Propagating Azaleas and Rhododendrons
The primary methods of propagating Rhododendrons and Azaleas are seeds, layering, grafting and cuttings.
As grafting is a very difficult method of propagation, particularly to the amateur, this method of propagation has has not been covered in these cultural notes.
Normally larger leaved rhododendron species are raised from seeds or by layering, whereas,hardy hybrids are propagated by cuttings, and deciduous azaleas by layering.
Alpine species and half-hardies are usually propagated from cuttings.
Cultivation / Propagation
Sow seeds thinly in pots/trays of moist, finely sifted horticultural peat.
Cover seed with a sprinkling of silver sand or fine vermiculite, and cover the pot/tray with a piece of glass until they germinate, and maintain a minimum temperature of 16°C (60°F)
Place the pots/trays in a heated propagator and cover them with glass.
Alternatively in Week 13,
Sow them in a cold frame or unheated greenhouse.
Keep the seedlings well shaded and the compost surface moist.
Prick out when they make their first two true leaves 25-50mm (1"-2") apart in boxes /trays or 75mm (3") pots of ericaceous compost.
Protect the seedlings from strong sun and keep the compost moist but not soggy.
Rhododendrons and azaleas do not need regular pruning, however, spindly young plants may he lightly pruned or disbudded to make them bushy.
Older plants that have become somewhat untidy and or straggly may be cut back hard, if necessary to within 300mm (12”) of the ground.
Some of the pruning offcuts may be suitable to use as cuttings!
Take 100mm (4”) cuttings from the new growth of deciduous cultivars, cut just under a node (leaf joint) alternatively cut at the point where the new softwood has just begun to firm up.
It is advisable to collect cuttings early in the morning when plants are fully turgid and place in polythene bags to reduce wilting.
Retain the tip and reduce the number of leaves on the cutting, slice off a wedge of stem approx 20mm (¾”) long, dip the bottom 6mm (¼”) into a rooting hormone powder / gel.
Insert cutting into a 50-50 sand/ ericaceous compost mix, and place into a humid environment.
Maintain bottom heat at 15°-20 °C (60°- 68 ° F) shade the cuttings to prevent foliage scorching.
Regular misting will improve the rooting cycle.
This can be done with an electronically timed, purpose made mister unit, failing that use a pump / hand spray.
Cuttings should root in eight to ten weeks.
When rooted pot up into 75mm (3”) pots of ericaceous potting compost and grow on in a protected environment until planting out time the following year.
If conditions allow, plant out the seedlings taken the previous year in light, peaty soil in a nursery bed or cold frame.
Circa Week 20:
When the seedlings make their first two true leaves, prick them out 25-50mm (1”-2”) apart or use thumb pots or cell trays, containing a low nutrient seed compost, then grow on in a shaded glasshouse.
Pot on again in their second year and plant out in their third.
Rhododendrons can be propagated from semi-ripe cuttings.
Take 50-75mm (2”-3”) long half-ripe cuttings and insert them in equal parts (by volume) peat and sand and place on a hot bench set to give a soil temperature of 16°C (60°F).
Cuttings are often erratic in rooting, and can frequently fail altogether, the initial use of a hormone rooting compound, and a misting facility will give the best results.
Rooting can take several months, and cuttings from young shoots generally root more easily.
When they are rooted, pot the cuttings into 50mm (2”) pots containing a low nutrient seed compost, and grow on in a frost-proof place until the following April, then move them into 75-100mm (3”-4”) pots.
Protect the cuttings from strong sun and keep the compost moist but not water-logged.
In summer the plants can be placed in the shade outdoors, but must be brought back under glass in autumn.
Liquid feed rooted cuttings with a dilute balanced feed at half the manufacturer's recommended rate.
Repeat this procedure monthly during the growing period.
Layering can be done at any time of the year however an early start can often increase the success rate.
Select young stems which are low enough for bending to ground level.
Make a small lengthwise slit where the stem is to be buried, this will hasten rooting.
Anchor the stem with pegs and keep its end vertical by attaching it to a short stake.
Alternatively, peg the layer into a 100mm (4”) pot sunk into the soil and filled with seed compost.
This method causes the least disturbance to the new root system after the layer has been severed from the parent plant.
Sever the layer from the parent plant after two years, when it should be well rooted.
Plant out mature plants, planting them firmly with the top of the root-ball level with, or slightly below, the surface of the bed.
Mulch plants with peat, compost or leaf-mould after planting.
Now is the time to move any mature plants to a new location if required.