Excerpts From Talk Given by Paul R. Bosley, Bosley Nurseries, Inc. to Great Lakes Chapter
Ericaceous plants and particularly rhododendrons and azaleas have one definite characteristic in common, namely, that the roots of the plants must breathe. Even though the pH of the soil is correct and the location ideal, Rhododendrons and Azaleas can literally drown in heavy soil. Heavy soil very often means lack of drainage and they will drown in water as well. These plants like free moisture but not standing moisture. They are very often spoken of as surface rooting plants and people are urged not to cultivate their garden, but if the area was properly prepared, then cultivation is desirable. Rhododendrons and azaleas will develop a pancake type of root system in order to survive, in other words, in order to breathe at their roots.
The use of sulphur incorporated into the soil (not scattered on top of the ground) will create long range acidity where it is needed. Both iron sulphate and aluminum sulphate will give a quick emergency change of pH, but iron sulphate is to be preferred because the plants will have the benefit of the iron as well. Both of the latter are very water soluble and can leach away very quickly, therefore the need to use a more permanent type of acidifier such as sulphur.
Brown leaf margins on the rhododendrons are indicative of external trouble such as too hot a sun or bright winter sunlight when the plants are frozen and other external factors. A browning near the midrib of the leaf usually beginning near the petiole is indicative of internal trouble such as over-fertilizing or incorrect fertilizing.
Rhododendrons are the most expressive plants with which I am familiar because they at all times will tell you their state of health and how they feel. Green veins with a yellowish-green background is usually indicative of an incorrect pH. Leaf color, length of new growth and hang of the leaves all tell a story of condition and health.
All of the new current growth on a rhododendron should have its whorl of leaves standing above a horizontal plane. If they drop below that point the plant is in trouble, except that in the Fall (usually about the first of November) the leaves will drop below a horizontal plane and it is indicative that the plant has gone into a slower tempo of growing. The leaves will remain below horizontal until about the second week in May in the latitude of Cleveland. After this time the uppermost leaves will again move to an above-horizontal position indicative of a stepped-up tempo of growth.