H. Edward Reilly's book, Success with Rhododendrons and Azaleas has a good section on grafting. The basic techniques are the same as grafting any woody plant, but the after care can be tricky and you'd generally be dealing with smaller diameters of both scion and rootstock than you would be with apples, for example.

The standard understock in Europe is Cunningham's White because of its disease resistance and tolerance of higher pH soils. Recent research has shown that the older Joseph Gable hybrid Caroline, however, is even more resistant to fungal root diseases, so that might be the one to use.

Germany, and most of Europe in general, raises most of their rhododendrons on grafted plants. Many of their plants, if grown on cuttings on their own roots, as would be done in the US where grafting of rhododendrons is virtually unheard of, would be 1) too difficult to root to be commercially successful, 2) too tall and leggy, and/or 3) too difficult to raise due to low pH demands, susceptibility to root-born diseases, or poor root structures.

One thing that I found interesting in Germany was that there are basically two different methods of raising grafted plants. One method, the more conventional method, has something like Cunningham's White or Inkarho patented rootstocks that are rooted from cuttings and grown for 2 years. Then the scion is grafted on to the root stock using a side graft. The graft is wrapped with a rubber band type material until the graft takes.

The second and newer method takes cuttings of both the scion and the root stock and grafts them together and roots them at the same time. For about 1.5 to 2 inches each cutting is cut from one side to the center of the base. Then the cut areas of each cutting are mated with green tissue touching green tissue. Then the two cut areas of mated cuttings are wrapped with raw cotton string and dipped in a powdered rooting hormone and stuck in a rooting soil-mix. The raw cotton string will eventually rot in the soil.

As far as I know, all sources of Inkarho rootstocks are in Europe now. Cunningham's White has been used for over a century and is a good rootstock.

Here is a link that might be useful: Inkarho patented rootstocks

Thank you -- I think I'll give it a try.

Here are photos of the above two graft types.

First, the conventional side graft at the Rhododendron Park in Bremen, Germany:

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Second, the graft & root process after the rooting has completed at Timo Schröder's Nursery in Wiefelstede, Germany:

They look very similar. The first uses the elastic band, the second the raw cotton string. The first takes about 3 years, the second, one year. With the Graft & Root method, the top of the root stock is cut off after the graft has taken. They don't care if the scion forms roots, but they don't want the root stock to form suckers (top growth competing with the scion).