They are basically what Europe has been using for over a century. They are grafted on a hybrid rootstock similar to Cunningham's White. They will not tolerate alkaline soil but will tolerate neutral or just slightly acidic soils which many rhododendrons won't tolerate on their own roots. They still need good drainage, so I would recommend a raised bed if you have clay soil with poor drainage. They are definitely more tolerant but not fool proof.

The downside is that if the graft develops a mismatch in size of the two stems forming the union, it may eventually breakdown. Hence, these plants may have a finite life.

I talked to Kathy Van Veen and my impression is that she just grows and retails plants sent to her by the Inkarho people from Germany. She doesn't do the grafting.

They were patented about 25 years ago in Europe. The reports and field trials are quite promising. I agree, there is no such thing as foolproof Rhododendron unfortunately. But I am going to give them a try... In a couple of years I might be able to make my own report on their perfomance. I planted Inkarho and regular Rhodies on the same bed at the same time. Time will tell...
The root system looks stronger indeed. The graft unions look well healed, so I hope that they will not break down.

The problem that develops with graft unions when the plants get older is that the growth rate of the scion and the rootstock must be the same or one becomes much larger than the other and the cambium layers don't match.

To avoid this Inkarho has patented several rootstocks so they can match more closely the growth rate of the scion. Two of them are rhodunter 37 and rhodunter 48. Both are the result of a planned breeding program between unpatented Rhododendron fortuneiÃcv.ÂCunningham's WhiteÂ.

The development of Inkarho rootstocks is interesting. The rootstock was developed through an extensive breeding programme at the German Federal Institute of Horticultural Breeding at Ahrensberg. In the selection procedure the seedlings were subjected to growing conditions at a pH level of 7 for a period of six months. Seedlings which survived and were selected were then propagated. 20 rhododendron varieties were grafted onto the clonal rootstocks for screening trials for compatibility and growth rates. . The result is a clonally propagated rootstock with a high lime tolerance which can be grown satisfactorily at relatively high pH levels - pH 6.0-6.5. The claim is that they will establish on clay soil with a pH as high as 7.0. Although not particularly alkaline, the majority of other rhododendrons rarely do well above a pH of 5.5 - 6.0.