A Riot Of Rhododendrons
Rhododendrons are big, beautiful and run riot in the Lake District
Rhododendrons are one of those plants you either love or hate, their big blousy flowers being too much for a lot of people. They really need to be in a large garden as they only flower for a few weeks in spring and for eleven months of the year there is no interest. However there is a variety to suit most gardens except a hot, dry, sunny position. They range in size from 10cm (4”) to 30m (98’) tall, and from neat compact dome shapes to huge straggly trees. There is a huge diversity of colour from pure white to darkest burgundy, with some having felty undersides to the leaves in shades of rust and silver. They are widely distributed in the wild with the greatest species diversity being in the Himalaya region; China, Tibet and Nepal, where they are the national flower. They also occur in: Burma, Korea, Taiwan, North America and Russia. There are an estimated 1,000+ species and about 20,000 man-made varieties, including Azaleas.
They are all spring flowering and need moist but well-drained soil with plenty of organic matter incorporated and with a pH of 5.0 – 6.0. If you have alkaline soil choose one of the smaller varieties and grow it in a container or opt for one of the lime tolerant varieties. It is no good trying to change the pH of the soil by adding ericaceous compost or using chemicals as this will only work for a very short time and in the end you will always have alkaline soil. They really need to be grown in an area of high rainfall as they will take a lot of watering in summer especially if they are in dry soil. If you are watering them they need to be watered with rain water as this is slightly acidic whereas tap water is usually slightly alkaline, this would do for a few weeks but not as a long term prospect.
They can be grouped into several categories:
These are the hardiest and the smallest, growing to a height of only 80cm (32”) after 10 years. They are ideal for small gardens or for growing in containers. Varieties to try include: ‘Blue Steel’ a lavender/blue, ‘Patty Bee’ a delicate cream/primrose or ‘Scarlet Wonder’ a good strong red.
These are the dome-shaped varieties often growing to be wider than they are tall. A group of three of the same variety planted together will produce a show-stopping mound of colour in spring. They typically grow to a height of 75 – 100cm (30 – 40”) after 10 years. These include the yakushimanum, which flower in May, and williamsianum species, which flower in April. They are ideal for small gardens and containers. Recommended varieties include: ‘Cowslip’ a lovely ceamy white, ‘Bow Bells’ a soft baby pink, ‘Dopey’ a vibrant red/pink and ‘Golden Torch’ which is a lovely cream flushed with pink.
These varieties really need a large garden as they can reach a height of 140 – 200cm (56 – 80”) after 10 years. Recommended varieties include: ‘Albert Schweitzer’ which is pink with a magenta spotted throat, ‘Avalanche’ a good pure white with a dark burgundy throat, ‘Black Magic’ a really dark burgundy and ‘Firelight’ which is a gorgeous cream with dark peach/red speckling.
'Half Dan Lem'
These Rhododendrons are not recommended for the beginner. They are large shrubs/small trees reaching a height of 150 – 180cm (5 – 6’) after 10 years and becoming considerably larger with time. Most of them take several years to flower and need a sheltered woodland position. Many of them are scented, but the colour can be variable on the same plant. Recommended varieties include: Rhododendron arboreum a pale and dark pink, R. arboreum ‘Rubaiyat’ a hybrid which has a large dark red ball of flower, R. decorum a pure white scented shrub which is a little more tolerant of a less acid soil, R. irroratum a pale pink with dark pink spots, but this is one species which can have variable colour.
These typically have leaves up to 60cm (24”) long and need quite a large space in a sheltered woodland situation. They will reach a height of 180cm (6’) after a period of 10 years but will eventually grow to be considerably taller. These species are slow to start flowering, sometimes taking up to 10 years. They are not completely hardy so are best grown in frost free areas of the UK; young plants may need to be protected from frost. Recommended species include: R. falconerii which has white or the palest pink flowers with a dark burgundy throat, R. kesangiae whose flower is a huge dark pink ball.
Lime tolerant species
These are grafted onto an ‘Inkarho’ rootstock and can tolerate pH 7.0. They do need more feed than the acid loving species, and we recommend an ericaceous controlled release fertiliser. They can reach a height of 150cm (5’) after 10 years but will become taller after a period of time. Recommended varieties include: ‘Azurro’ a deep purple/blue, ‘Bellini’ with yellow flowers, ‘Fantastica’ is a lovely red/pink with a paler centre; this is a yakushimanum type so is ideal for growing in containers, ‘Old Port’ is a deep red/purple which eventually is wider than it is tall.
These are best grown in a cold greenhouse or conservatory or in mild areas of the UK; if you are in an urban environment and have a sheltered southerly aspect they could be worth trying. They grow best in very well-draining compost with their roots constrained in a pot. If grown inside they may put on a lot of growth so will need pruning hard back immediately after flowering. They reach a height of 100cm (40”) after 10 years. The pink and white varieties are often scented. Recommended varieties include: ‘Countess of Haddington’ scented with pink/white flowers, ‘Lady Alice Fitzwilliam’ a white, scented variety, ‘Saffron Queen’ has yellow flowers but unfortunately is not scented.
Most of the varieties available in garden centres are originally from montane woodland so do best when they are in dappled shade. They need organic rich, moist soil, but not boggy in winter, with a pH of less than 6.0. They are not suitable for light sandy soils.
Plant to the same depth as it is in the pot; rhododendrons are shallow rooted and need their roots close to the top of the soil. Enrich the planting hole with some ericaceous compost. Water well after planting.
Feed in spring with a controlled release fertiliser especially for rhododendrons. Don’t feed any later than June as the plant may concentrate on growing rather than making buds for next spring. Don’t use a fertiliser which is high in nitrogen as it will result in lots of leafy growth and no flowers.
Make sure they are well watered the first couple of summers after they have been planted, especially if there are prolonged periods of drought. As they are shallow rooted they cannot cope with low rainfall. If the plant is in a pot move to a shady position to reduce moisture loss, you may have to water twice a day when it is hot, try and use rain water as tap water is too alkaline. If your rhododendrons are planted on a slope you will need to keep checking the moisture levels even if it is raining as sometimes the water can just run off without penetrating the soil. It could be worthwhile building a little dam on the down-side of the plant with compost; this will hold the water long enough for it to soak into the soil.
Just remove any dead wood or take some stems out of the centre if it becomes congested, as you ideally need a good airflow through the plant to discourage fungal diseases. Deadhead after flowering taking care not to remove the growing tip from besides the dead flower.
Growing in containers
Choose a container which is just a little larger than the pot it is already in, as if it sits in a lot of sodden compost over the winter it is more likely to succumb to frost. Put a good layer of crocks in the bottom, at least 5cm (2.5”,) and raise the pot off the floor on pot feet or stones to ensure the water runs clear away from the container and doesn’t block the drainage hole with silt. Use ericaceous compost.
Every other year take the plant out of the pot and tease the compost away from the roots as peat-based composts eventually break down and become compacted, causing the leaves to turn yellow and brown and eventually die back; replace with fresh compost. If the plant is root bound plant it in a container just a little larger than the one it has come out of, if you want it to stay in the same container trim the roots back by a third and re-pot back into the same container using fresh compost. When you are not completely repotting replace the top 5cm (2.5”) with new compost.
Diseases and problems
Scale insects are flat, small disc-like insects that exude a sticky honey dew which in turn attracts a sooty fungus. You may have to treat the fungus with a fungicide before you can treat the insects with an insecticide. If the infestation is particularly heavy you will have to prune out the infected branches. You can wash the sooty fungus off with a soap and water solution and scrape the insects off with your fingernail if the infestation is only light, but if the plant is fairly large this would not be feasible.
Aphids cause the new leaves to become wrinkled; apply an insecticide, a soap and water mixture or wash them off with a jet of water. Don’t use a high nitrogen feed as this only encourages a lot of soft, sappy growth which aphids love.
Phytophthora ramorum (Sudden Oak Death) manifests itself as die-back of whole branches, particularly of new soft growth. There is no remedy, the plant and any surrounding leaves and twigs must be burned. Don’t feed a high nitrogen feed as this only encourages soft new growth. It is exacerbated by poor drainage and high soil temperatures so make sure the soil does not become compacted and ensure there is adequate airflow around the plant, container plants are particularly at risk. Keep an eye on nearby woody plants to make sure it doesn’t spread. If you do get the disease you have to notify DEFRA.
Mildew is characterised by pale spots on the upper leaf and darker spots underneath. Spray with a fungicide, such as Roseclear. Make sure the plant is kept healthy and has an adequate airflow through its branches.
Rust is a fungal disease shown by orange patches on the leaves, treatment is as for mildew.
Crinkly leaves are usually caused by sap sucking insects, such as aphids or occasionally the frost. Spray with an insecticide and don’t feed with a high nitrogen fertiliser.
Yellow leaves can be caused by a number of factors: compost too wet or too dry, starved of nutrients, soil too alkaline or too compacted.
No flowers can be the result of a number of causes:
Too much shade can cause the plant not to form flowers; it must be moved to a sunnier spot.
Fertiliser applied too late will result in vegetative growth and not flower formation.
Some species can take a number of years to flower so the plant may simply be too young.
The growing point may have been snapped off when deadheading.
If buds form then turn brown they may have been struck by a late frost or more probably have suffered from lack of water.
Plant death can be the result of several factors:
Poor soil conditions; such as too dry, heavy clay soil becoming too wet and boggy or becoming too compacted.
The plant has been planted too deeply; the roots need to be just under the surface of the compost. Make sure that if you have applied mulch it is kept well away from the stem and root ball.
The variety isn’t hardy.
Vine weevils are a common cause of death; the plant will look wilted even though it has been watered. Check around the stem where it meets the soil, the bark will be eaten. Check around the roots for small cream grubs with a brown head. There is not usually any cure if the plant has wilted, pull it up and destroy. If you see half-moon notches in the edges of the leaves, this is the adult beetle; before the plant wilts there is still time to save it, water in a solution of Provado vine weevil killer.
Honey fungus is fatal as there is no cure; it spreads by underground mycelium and occasionally by large clusters of golden fungi above ground. The shrub must be taken up and burned, make sure you don’t plant a woody shrub in its place.
Best places to see Rhododendrons:
Benmore Botanic Garden, Argyll & Bute, Scotland
Bodnant, Conwy, Wales
Mount Stewart, Co. Down, Ireland
Muncaster Castle, Cumbria
Lea Gardens, Derbyshire
Westonbirt Arboretum, Gloucestershire
Just a word at the end about Rhododendron ponticum; this is the violet/pink one you see growing wild. It looks pretty but is an absolute thug so don’t be tempted to put one in your garden. It is highly invasive and is laying waste to huge swathes of our beautiful countryside. It sends out suckers and forms a huge shrub up to 5m (16’) tall and effectively cuts out the light so nothing else can grow. It is not beneficial to insects, which has a knock-on effect on the birds. As there is no grazing underneath this means no food for mammals either, so all in all it is the equivalent of a barren desert. There is some evidence that it exudes a toxin into the soil which prevents other floral growth. The leaves are toxic to farm animals and honey produced from the flowers is toxic to humans. It is extremely difficult and time consuming to eradicate, with environmental agencies spending hundreds of thousands of pounds on eradication programmes in places such as Arran and Snowdon where it has colonised huge areas. In the main it is cut down and sprayed with herbicide, but this has to be repeated several times, there is more success in injecting each individual plant but this is extremely time consuming.
Click the links for more gardening information: 'Best choice of shrubs to give you spring colour in the garden', 'How to get your greenhouse ready for spring' and 'How do I use spring bulbs to add some colour beneath my deciduous trees and shrubs?'