Color Variations of Rhododendron Flowers From Year to Year
G. M. Idorn, Naerum, Denmark
Some years ago we replanted a group of
in new sphagnum with 10-20% compost. They achieved much more intensive blue flowers the next summer, and we wondered whether the new soil, or the winter climate of that year had caused the change.
P.A. Cox observed paler yellow rhododendron in 1970 than usual at Glendoick, and blue rhododendron in Edinburgh Botanic Garden were referred to as most blue after hard winters (Rhododendron & Camellia Handbook 1971, p. 93). Were these variations maybe caused by a change of pH in the soil rather than by the more conspicuous climatic variations?
A porous, gravel/sand soil, well-drained, will have a pH value which follows the balance between the precipitation of rain and snow (increasingly acid in urbanized regions) and groundwater (acid/alkaline dependant upon the character of soils - alkaline in East-Denmark). The pH may therefore change readily with the quantity of precipitation.
When an acid, spongy sphagnum-soil with a pH of 4½-5½ is made for rhododendrons, one has got a "buffer-effect". Acid rain or snow as alkaline groundwater can be absorbed and held in quantity with only minor influence on the pH as long as there is a surplus of water in the pores of the soil. At the same time the sphagnum soil has a considerable ion-exchange capacity for dissolved substances from minerals. Also, the temperature has a significant effect in this geochemical picture, because there is much less chemical reactivity at low temperatures than at higher ones.
In recent years, also, the increasing air-pollution, particularly with sulphur oxides from coal fired power plants and industry works has an effect. In Scandinavia, for instance, ever increasing acidity of the precipitation is now a remarkable feature both on cultivated and virgin land, and in lakes and rivers.
Accordingly, a good "buffer-soil" is important, if constant growth conditions are wanted. On the other hand, if the soil has very limited buffer-capacity, the climate variations (temperature, precipitation) may well change the pH so much that colour variations could result.
The pH as a measure of chemical conditions seems more likely than temperature per se, as a pure physical measure, to affect the colour of flowers. And it is well known that hydrangea macrophylla changes from blue to red with the acidity of its soil.
Much more than this does not seem well known, although it would be interesting for hybridizers to know whether their first bloom with a new creation is an exact expression of this deliberate chromosome-mix, or next year may prove different from the first sublimity or disappointment.
It would also be interesting to learn whether, for instance, shredded bark, which assumingly concentrates alkaline salts from the groundwater (and thus leaves an increasing acidity in the soil in cultivated forests), may provide another tone of dye to rhododendrons than does sphagnum and whether maybe only certain colour pigments in flowers are sensitive to pH-changes.
If so, then some observed colour varieties of rhododendrons in the wild could be a climate-conditioned phenomenon, or partly so, and a captured beauty might fade not out of grief over the lost freedom, but for chemical reasons.
Facts seem to be: Rhododendrons and other plants with preference for acid soils do show variations in intensity of the colour of flowers. Assumption is: Change of pH-value of pore water in the soils causes this to happen. Question is: Is this of any significance in general and for hybrid breeders? Answer is: If so, provide a good buffer-soil.