Rhododendrons have a preference for acidic soil, which increases the intensity of the blooms, but soil acidity does not usually cause a flower to become white. Rhododendrons and their companion plant, the azalea, are prized in the home garden because of their abundant, long-lasting blooms and bushy growth. They are not always easy to grow, however, and their flowers change colours, or appear to change colours, for a variety of reasons.
A rhododendron may bloom white because it actually is a white variety. But a rhododendron bud may appear to be a different colour, usually pink or lavender. The White Lights variety, according to the University of Minnesota Extension, has "a delicate pale pink cast but opens to a white flower with a slight yellow blotch." The Northern Hi-Light has a creamy white flower, though it has a yellow upper petal, and Roseum has flowers that range from white to rose pink, even on the same plant.
Precipitation and pollution can change the pH of the soil, according to G. M. Idorn in the Journal of the American Rhododendron Society (JARS). When acidic precipitation or other factors change the soil's acidity, the pH can change to such a degree that it alters the colour of the flowers. Adding acidic material--such as pine needles or specific additives made for acid-loving plants--to soil adds a hydrogen atom to a coloured molecule, turning it a lighter colour. To test the soil's pH, a sample can be taken to the nearest extension office.
Shredded wood and bark used for mulch around rhododendrons might also contribute to colour changes. According to Idorn, "shredded bark, which concentrates alkaline salts from the groundwater ... may provide another tone of dye to rhododendrons." Most soil filters minerals and pollutants, reducing the effect on plants. But if the mulch contains pollutants, it can affect flower growth and colour.
"Rhododendrons change colour from bud stage to fading, which is glorious in itself, but it can create problems in placement for colour harmony," says Bob Boveee, another JARS writer. Some of the most reliable rhododendrons are those that fade to white. They are hybrids, sometimes called Catawba hybrids. These include "album elegans," a soft lilac that fades to white, and "album grandiflorum," which is white with a tinge of lavender that later fades to white.
Reduced fertility and hybridisation among species can cause the reduction of colour in a flower, notes Gustav Mehlquist, a JARS writer. A white flower is absent the pigment responsible for colour in rhododendrons. A white parent plant can produce coloured blossoms, and a rhododendron with coloured flowers can change to white.