

Why is rhododendron a problem?

Rhododendron ponticum is the only species of rhododendron that causes problems. Native to the Mediterranean, Asia, and China, it was introduced to Britain in the 19th century and planted in parkland as cover for game birds. It's highly invasive and has spread into woodland where it out-competes native plants and significantly reduces biodiversity. Rhododendron ponticum is also host to Phytophthora fungus species that cause Sudden Oak Death which has caused the death of thousands of oak and larch trees. It is hard to eradicate as this shrub spreads readily by seed, and by layering of the stems where they touch the ground.

Where is the best place to plant rhododendron?

Grow rhododendrons at the edge of a woodland border or shady area. Dwarf species work well in rock gardens and smaller varieties work well at the front of a border, or in pots. Remember that rhododendrons need acid soil, so check your soil's pH before planting.

How to look after rhododendrons?

Mulch annually with an acidic mix of leaf mould, peat-free ericaceous compost or conifer bark chippings. For rhododendrons in pots, replace the top layer of compost with fresh peat-free ericaceous compost, or repot completely in spring and feed with an ericaceous fertiliser. Water with rainwater as much as possible – tap water might contain too much calcium.

Why has my rhododendron died?

Rhododendrons sometimes succumb to over-watering. Too much water around the rootball prevents the roots from being able to take up vita nutrients and can, effectively 'suffocate' the plant. However, under-watering can also kill rhododendrons. Their roots are shallow, meaning they don't have a large tap root with which to draw water up from deep in the soil. Other reasons for sick or dead rhododendrons include planting too deeply (remember rhododendrons are shallow-rooted) and applying too much fertiliser.

To revive a sick rhododendron, first check its roots, and replant it if it's growing in too wet or dry soil, or if planted too deeply. Then, prune away damaged stems. Water sparingly and apply an ericaceous liquid feed as the flower buds are developing in spring, only.