Growing rhododendrons – everything you need to know

June 7th, 2020
Posted In: Gardening know how

You may not have thought about growing rhododendrons in your garden.

They come in brilliant colours, they’re easy care and they love shade.  If your soil isn’t right for them, they grow beautifully in pots, either wedged into your border in the right place or on a terrace.

A rhododendron could be just the plant we need for that shady spot.

So, to find out all about growing rhododendrons in our gardens, I went to the magnificent Grade 1 Listed Leonardslee Lakes and Gardens in Sussex to talk to head gardener Stephen Herrington.

Leonardslee Lakes & Gardens are now open to the public. You can buy tickets online.

Leonardslee have 240 acres of lakes, woodlands and gardens. They are known for their spectacular rhododendrons, some of which were bred by Edmund Loder, a famous Victorian plant hunter who previously owned Leonardslee.

Stephen Herrington, head gardener at Leonardslee Lakes & Gardens gives his tips on growing rhododendrons

The gardens have been famous for more than 100 years, but were closed for ten years in the mid 1990s. They were re-opened after what has been described as the biggest garden restoration in Britain. They are now well known for their spectacular spring colour, glorious woodland summer walks and outstanding autumn foliage.

There’s also a Victorian rock garden, a colony of wallabies that have lived here for 100 years and several species of deer.

Leonardslee Lakes and Gardens has extensive woodland paths and gullies – in autumn, the trees are a blaze of colour.

How to choose rhododendrons for your garden

I asked Stephen what we need to know about growing rhododendrons on our very much smaller patches of land.

‘Think about the size of your garden and how big you want your rhododendron to grow, says Stephen. You’ll need to check the ultimate size on the label or ask the grower! Some rhododendrons eventually tower 100ft up into the air!

Check what height your rhododendron will grow to before buying it. Some can literally grow to 100 feet high, like these in a woodland glade at Leonardslee.

‘Think about what you want the rhododendron to achieve in your garden. And what time of year you want it to flower.’

Rhododendrons can flower any time between January and the end of June (that’s mid-winter to early summer).

But in winter, you won’t be out in the garden very often, so you’d want to be able to see it flowering from your house.

Whereas you can enjoy later flowering rhododendrons while you are in your garden.

Rhododendrons flower from mid-winter to early summer – this pretty pink was in flower when I visited Leonardslee in early June.

‘And check whether you want it to lose its leaves in the winter or be an evergreen presence in your garden,’ says Stephen.

And also do you want it to be scented? Some rhododendrons (mainly the deciduous ones that lose their leaves in winter) are scented.

Where do rhododendrons grow best?

Rhododendrons grow best in acidic soil, says Stephen. They like dappled shade or even full shade. .They don’t want a really sunny spot.

And they like lots of water.  But they do well in pots, so if your soil isn’t right for them, you could grow them in a pot. Either wedge it in a border or put it on a terrace.

Although rhododendrons have brilliantly coloured flowers, they prefer shade or dappled shade to full sun.

When to plant rhododendrons?

Stephen mainly plants his rhododendrons in autumn now. ‘Historically, you planted rhododendrons in either spring or autumn, but there seems to be some very dry springs happening at the moment. So we think autumn works best. Don’t plant rhododendrons in frosty weather.’

Plant your rhododendron where you will most enjoy it. Here a bench at Leonardslee overlooks the lake and its rhododendrons.

Do rhododendrons grow in shade?

One of the big advantages of growing rhododendrons is that they really do like shady or semi-shady spots.  Certain varieties will take some sun, but not full sun, says Stephen.

This rhododendron at Leonardslee flowers in early spring – it is so beautiful. Its pale pink flower would stand out in a shady or semi-shady spot in your border.

Can rhododendrons grow in pots?

Rhodendrons grow very well in pots. A lot of people use pots to grow them if their soil isn’t suitable.

Plant in a good sized pot, about 15cm wide than the root ball, advises Stephen. Use an ericaceous compost and feed with a fertiliser suitable for acid-loving plants throughout the growing season. Re-pot into a larger pot every 2-3 years.

What about growing rhododendrons indoors or as house plants?

Stephen says that the Bureavii rhododendrons (Bureau rhododendrons) and the Maddeni rhododendrons come from temperate regions. ‘They have an amazing scent and will grow indoors, preferably in quite a cool room, such as a conservatory.’

But most rhododendrons are better outside.

What plants go well with rhododendrons?

At Leonardslee, they have echoed the rhododendrons’ past in the mountains of Asia. ‘We’ve planted them with conifers, azaleas and primulas,’ says Stephen. ‘But you could also use a rhododendron as a focal point in a herbaceous border, especially if you have a shady spot.

Rhododendrons work well with conifers, azaleas and other woodland plants. Seen on the stand of specialist rhododendron grower Burncoose Nurseries at RHS Chelsea Flower Show 2019.

Can rhododendrons be moved?

Yes, rhododendrons can be moved. ‘We’ve moved quite a few during the renovation of Leonardslee,’ says Stephen. ‘Do it in autumn and give the plant a really good water before you start. Check which rhododendron it is – if it’s one that can be cut down, then cut it back a bit.’

‘You can use a spade – we’ve used diggers quite a lot – and dig it up, making sure you leave lots of soil around the rootball.

Plant with lots of good organic matter and acidic compost. And make sure they’re well watered for a couple of years afterwards.’

Can they be cut back hard?

Some can. Others will die if you cut them back too hard. It depends on which one you have so if you know the variety, check it, advises Stephen.

One clue is to look at the base down the centre of the tree, he says. See if there is new growth coming up, and if there is you can probably cut back down to above a leaf node near the base.

If you don’t know what kind of rhododendron you have, then check around the base. If there is new growth emerging, then it’s probably safe to cut it down to a node above the new growth.

How to prune rhododendrons?

It depends on what type of rhododendron you have, so if you’re buying, keep a record of the name so you can look up the pruning details. Always prune after flowering, because otherwise you’re likely to cut the emerging flowers off.

Stephen says that they prune the azalea types with secateurs or a hedge- cutter. ‘We trim them back into the shapes we need, to keep them off paths or down to size in the historic Rock Garden. And we’ll even take a chain saw to some of the bigger, older rhododendrons.

There’s a rhododendron here called Rhododendron Luteum, which has semi-naturalised here and we cut clumps down to the ground every five years, then it springs back up again.

How to care for rhododendrons?

The main issue is watering, especially in the first few years after planting. And if they’re growing in a limited space, such as a pot, they’ll need feeding too, says Stephen.

But otherwise, they’re very easy care plants.

Are rhododendrons evergreen?

Some rhododendrons are deciduous and lose their leaves in winter. Others are evergreen.

So if you’re buying a rhododendron for some evergreen structure in your garden, check this.

If you want a scented rhododendron, then they are mostly deciduous. ‘But there is a group of rhododendrons called the Loderi, which were bred here in Leonardslee Gardens in 1901. They have big showy flowers and are highly scented.

This is one of the Loderi rhododendrons, bred a 100 years ago at Leonardslee. It’s evergreen and highly scented.

Can you take cuttings from rhododendrons?

Yes, we do take cuttings from some of our rhododendrons. Take them from new growth, and place in a free draining compost – we use a bark compost because that’s closest to what they would grow in naturally. Keep them moist – you can put a plastic bag over them for a mini greenhouse effect.

Quite a few rhododendrons have to be grafted, however. Once again, it depends on which rhododendron you have.

What causes a rhododendron to die?

Stephen identifies the number one cause as ‘not being watered.’

‘But there’s also a disease called phytophthora.’ This is one of two different fungus, which affect several trees and shrubs and are most severe on rhododendrons. The symptoms are brown, spreading lesions on leaves, wilting and dieback.

Phytophthora is a notifiable pathogen, which means that if you think your rhododendron has it, you should notify DEFRA’S Plant Health Portal.

Or your rhododendrons could die of old age. Some rhododendrons at Leonardslee are 100-150 years old. ‘That’s why we have to keep propagating them and re-planting new ones.’

Why isn’t my rhododendron flowering?

‘There are a number of reasons,’ says Stephen. ‘It may not be old enough to flower yet as rhododendrons don’t flower until a certain age (this varies, depending which one it is).

Or you may not be watering it enough.

Or it may be short of nutrients. ‘If you’re having trouble getting your rhododendron to flower, give it some potash and that will push it into flower,’ says Stephen.

Or finally, you may have pruned it at the wrong time. Always prune or cut back immediately after flowering, which is usually early to mid summer. Cutting back at any other time risks losing next year’s flowers.

Prune rhododendrons and azaleas immediately after flowering. This one at Leonardslee is flowering in June, so it won’t be pruned until the end of the month.

Are rhododendrons poisonous to cats/dogs/sheep?

Cats, dogs and sheep wouldn’t normally try eating them, so they’re unlikely to cause any problems.

Are rhododendrons deer resistant?

The deer only nibble our rhododendrons if they’re very hungry, says Stephen.

What about growing rhododendrons for bees?

Some rhododendrons are good for bees, but others have poisonous nectar. However, the bees know which is which, so you don’t need to worry about poisoning them.

There’s quite a lot of information available when you’re buying rhododendrons, so check this when you’re buying. ‘The most wildlife friendly rhododendrons are usually the deciduous ones,’ says Stephen.

A bank of rhododendrons growing opposite meadow grass at Gravetye Manor Hotel in May. It’s not far from Leonardslee, and also has wildlife friendly garden policies.

Are rhododendrons invasive?

‘Rhododendron ponticum (the Pontica rhododendron) is the invasive type,’ says Stephen. ‘It was brought over by the Victorians and planted on big estates as cover for pheasant and grouse shooting. Then it got out of control and spread everywhere. And it does poison the soil around it so nothing else grows. We have some here and we dig it out and burn it whenever we can.’

So it’s important not to buy Rhododendron x ponticum or any variants bred from it, when choosing a rhododendron for your garden. Pontica rhododendrons are now listed as an ‘invasive, non-native species’, but you can still buy them.

More about Leonardslee Lakes & Gardens

Leonardslee is now open to the public every day, observing social distancing guidelines. You can buy day entrance tickets in advance online, or take out annual membership. And takeaway snacks and drinks are available to eat within the gardens.

With 240 acres, it’s easy to maintain social distancing when visiting Leonardslee. Here the last of the rhododendrons is reflected in one of the lakes.

Summer opening hours are daily 9am-5pm, with the last entry at 3..30pm

As well as the woodland walks, lakes and Edwardian rock garden, the estate is also a wildlife habitat. There are deer, badgers, weasels and stoats, plus the famous wallabies and a range of birds, including ducks, green woodpeckers and kingfishers.

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