Rhododendrons grow best in soils with a pH of 4.5-6.0. They do not grow well in soils with appreciable levels of calcium. This is principally due to iron getting ‘locked up’ and becoming unavailable to the plant.

Soil pH is easily measured using a home soil test kit available from garden centres. Alternatively, the RHS provides a comprehensive soil analysis service.

Generally, it is difficult to alter soil pH in the long term. A high level of organic matter in the soil is advantageous, particularly an acid medium such as peat substitute, chopped bracken or decayed pine needles but this alone will not bring about any appreciable reduction in soil pH, especially where the soil and ground water is alkaline.

With neutral or slightly alkaline soils chelated iron can be used. Ericaceous compost, chopped bracken and pine needles at planting, plus a dressing of sulphur, may also enable rhododendrons to be grown. Aluminium sulphate (found in hydrangea 'blueing' agents) should not be used to acidify soils where rhododendrons are to be grown.

On soils approaching pH 7.0 (i.e. neutral) the most reliable method is to grow rhododendrons on raised, well-drained beds. For successful cultivation a minimum depth of 30cm (1ft) of ericaceous compost or soil is needed above a layer of isolating lime free drainage material such as sharp sand. Alternatively, line the bed with polythene with holes punched through. Raised beds dry out quite rapidly, needing thorough and regular watering in dry periods.

Smaller rhododendrons can also be successfully grown in containers of ericaceous potting compost.

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. ciliatum
R. hippophaeoides
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'
Bo-peep Group
'Chink'
Crossbill Group
Eleanore Group 'Eleanore'
'Saint Tudy'

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. ambiguum
R. decorum AGM
R. dichroanthum subsp. scyphocalyx
R. insigne AGM
R. lutescens
R. oreodoxa var. fargesii AGM
R. rubiginosum AGM
R. traillianum
R. vernicosum
R. wardii

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.