The many Cox books are probably the best source of in depth information about how to grow Rhododendrons and azaleas. But the fundamentals are pretty straightforward and this is a concise summary of the salient points from Glendoick Nursery:-

SITE & SOIL. Soil pH (acidity of soil) is ideally pH 4.5-6. Almost all soil in Scotland is acidic. If it is not, it may have been limed for growing vegetables etc. This is easily remedied by adding a percentage of peat into the soil. One alternative is to use sulphate of ammonia. (you can’t use much of this when plants are in situ as it will burn lvs, so it is best done a few months before planting.)

SOIL PREPARATION. Rhododendrons need an open soil mixture. Very heavy (clay) and very fine particles are not suitable. To render soil more open (i.e containing air pockets) organic matter is added: leafmould is the best. Alternatives are compost (own or bought), composted bark, conifer needles etc. There is no 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 be less than 12” (30cm) deep. You do not need peat: it has no structure, no feed and no mulching value. It is useful as an acidifier and for containers.

CLAY SOIL. If you have heavy clay soil, the best thing to do is make up a bed on top of the clay soil with compost, bark, peat etc and plant into this. This is what we did in the Glendoick Garden Centre Pagoda garden.

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 will kill the plant.

PLANTING Make sure plant is well-watered before planting. For bare rooted stock, October to early April is the planting time. Container stock can be planted at any time but if planted May-August must be well watered in the first growing season. Soil must be firmed up around the roots but do not stamp on the rootball. This only compacts the soil and buries the plant

CONTAINERS: Evergreen azaleas, yak hybrids and compact hybrids are best subjects for containers. Tender scented varieties can be grown in conservatory and brought in to house in flower. Use ericaceous compost with added perlite. Rhododendrons do not like central heating and will die if kept as house plants whereas Indica Azaleas are of course perfect. Make sure you have good drainage and do not allow compost to get too dry. Feed and repot when plant becomes rootbound. Do not over pot.

SHADE: Rhododendrons will not grow and flower well under trees: the roots will take the moisture and the lack of light will make plants straggly and shy flowering. The worst trees are 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 all books/advice which say shade or part shade. Maximum light = maximum number of flowers. Good trees to grow with rhododendrons: Maples, Japanese and others, Cherries, Sorbus, Conifers such as Larch and Spruce, Hawthorn, Eucryphia.

Plant dwarf rhododendrons and evergreen azaleas in full sun in Scotland. Deciduous azaleas, larger hybrids and species can take some shade.

DEADHEADING & PRUNING. This is largely a cosmetic exercise: only a few varieties produce seed at the expense of growth. Rhododendrons and azaleas to not require any regular pruning. All azaleas and small-leaved rhododendrons can be pruned. 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.

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 ground covers like heathers, grasses. 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, Lilies, Rheum, Orchids, 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 tender require shelter from wind, particularly from south westerlies and north easterlies. If you have no shelter there are several options. 1. Plant a shelter belt of vigorous trees and shrubs. 2. Use rokolene 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 fertiliser but beware of high nitrogen mixes as they can burn foliage. A small handful (granular) 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. You can also use liquid feed. We don’t use sequestrene: it is not required unless there is iron deficiency.

CAN I PROPAGATE MY RHODODENDRONS AND AZALEAS?
Dwarf rhododendrons & evergreen azaleas are quite easily rooted in a propagator. With 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 need to be grafted. Don’t waste time with seed unless it has been control-pollinated, otherwise it will be hybridised.

HARDINESS Measured in our catalogue as H1-5. H1 for frost free/greenhouse, to H5 the hardiest.

H5. Hardy hybrids, some species & dwarfs, yak hybrids and most evergreen and deciduous azaleas. H5 areas tend to be well inland and tend to suffer late (and early Autumn) frosts, so choose most varieties which flower in mid May-June to avoid damage to flowers.

H4 Glendoick, Perth, Dundee, Coastal Fife, Edinburgh etc, not too far from the sea or with plenty of shelter inland: woodland garden, or on slope with good frost drainage. Lots of hybrids and species are H4.

H3. Glendoick in sheltered woodland site. Some protection from trees, or on a south or west wall. 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 Argyll and similar mild climates. Scented Maddenii species for conservatory/greenhouse.

H1 Indoors (frost free) only. This is for the Vireyas.

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?

• This can be caused by late Spring or early Autumn frosts or sap sucking insects.

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: systhane, fungus fighter, roseclear etc.)
• rust black spots on upper surface, lower surface with orange patches: use fungicide
• black spots with no patches on leaf undersurface: some varieties eg Mrs GW Leak suffer from this; it is nothing to worry about and is not a disease.

Why does my rhododendron not flower?

• if 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. There is also a disease: bud blast fungus which is characterised by black bristles on the dead buds)
• if 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.

Why has my rhododendron died?

• drainage/depth of planting: soil too heavy or compacted or rhododendron planted too deep. Dark brown dead roots = phytopthora caused by poor drainage. Some varieties require really sharp drainage. Larger yellow hybrid rhododendrons require particularly good drainage.
• vine weevils? Examine the stem at ground level. Vine weevils tend to girdle the stem, eating off the bark. They can also eat the roots.
• honey fungus (roots are full of black bootlaces with white core) comes from old treestumps.
• the variety may not hardy enough? Check the hardiness rating and look for signs of bark split.

Why have my old rhododendrons reverted to ponticum (wild rhododendron)?

• This is because prior to 1950 all were grafted onto R. ponticum which sends up numerous suckers. If these are not cut or broken off, the R. ponticum will eventually take over. No Glendoick rhododendrons are or have been grafted onto R. ponticum For the few varieties we graft, rootstocks which throw few suckers are used.

Can I grow rhododendrons without peat?

• Yes you can, though I would not recommend growing in containers without a percentage of peat added. At Glendoick in our nursery cultivation we use very little peat. We prepare soil by adding a mix of organic matter including composted bark, needles, leaf-mould and topsoil which encourages rhododendrons to produce very healthy root-systems. Peat is an acid, moisture-retentive substance and is cheap. But it is certainly not a requirement for happy, healthy rhododendrons.

CHOOSING VARIETIES FOR DIFFERENT PURPOSES

EASY DWARFS: To 2ft Curlew, Crane, Dora Amateis, fastigiatum, Intrifast, calostrotum ssp. keleticum, Patty Bee, Ptarmigan, Ramapo, Scarlet Wonder.

EASY SEMI-DWARFS & 'YAKS': 3-4ft Bruce Brechtbill, Elisabeth Hobbie, Fantastica, Linda, Percy Wiseman, Praecox, Unique.

EARLY-FLOWERING: Nobleanum, dauricum Midwinter, Christmas Cheer. The following have frost-hardy flowers or buds: lapponicum, Ptarmigan, hippophaeoides, Blue Silver, anwheiense.

LATE-FLOWERING: hemsleyanum, Polar Bear, Azaleas: occidentale, nakaharae, Lemon Drop, Sparkler, Racoon.

BEST FOLIAGE: colour & leaf shape: Graziela, roxieanum, (narrow leaves), Elizabeth Red Foliage (red new growth), lepidostylum, campanulatum ssp. aeruginosum, pronum (blue leaves), Ever Red, Wine & Roses, (red leaves) Bluecalypytus (blue leaves)

BEST FOLIAGE: indumentum: bureavii, pachysanthum, rex, Golfer, Ken Janeck, Viking Silver, yakushimanum, falconeri ssp. eximeum.

BEST WHITE: decorum, Crane, Alena, Cunningham's White, Loderi (with shelter), Lucy Lou, Dora Amateis, Ptarmigan, Glendoick® Glacier, Panda (azaleas)

BEST PINK: Christmas Cheer, dendrocharis, orbiculare, Linda, Pintail, Fantastica, Canzonetta (azalea).

BEST YELLOW Dwarf/semi dwarf: Curlew, Chiff Chaff, Patty Bee, Swift, Loch Earn.

BEST YELLOW LARGER: campylocarpum, wardii, Goldkrone, Nancy Evans, luteum, Klondyke, Lemon Drop. (dec. azaleas) Note: larger yellow rhododendrons need perfect drainage. Add grit or coarse bark or plant on top of rather than in heavy soil.

BEST RED: Dopey, Elisabeth Hobbie, Erato, Grace Seabrook or Taurus, Jean Marie de Montague, Vulcan. Evergreen azaleas: Squirrel, Glendoick Crimson, Glendoick Garnet, Racoon.

BEST BLUE-PURPLE dwarf: fastigiatum, calostrotum ssp. keleticum, russatum, augustinii, Night Sky, Penheale Blue.

BEST DEEP PURPLE, Azurro, Glendoick TM Velvet.

BEST ORANGE: citriniflorum Horaeum orange, cinnabarinum Concatenans, Fabia, September Song, Sonata, calendulaceum, Gibraltar (azaleas). The only true orange is in the azaleas.

BEST EXOTIC MULTICOLOUR: Lem’s Cameo, Jingle Bells, Naselle, Many vars of Vireya (indoor) species & hybrids.

SCENTED +/- hardy: decorum, fortunei, glanduliferum, hemsleyanum, Loderi, Tinkerbird, Polar Bear. Deciduous azaleas: arborescens, atlanticum, luteum, occidentale, Lemon Drop, Exquisita, Irene Koster, Rosata. Mild gardens or conservatory: edgeworthii, formosum, 'Lady Alice Fitzwilliam'.

NEUTRAL OR SLIGHTLY ALKALINE SOIL: decorum, hirsutum, rubiginosum, vernicosum, Cunningham's White.

COLD/EXPOSED SITES, Cunninghams's White, Fastuosum Flore Pleno, Gomer Waterer, Azurro, Goldflimmer. Hardy deciduous azaleas such as exbury hybrids.

USER INSTRUCTIONS FOR THIS GALLERY.

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