Rhododendrons require a soil that is not alkaline, though not all varieties need a very acid soil, and if your soil is near neutral it is worth experimenting. They must have good drainage and will not tolerate waterlogging or being planted directly into heavy soils. This is especially true of the smaller alpine species. In wet and heavy conditions, work in drainage material such as sand or gravel and organic matter and mound plant, so that excess water drains away. The shallow roots must not be allowed to dry out: in very light sandy soils plant in a depression that will collect rainwater.
Mulching is essential to the successful growth of almost all varieties - do not skimp on this. Use leaf mould, dead leaves, chippings, twigs etc, but not fresh manure. Do not mulch right up to the trunk. Rhododendrons, in general, require little fertiliser, and none should be given after July.
Many varieties, other than the Hardy Hybrids, require some shade, but too much shade will deter flowering, and it is best to try high light levels - if leaf scorch occurs the plant can be moved. On the other hand, big leaved rhododendrons will show reduced leaf size if they are not shaded. As with most evergreen shrubs, all rhododendrons must be protected from cold drying winds.
Rhododendrons have a shallow spreading root system, and are easy to move at any time other than in hot weather provided they are well watered in. Experimenting to find the ideal planting position is therefore much easier than with many other genera.
The roots of plants that have been in containers for any length of time may need spreading out when planted. Be careful to purchase plants with pale roots - red roots near the container surface are often dead. Plants grown in the open ground are to be preferred.
Varieties that come into growth early will need good air drainage in gardens which are subject to spring frosts.
Elepidote rhododendrons, and some lepidotes, have the advantage of being disliked by deer.
Compost. Use sieved medium grade moss peat. Pass through a 1cm riddle to remove coarse pieces.
Containers. Shallow trays approx. 4cm deep seem to give the best results
Method. Fill containers with compost to within approx. 1cm from the top. Gently firm. Stand containers in water mixed with half-strength Phostrogen or Miracid and leave until completely wet. Remove and allow excess water to drain away. Use a small piece of wood to firm compost and produce a level surface. Sow seed very thinly on the surface of the compost. Do not cover the seeds, as light is needed for germination. Using a small hand-held sprayer, gently spray to settle the seeds into the compost. Cover containers with glass or clear polythene; this should be removed a few days after germination otherwise Botrytis might set in.
Timing. If artificial lighting is not available, sow seeds around mid-March, after which time it is probably advisable to keep the seed in the freezer until the following year. Place containers in a well lit situation protected from direct sun. A north facing windowsill is ideal. Temperatures over 65F will cause poor germination. Seed can be sown in Jan/Feb under artificial light. .
Pricking Out. This should be done as soon as the seedlings have produced their second pair of true leaves. The time taken to reach this stage can be quite variable, but most take between 3 and 4 months. With March sowings it is probably best to postpone pricking out until the following spring. Use standard seed trays filled with medium grade peat, incorporating a Vine Weevil control if you have experienced problems with this pest. After transplanting to 4cm apart, soak the trays in water containing full strength Phostrogen or Miracid . If your water is "hard" use Miracid.
Watering. Whilst the seedlings are establishing, it is best to water by soaking from below, especially during the growing season - April to September - but keep just moist during the winter months. A few days before pricking out, cease watering to allow the compost to become just moist and crumbling, thus facilitating separation of the young plants, thereby avoiding fatal root damage..
The Next Stage. Around 12 to 14 months after pricking out, transplant the seedlings into 9cm pots or, better still, about 20cm apart in a peat bed approx. 15cm deep. The latter method enables plants to grow on undisturbed for 2 or 3 years without further work until planted in their permanent positions.
And finally, never allow young Rhododendrons to dry out!
Some rhododendrons will root from half ripe cuttings; the time to take these varies very considerably with variety and the weather conditions, and can be from June until late October. Thin side shoots with no flower bud on them are best. A compost of 50% Cornish grit and 50% peat gives reasonable results. Rooting is likely to take at least 6 months. Bottom heat is not essential, but maintaining high humidity is.