Lime tolerance in rhododendrons
It is occasionally suggested that some rhododendrons are tolerant of alkaline soils. The species most usually mentioned, Rhododendron hirsutum, is common on limestone ranges in the Dolomites but this on dolomitic or magnesium limestone rather than calcium limestone, which may explain its apparent tolerance of alkaline conditions. It seems doubtful it would thrive in soils much above neutral (pH 7.0).
Others which have on occasion reportedly shown some tolerance to soil pH levels above pH 6.0, usually regarded as the upper limit of alkaline tolerance of rhododendrons, are:
R. augustinii (Electra Group) 'Electra' AGM
R. 'Praecox' AGM
R. triflorum (and others in the Triflorum series)
R. williamsianum AGM
If attempting to grow rhododendrons where pH values are higher than pH 6.0 it may be advisable to limit planting to these species and hybrids which may show some tolerance as a result of their parentage. Having as one parent a species from the Triflorum series, the following cultivars might be considered:
Blue Diamond Group
Blue Tit Group
Bluebird Group 'Bluebird'
Eleanore Group 'Eleanore'
Other species which have sometimes been suggested, in various articles and writings as being more or less tolerant of calcium carbonate and which might be experimented with are:
R. decorum AGM
R. dichroanthum subsp. scyphocalyx
R. insigne AGM
R. oreodoxa var. fargesii AGM
R. rubiginosum AGM
Lime tolerant rhododendron rootstocks
Rhodendrons grafted on the rootstock ‘Cunningham’s White’ appear to tolerate pH levels in excess of 6.0. This cultivar has been used subsequently to develop the Inkarho rootstock. Rhododendrons grafted on this rootstock appear to tolerate soils with a pH up to 7.5.
See RHS Find a Plant for more choice rhododendrons.