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.
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 or perlite/grit. 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.
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 e
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.
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 well watered through the first growing season.
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.
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 with overhead light or at least part day sun in Scotland. Deciduous azaleas, larger hybrids and species can take some shade.
Deadheading is largely a cosmetic exercise: only a few varieties produce seed at the expense of growth.
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.
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.
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.
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 have our own Rhododendron feed available at our garden centre. Growmore is OK but it it can burn so dont put on too much. Bone meal is of limited value.
Well.. we aren't going to tell you how to do that, are we?
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.
