How to Grow Rhododendrons

Rhododendrons light up woodland gardens with abundant blooms in bright shades of pink, purple, red and orange. Their shapely branches and leathery green leaves add structure and height throughout the year. Rhododendrons are easy to grow, as long as your garden has the acidic soil they need to thrive.

Rhododendrons are flowering evergreen shrubs with trusses of large blousy flowers that bloom in early summer. Most rhododendrons are medium to large shrubs, with some growing up to six metres high and wide, though most garden varieties top out at around 2.5 metres. You can also buy smaller cultivars to grow in smaller borders and pots. Rhododendron ponticum is classed as an invasive species, but there are plenty of other less harmful varieties available.

Gardeners often get confused about the difference between azalea and rhododendron shrubs. Find out how these two plants differ and which is best for your garden in our azalea vs rhododendron guide.

 

Where to plant rhododendron

Rhododendrons prefer dappled shade and shelter from cold winds. Their ideal environment is a woodland garden or border, though they can cope with more open, sunny locations as long as they're not too exposed. Avoid deep shade, as plants will get leggy and produce fewer flowers.

If you have the space, rhododendrons look spectacular when planted in groups and when interspersed with hydrangeas for flowers from late spring through to autumn. They're excellent screening plants and effectively block noise. You can plant upright varieties like Rhododendron 'Nova Zembla' to form a rhododendron hedge.

Rhododendrons need acidic soil, ideally between pH 4.5 and 6.0, though plants grown on Inkaro rootstocks (e.g., Rhododendron 'Bloombux' cultivars) are more tolerant of neutral soils. Don't try to plant a rhododendron bush in alkaline soil – it will just die. The soil needs to be moist but well-drained. Rhododendrons can suffer in waterlogged ground, so you may need to dig drainage channels to help water drain away if the soil is too wet and heavy.

As rhododendrons are shallow rooted, you can plant them in raised beds filled with acidic soil or ericaceous compost, leaf mould and top soil. You can also plant dwarf varieties, such as Rhododendron keleticum or Rhododendron yakushimanum cultivars like 'Dusty Miller' in pots filled with peat-free ericaceous compost.

 

When to plant rhododendron

Mid autumn (October) or spring (March or April) are the best times to plant rhododendron bushes, as the soil tends to be moist and warm. Avoid planting if the soil is waterlogged, frozen or very dry.

 

How to plant rhododendron shrubs

Rhododendron roots need to be near the surface, so don't plant them too deeply. Dig a wide hole and position the rhododendron plant so the roots are just covered with soil. Fill the hole with a mix of acidic soil or compost and leaf mould and water the plant well. Add a three-inch layer of acidic mulch, such as conifer bark chippings. 

When planting a rhododendron in a pot, choose a container that has space for the roots to grow and good drainage (you can add some broken crocks at the bottom to help with this). Mix some fertiliser into peat-free ericaceous compost and plant the rhododendron so the top of the rootball is at the compost level. 

 

How to care for rhododendrons

Rhododendrons need plenty of water to thrive. Use rainwater to water plants during dry spells, as tap water is too alkaline. That said, if you have nothing else, it's better to water them with tap water than leaving the soil to dry out. The flower buds develop in late summer, so it's particularly important to ensure rhododendrons have plenty of moisture at this time of year.

Apply a thick layer of acidic mulch annually. You can use conifer bark chippings, leaf mould, bracken mulch or ericaceous compost. Rhododendron bushes grown in borders shouldn't need additional fertiliser.

Rhododendrons in pots will need repotting every other year in early spring. This prevents the plant becoming pot bound and ensures good drainage. In the intervening year, you can remove the top couple of inches of soil and replace it with fresh compost. Feed rhododendron pot plants with an ericaceous fertiliser.

Rhododendrons are hardy and can be grown outdoors across the UK. You may see leaves drooping on rhododendron bushes when it's very cold, but they usually recover when the weather turns milder.

How to prune rhododendrons

Rhododendrons don't generally require any pruning unless you need to remove dead wood, renovate an old shrub, or you want to keep a neat shape. You should carry out rhododendron pruning after flowering. Remove any dead or diseased branches and lightly cut back wayward shoots if necessary for shaping.

Rhododendrons form their buds the summer before flowering, so deadheading rhododendrons isn't necessary to encourage more blooms. However, removing spent flowers also removes the seedheads, which means the plant puts its energy into growing and developing flower buds, so deadheading may mean you get more flowers the following year. If you want to restrict the growth of your rhododendron shrub, you may prefer not to deadhead flowers.

If you need to renovate a rhododendron bush, cut it back hard during March. This will likely affect flowering for a few years, but you should end up with a smaller, bushier shrub. Some smooth-barked rhododendrons don't respond well to hard pruning. 

 

How to take rhododendron cuttings

You can propagate rhododendrons by taking semi-ripe cuttings or by grafting or layering. Rhododendron propagation from seed is possible, but only species rhododendrons will grow true.

Take semi-ripe heel cuttings in late summer. To take a heel cutting, snip the stem part way through then peel it off to leave a "tail" of bark. This helps the cutting to root. Remove lower leaves and cut large leaves in half to reduce moisture loss. Dip the "heel" in hormone rooting powder and insert it into a pot of ericaceous compost mixed with perlite. Cover with a plastic bag and keep in a warm, light place.

 

Rhododendron problems

If your rhododendron is not flowering, this is most likely because there was a dry spell the previous summer when the flower buds were forming. There isn't much you can do other than making sure you water and mulch throughout dry spells in late summer, so it flowers better next year.

Yellow leaves on a rhododendron usually indicate the plant is suffering from nutrient deficiencies. This can happen if the soil pH is too high. To prevent nutrient deficiency, water plants with rainwater rather than tap water, especially if you're in a hard water area, refresh the compost for container-grown plants or add some specialist fertiliser. Over fertilising can cause problems, so don't give plants too much food.

Rhododendrons are sensitive to drought, and hot, dry weather can cause rhododendron leaves to drop if the plants aren't watered. Waterlogging can also cause leaf drop – if this is an issue, you may need to dig drainage channels to allow water to drain away from the shrub.

Bud blast is a fungal disease spread by rhododendron leafhopper insects that causes flower buds to turn brown and die. Remove any affected flower buds immediately.

Powdery mildew can cause pale or reddish-purple patches to form on leaves. Fungal infections can also result in purple or leaf spots on evergreen rhododendrons.

Rhododendrons can also be susceptible to honey fungus and root rot, both of which can kill plants. The best preventative measure you can take is to ensure the plant isn't sitting in waterlogged soil.

 

Our favourite varieties of rhododendron

Rhododendron 'Nova Zembla' is a beautiful deep pink rhododendron that grows to around two metres. It's a sturdy upright shrub that's good for screening and rhododendron hedges.

Rhododendron 'Percy Wiseman' is a popular yak hybrid that's renowned for its consistent flowering. It produces trusses of pretty pink and cream flowers and will grow well in sun as well as partial shade. 

Rhododendron 'Cunningham's White' is a white rhododendron that's more tolerant of neutral soils than other rhododendrons. Its compact habitat makes it a good screening or hedging plant. 

Rhododendron 'Catawbiense Grandiflorum' is a large, vigorous purple rhododendron that has orange speckles on the upper parts of its lilac petals.

Rhododendron 'Scarlet Wonder' is a dwarf rhododendron that produces bright scarlet flowers in late spring and early summer. It's suitable for growing in borders or pots.

 

FAQ

When does the rhododendron bloom?

Rhododendrons flower in May and June, with some early-flowering varieties, such as Rhododendron 'Scarlet Wonder', blooming in April.

How do you prune rhododendrons?

Rhododendrons don't need regular pruning, but you can deadhead spent flowers and trim any long shoots to maintain the shrub's shape immediately after flowering in late spring. If you need to hard prune a rhododendron, the best time to do this is in March.

Are rhododendrons poisonous to dogs?

All parts of the rhododendron plant are toxic to dogs, cats and humans. Chewing on leaves or flowers can make pets very unwell, and in severe cases, could be fatal. If you suspect your dog has consumed part of a rhododendron shrub, contact your vet immediately.

Do rhododendrons like shade?

Rhododendrons like dappled or partial shade, such as that found on the edge of woodlands, but they won't thrive in deep shade. If planted in too much shade, the plants will grow taller and won't produce as many flowers.

Should you deadhead rhododendrons?

Deadheading rhododendrons helps encourage the plant to put its energy into growing bigger and producing flower buds for the following year. It also makes the plant look neater. However, you don't have to deadhead rhododendrons, and it won't have any effect on the current year's blooms.