JARS v38n2 - Rhododendron Size

Rhododendron Size
Herbert A. Spady, Salem, OR

Most people plant rhododendrons too close to each other, other plants or buildings. Most people plant rhododendrons that grow too large for the site where they are planted.
Those two statements may be hasty generalizations, but they do deserve some discussion. Some people want instant landscaping or at least some immediately desirable landscape effect. Considering the short time that most families live at one residence perhaps they should plant relatively rapidly growing, showy plants. "Let the next occupant worry about the fact that the plant will become too big," they might say. Some nurserymen take the attitude that it is best to re-landscape after a few years. With these facts and opinions in mind perhaps there is something to be said for close plantings.
On the other hand, many rhododendron enthusiasts can hardly be separated from their gardens for many years and then only by death or illness. These gardens and public gardens require some different thinking. What alternatives are there to the wide open spaces left for future growth of rhododendrons? Here are three suggestions:
1.  Plant three of the same variety and remove the outer two as the plants become crowded. Or use any other grouping with
planned removal.
2.  Plant one highly desirable plant with less desirable or less expensive plants surrounding and plan removal of the less
3.  Fill the surrounding space with plants of other genera that can be progressively removed.
Even so, it would be desirable to have some idea of how big the plants will grow. Remember that they never stop growing as long as they are alive. Be careful to pay attention to the growth habit of the particular variety, i.e. up or out. Information about growth usually indicates height in ten years. Is this useful data? What is magic about ten years? There are several reasons why this is useful information.
Remember that a cutting grown plant will double its size the first year. The second year it will increase by one-third; the fourth year by one-fourth; the fifth year by one-fifth. By the tenth year it is adding only one-tenth to its height. That is height, not volume. The volume will increase by the cube of the height increase. The outcome is that by the tenth year and beyond it is not adding much to its size in proportion to the size it has already attained.
Also the plant tends to put out several flushes of growth each growing season as an immature plant. As it gains size and begins blooming it is likely to make no more than one flush of growth each year.
There are other factors to consider. Plants that are given abundant nutrients will grow with longer shoots than those that are not stimulated with such good nutrition. Plants grown in shade will have longer shoots than plants grown in full sun.
A good way to judge the "ultimate size" of any rhododendron is to measure its yearly growth. Careful pruning and pinching can to a limited degree regulate growth and habit. Definitive pruning, of course, is to cut that large overgrown plant back to stubs. In a few years it will probably recover to a shapely young looking plant. Winters like 1972 are effective natural pruners and thinners of rhododendron plantings.