Rhododendrons are very self-supporting but they still need occasional maintenance.
Weeds are competition that your plants don't need. Because rhododendrons have so many surface roots, they are easily damaged by chemical sprays and hoeing. Hand weeding is fairly safe but the best idea is to use mulch, which stops many weeds becoming established and makes it far easier to remove those that do.
Mulching also helps to conserve soil moisture, lessens soil compaction and insulates the surface roots from temperature extremes.
Compost, rotted sawdust, rotted straw, used potting mix and bark chips are all suitable mulches. A mixture of fine-and medium-grade bark chips is attractive and functional. The larger pieces tend to rise to the surface and prevent the fine moisture-holding bark from blowing away. Avoid mounding excessive mulch up against the trunk or main stems of larger rhododendrons. It may damage or soften the bark, leading to fungal or viral diseases.
Rhododendrons don't need regular pruning; all they require is an occasional trimming to keep them neat and compact.
Dead-heading -removing of old flowers and seed heads - is often all the trimming required. Removing the seed heads allows the plant to channel its energies into growth rather than seed production and if while dead-heading you remove any crowded, weak or diseased branches close to the main stem and pinch out the tip buds from any non-flowering stems, you'll produce an evenly shaped bush with strong lateral branching and dense growth.
Any pruning is best done immediately after flowering because this allows the maximum time for regrowth and lessens the effect on the following season's flowering. Because the flower buds form in the late summer and fall, late pruning limits the amount of bud-bearing regrowth that can develop before the buds are initiated.
Start by thinning out the center of the bush to improve the air circulation. Cut back any spindly or diseased branches and remove any congested growth. Shaping the bush is now just a matter of cutting back any overly long branches and evening up the growth. Always cut back to a whorl of leaves because that is where the most vigorous buds are located.
Evergreen azaleas will usually re-shoot from anywhere on the plant and can be sheared to shape. Compact small-leafed azaleas are ideal subjects for hedging and topiary. More precise shaping is possible but it is seldom necessary to do anything more than head back any extra vigorous growth and remove damaged or weak wood.
If necessary, prune deciduous azaleas immediately after flowering. Cut the bush back to four or five main branches. You can be severe, as the new growth that develops will often be very vigorous. Pinch it back before it gets too long. This encourages lateral branching and helps to produce strong, healthy branches rather than the spindly twigs often seen on deciduous azaleas. Very overgrown plants can be cut back to stumps but heavy pruning should be done with care because rhododendrons are sometimes reluctant to re-shoot from bare wood. Although a well-established plant will generally recover, it is often better to prune in easier stages over two seasons. Hard-pruned plants can take several years before resuming normal flowering and further shaping is almost certain to be required as the growth develops.
Rhododendrons are not heavy feeders. Most of their needs can be met by incorporating an all-purpose acidic plant food in with their mulch. Slow-release granules are also an effective feeding method and mild liquid fertilizers can be applied during the growing season.
Any fertilizers should be watered in well because it is very easy to burn the surface roots, resulting in scorched leaves or foliage and even stem tip die back if the burning is more severe.
While fertilizers can be used throughout the growing season, cease feeding at the end of summer or the plants may still have soft growth when the first frosts occur. Commercial growers often apply sulfate of potash in the fall to help ripen the soft growth before winter. Although this is important for commercial growers, who feed their plants well into fall to get the maximum, it is unnecessary for home gardeners provided a natural fall ripening period is allowed for.
Because rhododendrons have compact, shallow root systems with few heavy roots, they are very easily transplanted at almost any time of the year. Other than your ability to lift it, there is no restriction on the size of plant that can be transplanted.
The first step is to prepare the destination site. Dig a hole to what you think will be roughly the depth of the plant's roots, then increase the diameter and depth of the hole, working in compost as you dig. You'll regret not doing this first if you lift the plant and then find some unforeseen complication at the new planting site.
With the planting site prepared, you can lift the rhododendron. Dig around the plant well away from the main stem and take as large a root ball as you can manage. You should find that even a large rhododendron has most of its roots in the top 20 in (50 cm) of soil. The next step, lifting the plant, is fraught with difficulties. Underestimating the weight of a mature rhododendron and its ball of roots is a sure way to damage your back. Sometimes the plant can be slid onto a sack or tarp and dragged to its destination or you may be able to maneuver a wheelbarrow under the shrub, but often you will need assistance to hoist the plant onto a barrow or trolley.
Before replanting, check that the hole is about the right depth -you don't want to have to lift the plant again if you can avoid it. When the plant is in the right position, backfill around it, making sure to eliminate any air pockets under the root ball. Other than the usual staking, mulching and watering, little in the way of after-care is required. Most rhododendrons carry on growing apparently unchecked and can even be moved in flower.