Grafting rhododendrons
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If you have a large garden, there are few plants more spectacular than the fragrant, pink-flowered Rhododendron griffithianum hybrids. These plants need plenty of room because they grow to be small trees. The best known R. griffithianum hybrids are those of the ‘Loderi’ grex. This group of sister seedlings gave rise to such plants as ‘Loderi King George’, ‘Loderi Venus’ and ‘Loderi Sir Joseph Hooker’.
These plants can be difficult to obtain because they do not always grow well on their own roots and often have to be grafted. Although they are not always available in garden centres, there are quite a few specimens in older gardens and obtaining propagation material is not that difficult, but you have to know how to use it. Fortunately, grafting rhododendrons is quite straightforward; the only problem is getting the stock plants on which to graft.
‘Cunningham’s White’ is the best commonly grown stock. It is preferable to R. ponticum, which was widely used in the past, because it doesn’t produce basal suckers to the extent that R. ponticum does. ‘Cunningham’s White’ is sold in many garden centres and is easily propagated by cuttings or layers. Young, well-struck cuttings in pots are ideal as stock plants and one large specimen of ‘Cunningham’s White’ should yield plenty of cutting material. Alternatively you may find a nursery that sells small plants sutiable for use as grafting stocks.
The best time to graft is in early spring as the sap starts to rise, but before the new growth begins to develop. The saddle graft is the commonly used technique with rhododendrons and it is quite easy to master. Any grafting process requires that the cambium (the layer of cells below the bark) 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 is easy because it is just a wedge cut made to fit a matching V-shaped notch, but you will still need a sharp knife and a steady hand.
The stock and scion should be about the same diameter to ensure a good fit. Start with non-flowering stems of your scion and use 75–100 mm-long pieces of tip growth. Remove the lower leaves and trim back the remaining foliage by about half.
Next, trim the stock back close to ground level; the closer to the ground that the stock is cut, the less likelihood of suckers and buds developing, but you need to have enough stem to be able to work with — about 75 mm is adequate. The top of the stock should be cut to a wedge-shaped point by make shallow cuts on opposite sides of the stem.
Next, make a V-shaped cut in the base of the scion stem. Try to start as close to the bark as possible and gradually cut towards the centre, keeping the taper the same as the wedge cut of the stock. The cut base of the scion should slip neatly over the top of the stock. A little trimming may be necessary to get a good fit, but keep your cuts as smooth as possible and avoid jagged edges. Bind the stock and scion together with grafting tape if you have it, otherwise plumbers’ thread tape or adhesive tape will do. This process is fiddly at first and takes some practice to master, but you’ll soon get the hang of it.
Freshly grafted plants need to be kept in a warm humid environment to ensure good growth and to prevent the graft drying. I find the easiest way to do this with potted rhododendron grafts is to line the bottom of a trough or tub with a few centimetres of gravel and then pour in water until it’s just level with the surface of the gravel. The pots sit on the gravel and the water ensures that the atmosphere remains humid. It the trough can be covered with a few panes of glass, so much the better.
Once the scion is growing well and the graft has callused over, the tape can be removed. Don’t leave this too long or you may find the tape starts to cut into the stem.
Copyright Geoff Bryant
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