Research

History and development

It is well established that rhododendrons require acid soils to grow vigorously, and are intolerant of lime or calcium carbonate in the soil.

A new range of rhododendrons known as INKARHO hybrids has begun to appear in garden centres. What makes them special is their reported lime tolerance and ability to grow on heavy soils. The plants, which comprise a range of well known cultivars, are grafted onto special rootstocks developed and distributed by a German growers' consortium whence the plants take their name. The plants are made up of an INKARHO rootstock on which a scion of well-known cultivars is grafted.

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.

This experiment was set up to examine the lime tolerance of this new range of INKARHO rhododendron hybrids. In order to assess these claims of lime tolerance the RHS Horticultural Science section is undertaking an extensive experiment to look at the performance of these plants under different soil acidity conditions. To provide a comparison, another traditionally lime tolerant rootstock; Rhododendron 'Cunningham's White' is also being assessed as well as some common calcifuge (lime intolerant) cultivars grown as rooted cuttings.

Construction of limed beds

The plants are being grown in specially constructed beds in which the soil conditions are carefully managed to maintain specific, discrete pH levels. The beds are maintained at one of four pH or acidity levels; pH 5,6,7 and 8, by applying known quantities of lime.

The test plants are four lime intolerant cultivars 'F Daughter', 'Gomer W', 'P Wiseman', 'H Feuerschein', and the rootstocks INKARHO and 'Cunninghams W' with a scion of each of these.

Monitoring growth

The rhododendrons were planted in the spring of 2001. Their performance is being monitored over five years to study the effect of various pH levels on different rootstocks. The height and spread of the plants is measured, the leaf colour or vigour and the date of flowering.