How to Grow Rhododendrons
Glendoick Gardens Ltd, the UK's Best known Rhododendron Nursery.
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SITE & SOIL. Rhododendrons need acid soil. Soil acidity is measured according to the pH scale. Ideal is 4.5-6. Neutral is pH7 and higher than 7 alkaline. Most soil in Scotland is naturally acidic. It may have been limed for farmland, growing vegetables etc. Remedy by planting with peat and you can also use sulphate of ammonia applied before planting as too much can burn leaves. Be wary of cheap soil test kits: you may need to do several samples to get consistent results. If there are rhododendrons and other acid loving plants growing well nearby, then your soil is acid. Ask neighbouring gardeners.
SOIL PREPARATION. Rhododendrons need an open soil mixture. Very heavy (clay) and very fine particles (silt) are not suitable. To improve soil, making it more open (i.e containing air pockets) organic matter should be added: leafmould is the best. Alternatives are compost (own or bought), composted bark or conifer needles. There is little point in spending money on rhododendrons and azalea if you are not prepared to do some soil preparation. Improve the soil in an area much bigger than the rootball so there is room to grow. If drainage is good, then soil preparation need no more than 30cm (12in) deep. Peat can be used to improve the soil, but it is not as useful as products listed above. It is acidic and helps hold moisture but it has little structure, no feed and no mulching value. Ericaceous compost contains mostly peat.
CLAY SOIL. In heavy clay soil, a raised bed is best: 30-45cm deep on top of the clay soil. Make a soil-compost-bark-peat etc mix and plant into this. Glendoick Garden Centre Pagoda garden is an example.
DEPTH OF PLANTING Rhododendrons must not be planted too deep. The rootball should be just below the surface and no more. If you bury the rootball, you may kill the plant. Do not put thick layers of mulch on top of the rootball.
PLANTING Ensure plant is well-watered (but allowed to drain) before planting. Mix some organic matter (see above) into the existing soil as a planting medium. Soil should be firmed up around the roots but do not stamp on the rootball. This only compacts the soil and buries the plant. For bare rooted stock, October to early April is the best planting time. Container stock can be planted at any time but if planted May-August must be watered through the first growing season.
CONTAINERS & INDOORS Evergreen azaleas, yak & R. williamsianum hybrids are best choices for containers outdoors. You must ensure good drainage as rhododendrons hate being in waterlogged containers. Use ericaceous compost (with John Innes added if you can get it) and add some perlite, grit or bark. Ensure there are plenty of drainage holes and that they don't get blocked. Indoors: Tender scented varieties can be grown in greenhouse/conservatory and brought in to house in flower. Rhododendrons do not like central heating and seldom succeed as house plants. They need to be grown in a cool greenhouse. Make sure you have good drainage and do not allow compost to get too dry. Feed and repot when plant becomes very pot-bound. Do not over pot as Maddenia and Vireyas like to be a bit pot-bound. Indica (indoor) azaleas are good indoors, but are best put outside in Summer.
SHADE Rhododendrons tend not to grow and flower well under trees: tree roots will take most available moisture and lack of light creates straggly, shy-flowering plants. The further north, the more light is required: in Cornwall you can grow in more shade than Scotland. The worst trees are dense, greedy ones such as beech and sycamore. The roots of the tree will reach as far as the dripline (where the branches extend to). So you should be able to look up and see sky. If you can't, you have a problem. If you live in Scotland, ignore advice that advocate shade or part shade. Maximum light = maximum flowers. Good trees to grow with rhododendrons: Japanese maples, flowering cherries, Sorbus, Crataegus (hawthorn), Eucryphia, conifers: pine, larch, spruce (Picea), firs (Abies), cedar. Plant dwarf rhododendrons and evergreen azaleas in full sun in Scotland. Deciduous azaleas, larger hybrids and species can take some shade.
DEADHEADING Deadheading is largely a cosmetic exercise: only a few varieties produce seed at the expense of growth.
PRUNING. Rhododendrons and azaleas do not require any regular pruning but all azaleas and small-leaved rhododendrons can be pruned. (see example left of a hard pruned azalea with regrowth) This is best done immediately after flowering. You can prune most other rhododendrons back to where there is a circle of leaves (and therefore growth buds). Single growth buds can be pinched out in Spring to encourage bushiness. Larger varieties with smooth or peeling bark seldom respond to pruning.
WHAT CAN I PLANT WITH MY RHODODENDRONS? Anything you like as long as it does not take all the moisture from the roots. So avoid greedy trees, shrubs and ground cover such as roses, Vinca and heathers. In the wild rhododendrons grow with other Ericaceous plants such as Enkianthus, Kalmia (USA), Vaccineum, Gaultheria, Pieris, other shrubs such as Berberis, climbers such as Clematis, and perennials such as Aquilegia, Primulas, Meconopsis, Lilium Rheum, etc. For late summer colour, use Hydrangea, Eucryphia, Sorbus and other berrying plants.
WIND & SHELTER Varieties with large leaves, early growth or which are on the tender side for your climate require shelter from wind, particularly from south westerlies and north easterlies. Options if shelter is poor: 1. Plant a shelter belt of vigorous trees and shrubs. 2. Use rokolene (spun plastic membrane) or similar material to help plants establish. 3. Plant hardy wind-tolerant rhododendron varieties on the windward side and less hardy varieties inside these.
FEEDING Rhododendrons & azaleas do not need much feeding. If they look healthy and flower well, don't bother. If you are in a hurry or plants look yellow or sparse, you can feed with almost any granular fertiliser but beware of high nitrogen mixes as they can burn foliage. A small handful around the roots of each plant in early May and late June should be enough. Don't fertilise later as it encourages soft growth at the expense of flower buds. Liquid feeds are good for containers. We don't use sequestrene: it is only required to rectify iron deficiency usually because the pH is too high (alkaline). We like Vitax Q4 and Vitax Conifer and Shrub fertiliser. Growmore is OK but it it can burn so dont put on too much. Bone meal is of limited value.
CAN I PROPAGATE MY RHODODENDRONS AND AZALEAS?
Dwarf rhododendrons & evergreen azaleas are quite easily rooted in a propagator. With bottom heat, rooting will be quicker. In a cold frame rooting may take up to 6 months or more. Deciduous azaleas, hardy hybrids and species are difficult. Some species need to be grafted. Don't waste time growing seed from gardens: unless it has been control (hand)-pollinated, it will be hybridised and usually worthless.
Measured in our catalogue as H1-5. H1 tender (frost free/greenhouse) to H5 hardiest.
H5. Hardy hybrids, some species & dwarfs, yak hybrids and most evergreen and deciduous azaleas. Anyone who lives high up or in inland valleys: Clyde, Earn, Tay, Dee, Don etc. should concentrate on these varieties with some H4s. H5 areas tend to have late frosts, so choose mostly varieties which flower May-June to avoid losing flowers.
H4 Glendoick, Perth, Dundee, Coastal Fife, Edinburgh, Most of England, Coastal Norway, Belgium etc, not too far from the sea. With plenty of shelter inland, in a woodland garden, or on slope with good frost drainage. Lots of hybrids and species are H4.
H3. Glendoick in sheltered woodland site, or with some protection in cold winters. Coastal England and Wales, France, Spain, Italy. May suffer damage in severe winters or bark split from late frosts. Many big leaved species are H3.
H2. Indoors on east coast, fine outdoors in Cornwall, Ireland, Argyll and similar gulf-stream mild climates. Scented Section Maddenia species for conservatory/greenhouse.
H1 Indoors (frost free) only. This is for Vireyas (tropical species)
MOST COMMON RHODODENDRON PROBLEMS
Why has my rhododendron got yellow leaves?
-Drainage is poor: solution: lift plant and improve soil structure (see soil preparation) or move to better drained spot.
Plant is starved. Apply fertiliser May to Late June. (see under feeding)
Soil is too alkaline (unlikely in Scotland) apply sulphate of ammonia and plant with plenty of peat. Water with rain water, not tap water.
Why has it got crinkly leaves?
-Caused by late Spring or early Autumn frosts or sap sucking insects. The former is mainly cosmetic but if it keeps happening you may need to move a plant to a more sheltered site or cover it up in frosty weather. For insects use a contact or systemic insecticide.
I have spots on the leaves. What causes it?
<-Mildew: pale spots on upper leaf surface, brown/grey patches underneath: use fungicide. (Any rose fungicide will do: myclobutanil (systhane), fungus fighter, roseclear etc.
Rust black spots on upper surface, lower surface with orange patches: Control with fungicides rust->
-Black spots with no patches on leaf undersurface: some varieties eg ‘Mrs GW Leak' get this. Nothing to worry about, not a disease.
Why does my rhododendron not flower?
-Buds are formed and then turn brown, cause is usually frost. To avoid frosted buds, protect opening buds with fleece or plant later flowering varieties. Esp. azaleas.
-Flower buds do not form (flower buds are fatter than growth buds):
-Some varieties, especially species, take many years to flower.
-If planted in too much shade, will not flower well: move to sunnier spot.
-Fertiliser applied after late June: this encourages leaves, not flowers.
<- Bud Blast fungus Bud blast fungus: look for black bristles on the buds. It affects R. caucasicum hybrids such as 'Cunningham's White' in UK. Can be severe in Germany and other countries.
Why has my rhododendron died?
-Drainage/depth of planting: soil too heavy or compacted or rhododendron planted too deep. Dark brown dead roots= Phytophthora caused by poor drainage. (NOT the same as Sudden Oak Death.)
Variety not hardy enough? Check hardiness rating and for bark split. 2009-10 Winter was very severe all over Europe. Worst affected were plants in containers as the roots froze solid. Most rhododendrons and azaleas came through well and most of those damaged have grown from below, at Glendoick. Northern Europe had more severe conditions but what I observed in Germany in May 2010 was that most plants had come through well.
Honey fungus (roots are full of black bootlaces with white core). Fungus comes from old treestumps. Very little can be done about this.
<-Vine weevils? Look at stem just above ground. If bark has been eaten, usually girdling stem: vine weevil. White grubs in the soil.
Scale Insect. This is a serious problem in the UK now. Small white rectangular patches on the leaf underside and sticky black secretions on the upper surface. The only way to get rid of it is to prevent the adults hatching and laying on the young leaves in late summer and Autumn. Decis and petroleum oil are available to professional growers. For amateurs imidacloprid is the only legal weapon.
Further information on Rhododendron Pests and Diseases click on the links
www.rhododendron.org/disease.htm ARS website, disease control