Rhododendrons on Limey Soils
Rhododendrons are acid loving plants, and require a pH of between 4.0 and 6.0 to grow. For those on neutral or even slightly limey soils, improvements can be made to the soil, and there are varieties of rhododendrons which are worth trying. However, large scale changing of soil conditions can not be condoned, is not environmentally sound, and is likely to prove only a temporary adjustment of growing conditions. This leaflet aims to help those with marginal conditions to attempt to grow rhododendrons satisfactorily.
The Chemistry Lesson
Rhododendrons require a fibrous, organically rich open soil, and will perform best with a pH 4.5 to 5.5. On chalky, limey and alkaline soils, where the water is hard, and where there is a high pH level (above 6.0), rhododendrons will grow poorly with yellow leaves, or not at all. The reasons are still not fully understood, but high pH causes a toxic intake of calcium, which reduces the availability of nutrients and trace elements, and hinders photosynthesis. Some rhododendrons are found in limestone regions, but usually in areas of high rainfall on dolomitic (magnesium) limestone rather than carboniferous limestone.
The pH of soil can be lowered by the addition of very acidic compost in the form of a raised bed, or by the annual addition of sulphur in late winter, allowing time for the pH to lower before growth starts. Flowers of Sulphur is unpleasant but can be used, while Sulphur Chips are more expensive but easier. Preferable is Iron Sulphate which is readily available and also provides a useful source of iron. For each unit of pH that you wish to lower, iron sulphate should be used at the rate of 750 grams per 10 square metres. (eg. To change a soil of pH 7.5 to pH 5.5 would require 1.5 kg per 10 m2). Iron Sequestrine can also be effective in the short term, but is expensive and has little benefit over Iron Sulphate.
Rhododendrons are shallow rooted plants, so a 25cm raised bed of peaty acidic soil is quite feasible when using the most acidic moss peat. We recommend and can supply Bulrush Peat or Ericaceous Compost which have a pH as low as 3.5. Mixed with neutral soil, this goes a long way towards acidifying your borders suitable for rhododendrons.
Rhododendron growing on chalky soils can not be recommended, but a membrane can be used to isolate the plant roots from the high pH. Old baths, tubs and plastic have been used by those keen to persevere, but are likely to be only temporary solutions. In time there will be problems with limey water seeping in and poor drainage.
"Inkarho" Lime Tolerant Rootstocks
For more than 20 years, German research organisations have been developing and breeding lime tolerant rootstocks, onto which normal varieties of yakushimanum and hardy hybrids can be grafted. Many of the easy to grow popular varieties are now available on these rootstocks giving a full range of colours. Propagation is only allowed by licensed growers in Germany, but Millais Nurseries now stock these plants which represent an exciting development for those struggling with soils up to pH 7.5. Inkarho plants have been extensively tested in Germany for many years, and are also performing exceptionally well in Britain. They are ideal for neutral clay soils and effectively enable 30% more gardeners in Britain to grow rhododendrons. Healthy dark green leaves are assured even on neutral soils, but they do require more fertilizer than typical.
Other varieties suitable for neutral soil
Species
- decorum
- hirsutum
- makinoi
- sanguineum
- vernicosum
- williamsianum
Hybrids
- Cunningham's White
- Any hybrid grafted onto Cunningham's White
- Most williamsianum hybrids
Small Leaved Species
- augustinii
- lutescens
- eacemosum
- rubiginosum
- yunnanense