JARS v38n2 - Phosphorus

Phosphorus
Bob Badger, Seattle, WA
Reprinted from Susquehanna Valley Newsletter

Without it rhododendrons grow poorly, root development is slowed and flower buds fail to develop year after year. Phosphorus is the active ingredient in many complex organic compounds in the leaf of your rhododendrons. Its compounds provide the "ionic" pathway by which the sun's energy is converted to sugars which "fuel" the important processes leading to the formation of flower buds. Most all soils with too acid a pH (say 5.5) have nearly all the phosphorus content chemically "fixed" or complexed with metal ions such as iron and aluminum.

And since many of our soils (West Coast) and most all of our peat mosses have acid pH's below 5.5, that's why flower buds don't form. You need more phosphorus in your soils. Use rhododendron blended fertilizer, super phosphate, or bone meal. I use one teaspoonful of super phosphate per foot of height in late January or February and again in late March or April.

Also, phosphorus "filled" plants are much more winter hardy according to many recent experiments.