Raising plants from seed is the simplest propagation method, but unless you are interested only in species, the natural variation of seedlings means you will need to propagate rhododendrons vegetatively. In other words, by using parts from existing plants.
The most common vegetative method is the cutting. Small lengths of stem removed from the parent plant can be induced to develop roots if kept under controlled conditions and may then be grown as a new plant, identical to its parent.
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Other common vegetative techniques include layering, aerial layering and grafting. Although the methods vary depending on the type of rhododendron being propagated, cuttings are by far the most common.
Species and new hybrids must be raised from seed, but with the exception of some of the deciduous azalea strains, seed is rarely a practical method for propagating cultivars and selected varieties as they do not reproduce true to type.
Growing rhododendrons from seed is not difficult, but depending on the type, it can take 18 months to five years before the first flowers open. Tree-sized rhododendrons may not flower until they are over eight years old. Seed may be bought, produced by hybridizing or collected from plants that have been naturally pollinated.
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Determining exactly when the seed is ready to harvest is not always easy. The seed must be ripe, but leave it too late and the pods will burst and scatter the seed. Often there will be color changes or drying as the seed pods near maturity but occasionally you may have to harvest slightly unripe pods and ripen them indoors to avoid losing the seed. Tying a small paper bag over the ripening pods to catch falling seed is sometimes possible.
Sow your seed on a finely sieved 50/50 mixture of sphagnum moss and peat- or bark-based potting mix. Do not cover the seed, just sow it on the surface and gently moisten it with a fine mist. Kept lightly shaded and moist, it germinates in 10 days to six weeks depending on the type.
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While dry seed keeps reasonably well, it germinates best if sown immediately after harvest, which is usually in the fall. However, unless you can keep the young seedlings protected, it is best to delay sowing until spring because fall-germinated seedlings are rather tender and may collapse over winter.
Rhododendron seeds are very fine and the young seedlings are tiny and slow growing. They are generally not suitable for sowing outdoors either in trays or the open ground and will do far better if grown under controlled conditions. Aim for high humidity, steady, even warmth (around 64°F/18°C) and give them an occasional diluted liquid feeding.
Although most seedling plants are potted once they have their first true leaves, rhododendrons are frequently too small to handle conveniently at this stage. No harm will come to them if they are left to develop in the seed tray provided their nutrient requirements are met. Rhododendron seedlings normally transplant well and establish quickly.
Most rhododendrons can be grown from cuttings, the methods varying slightly with the different styles of growth. It is vital that the cutting environment is humid because any wilting will stress the cuttings, possibly fatally.
Softwood cuttings are taken from the new growth before it has become firm and often before it is fully expanded. Softwood cuttings strike and develop quickly because they are taken from the most actively growing part of the plant. They are, however, easily damaged and prone to wilting under the slightest moisture stress.
A semi-ripe cutting is simply a softwood cutting that has matured slightly. Its foliage (apart from the extreme tip) will usually be fully expanded and the stem will be firm yet pliable. Semi-ripe cuttings are usually available from late spring until mid-fall.
Tip cuttings strike best. The size of the cutting varies with the plant being propagated and the stage of growth. Softwood cuttings are seldom more than 4 in (10 cm) long, while semi-ripe cuttings tend to be larger, typically around 6 in (15 cm), because the internodal length (the stem length between the leaves) increases as the growth matures and expands. Very small softwood cuttings, such as those of evergreen azaleas, can simply be snapped off at the base. Larger softwood and semi-ripe cuttings are removed with pruning shears and usually strike best if cut at a node.
Carefully strip the leaves from the lower nodes. Although soft cuttings are easily damaged, most leaves come away cleanly if they are removed with a quick upward action after being pulled downwards just enough to break the joint between the leaf and the stem. If the leaves of the cutting are very large, it may be necessary to trim the foliage to cut down the moisture lost through transpiration and to enable more cuttings to fit in the container or tray. The foliage can be cut back by about half.
Before inserting a cutting into the propagating mixture, dip it in rooting hormone. This is available in liquid, gel or powder form and can be varied in strength to suit the type of cutting.
Some rhododendrons are slow to strike roots. Wounding - removing a small strip of bark - by making a shallow downward cut along the side of the cutting immediately above the base, exposes a greater area of cambium (the layer of cells below the bark from which new growth occurs) which can speed up the rooting process and may also produce a better root structure. It is generally restricted to shrubby large-leafed rhododendrons and is seldom used with alpines, small-leafed rhododendrons or azaleas.
Layering is a good way to propagate rhododendrons that are difficult to strike from cuttings, but it is slow and may take up to two years to produce results.
Low-growing rhododendrons, particularly evergreen azaleas, often self-layer where their stems touch the ground. These natural layers may be removed and potted or replanted elsewhere.
You can simulate this natural layering by keeping a stem in contact with the soil. With time, roots will form. Start by selecting a pliable stem that can be bent to ground level. Scrape out a shallow trench where the branch touches the ground. Make a shallow cut on the lower side of the stem, dust the wound with a little rooting hormone powder, then with wire hoops peg down the stem in the trench. Stake the growing tip with a bamboo cane to make it upright, then refill the trench, mounding the soil up slightly so that it doesn't sink.
Start by selecting a length of firm stem that still has green bark. Older wood will strike but it takes longer. Aerial layering can be done any time such material is available. Remove the foliage from around the immediate area and make a shallow upward cut into the stem. Lift the flap of bark and wedge it open with a matchstick. Lightly dust the wound with rooting hormone powder and wrap the stem with wet sphagnum moss. Then wrap the ball of sphagnum in black polyethylene and secure it with wire ties or tape. The moss will keep the wound from drying out and healing over, and roots will eventually form at the wound. The layer will take about 10-12 months to strike well. It can be left through winter, because if the plant can survive freezing, the layer should also. However, this method has not been well-documented, so it is a case of trial and error. When it is apparent that a good root system has formed, the layer, complete with the sphagnum, may be removed and potted.
The best time to graft is when growth begins in early spring, but before the new foliage expands. Grafting requires that the cambium of the stock and scion (the variety to be grafted onto the stock) is kept in contact long enough to fuse and this involves some trimming and fitting to get a good match. The saddle graft and the side wedge are commonly used techniques.
Practice making the cuts with a few old twigs before sacrificing any valuable plants. Bind the stock and scion together with grafting tape if you have it, otherwise plumber's thread tape or adhesive tape will do. This is awkward at first but you'll soon get the idea.
To keep the freshly grafted plants growing and to prevent drying out, keep them in a warm, humid environment. Fill a shallow tray with gravel, then pour in water until it's just level with the surface of the gravel. Sit the pots on the gravel and the water will ensure that the atmosphere remains humid. If the tray can be covered with a plastic tent, so much the better. Grafts in the garden should be covered with a plastic bag, making sure that the bag cannot dislodge the graft if it moves in the wind. Once the scion is growing well and the graft has callused over, the tape can be slit or removed. Don't leave this too long or you may find the tape starts to cut into the stem.