QBARS - v11n4 Rhododendron Notes

Rhododendron Notes

Rhododendrons and blueberries seem to have essentially the same nutrient requirements, hence any results of research with blueberries will be of great interest to the rhododendron fan. A report given at the recent meetings of the American Society for Horticultural Science at Stanford University indicated that blueberries were unfavorably affected if the nutrient medium were high in either calcium or phosphorus. So, if our rhododendrons aren't thriving we might suspect too high pH, too much nitrate or not enough ammonium, too much calcium or too much phosphorus. Progress is being made and any research such as this brings us a step nearer to being able to efficiently correct unfavorable conditions.

Writing in the June 1957, issue of the Journal of the Royal Horticultural Society, Mr. E. H. M. Cox has this to say about his recent visit to the northwest. "In Portland we were much impressed by the new Experimental Garden of the Rhododendron Society of America. This lies on two peninsulas in a lake. Apart from its charming setting it is a perfect example of public co-operation that might well be followed in this country . . . . Some of the most interesting plants for us were various forms of the native Rhododendron macrophyllum (californicum) . We saw stands of the type later in the Olympic mountains, but in this trial garden there was an attractive white and a charming small compact pink form that had been found right on the Pacific shore."

Many of our members have become acquainted with Mr. Eric Walther, long in charge of the Strybing Arboretum at Golden Gate Park, San Francisco. Mr. Walther, recently retired, has returned from a trip to England and will have an office at the California Academy of Science, San Francisco.

The use of leaf-soil in the compost or as a mulch to help acidify the soil is often recommended. Now comes a report from the National Botanic Gardens, Glasnevin, Dublin (Gardeners Chronicle, Jan. 26, 1957) indicating that leaves from trees grown on soil slightly above pH 7 made leaf-soil that was even more alkaline. Even when mixed with granulated peat of pH 3.8 the compost, after a year had a reaction of pH 7.2 and in other tests the leaf-soil was very effective in making the soil more alkaline rather than more acid. A very interesting sidelight to this work was that some 16 species of Rhododendrons were growing reasonably well on soils more alkaline than pH 7. The use of leaves as mulch or soil amendment for Rhododendrons undoubtedly has its merits, but apparently making the soil
more acid is not one of them.

- J. Harold Clark

For a fine yellow rhododendron, in flower, growth habits and foliage, it is hard to beat R. wardii , one of the parents of many of our best hybrids. There are many forms, all are good. Flowers are cup-shaped and vary from cream-yellow to bright-yellow. Leaves are more or less rounded. It seems to be quite hardy and so far as we know was not damaged by the freeze of November, 1955. It is well worth growing.

The rockery at the Rhododendron Test Garden, Portland, Oregon, is to be greatly enlarged. Plants were small when planted there some four or five years ago. They have grown amazingly and today more room is needed for replanting and for the addition of more plants. This rockery is one of the features of the Test Garden and is very beautiful in the spring.

A fine dwarf rhododendron of the Lapponicum series, not too well known is R. drumonium . When grown in the sun it makes a tight, twiggy hall literally covered with blue-purple flowers of a good color. Said to grow up to 2 feet, it will take many years to do so. Four year old plants will be only a few inches tall but are often a foot across. Another good dwarf is R. uniflorum which is closely related to R. pemakoense . The "Rhododendron Handbook" lists this as having "purple flowers." We have seen a good form with pink flowers, somewhat deeper than R. pemakoense . This form usually blooms two weeks later than R. pemakoense which is an advantage in this country.

Many have said they liked the American Rhododendron Society rating system, both quality and hardiness, as published in "Rhododendrons". We have often been asked how this was compiled. It is the result of a nationwide survey which was then reviewed by a committee of experienced rhododendron growers both amateur and commercial. It gives the opinions of several hundred people of how rhododendrons perform in the United States. While it is not perfect and must be corrected from time to time, it is certainly the most accurate and authoritative information available at present on this subject.

For fall color in the garden R. lutescens is wonderful as is the dwarf R. hanceanum var. nanum . Shrubs to grow with rhododendrons to give brilliant color are Knaphill azaleas, Acer ginnala and the dwarf Japanese maples and the different forms of a wonderful shrub called Enkianthus .

We are often asked do species rhododendrons come true from seed. You will usually get many variations and some will be better in flower, color and growth habits than others. Award plants of species are those selected from batches of seedlings. For example R. leucaspis is listed as a bushy under shrub up to two feet; we have seen a form that in five years has made a compact mat only four inches high and about a foot across, the flowers are the same as the more open growing form. R. pemakoense is listed as a shrub up to one foot and we have seen it taller than this. But one form in 15 years is only nine or ten inches tall and 24 inches across, every year it is a solid mass of flowers. This form seems to be quite hardy as it survived the November, 1955 freeze.

- Bob Bovee