Panel on the Growing of Rhododendrons
January 26 and February 23, 1961
Conducted by Messrs. Wertheim, Coe, and Pearce, California Chapter A.R.S.
When is the best time to plant?
Though rhododendrons (including azaleas) can be transplanted at almost any time of the year, even during flowering, the ideal period is while the plants are more or less dormant, in winter, spring, or late fall. The moving of plants in the fall may have the disadvantage of interrupting an active growing season that precedes extended dormancy. (Sumner)
Is it feasible to move plants in summer in a warm area like Orinda?
The transplanting of specimens during hot days is likely to require special protection. If, for example, rhododendrons are in the way of summer-time building programs, they can be taken up temporarily, mulched heavily in a protected location, and given frequent sprayings. (Sumner)
A plastic material of milk appearance can be used advantageously on the foliage of plants that are being moved during hot weather. The plastic spray, known as Wilt Pruf, helps prevent the loss of excessive moisture through transpiration. (Domoto)
How large a ball of earth should be included?
Naturally the size of the ball depends upon the species or variety, as well as on the age or size of the plant. Epiphytic rhododendrons such as the Maddeniis have weak, limited root systems whereas the more common kinds have a mass of fine, fibrous roots which extend to about the circumference of the foliage mass. In wet, heavy clays, root systems are ordinarily less extensive than in loose, sandy loams. Generally, large, old plants can be moved successfully with a ball of earth about three feet in diameter. In the Rhododendron Dell, we are preparing to move plants five or six feet high and each will have a large ball of earth. Ordinarily, the ball should include most of the root system and should be as large as can be handled readily. Each individual specimen is likely to require investigation as to the extent of its root system. One should begin digging at a fair distance from the trunk and then work inward until you begin to encounter masses of roots. (Hudson)
What type of roots do the Javanicums have?
They are similar to the Maddenii in having weak root systems; the majority of them are epiphytic. A five or six foot plant of this group may do well in a box a foot or a foot-and-a-half square. (Coe)
What about planting in heavy adobe soil?
Digging holes in heavy soil and filling them with loose soil, with or without shavings, pine needles, or gravel has not worked satisfactorily. The holes incline to become sumps for water and the plants grow poorly or die. The preferable method is to develop raised beds of good soil above the adobe. (Pearce and Long)
This experience has been confirmed by an investigator in North Carolina, as reported recently in the Quarterly. It was found that azaleas grew well in beds above the native red clay but not in made-soil below the clay-soil surface. (Martin)
Contradictory experience occurred in Maryland where holes in heavy red clay were filled with oak-leaf mold. Azaleas grew well under these conditions but they formed rather woody root systems instead of the more characteristic fibrous, fine roots. In this connection, attention was called to the native California rhododendron, R. macrophyllum. which grows well in almost solid granite in the Mendocino County highlands. (Coe)
How much mulch should be used?
One is not likely to use too much mulch of pine needles but excessive amounts of wood chips or similar materials tend to be harmful by becoming too compact and by shedding the moisture away from the root systems. (Hudson)
In England, among rhododendrons more than 100 years old, the pine needle mulch was so deep that a person would sink down to his knees in it, yet the plants were very healthy. This would seem to indicate that there is little danger of over-mulching provided there is adequate aeration, as among pine needles. (Sumner)
Last summer when temperatures rose to 116 degrees in my yard, the plants that had a heavy mulch of pine or redwood needles were not burned but those without such a mulch lost most of their leaves. (Osegueda)
What have been experiences with the so called "U. C." Mix (50% redwood shavings and 50% sand)?
The mix has not been satisfactory in the nursery business because plants such as roses grown in such a medium do poorly when sold and grown in less ideal soil conditions. Roots of such plants were not able to adapt themselves quickly to more typical soils and consequently numerous complaints resulted. (Tobler)
Used for rhododendrons grown in containers, the U. C. mix is difficult to keep sufficiently moist. (Giaque)
Tests have shown superior growth of roots in the U. C. mix but there can be no doubt that regard must be given to the soils where the plants are grown subsequently. (Wertheim)
What about other mixtures or materials?
Good growth has resulted from mixing equal parts of fir bark and redwood shavings, together with one third of manure, and allowing the combination to stand a year before using. (Osequeda)
A mixture of peat and sand together with an equal amount of fir bark has proved satisfactory in our nursery. (Kerrigan )
Perlite has proved easier to handle than sand and we have found it very satisfactory. (Smith)
Perlite has an additional advantage over vermiculite in that the latter tends to hold water excessively. (Roderick)
Since avoidance of weight was a prerequisite of gardens planned recently for the top of the Kaiser Center, resort was made to Vitabark together with Cinder Rock in various grades. some fine as sand, while coarser ones were about an inch across. Healthy plantings of rhododendrons and azaleas, as well as beautiful lawns, resulted from exclusive use of these materials. (Wertheim)
Is "feeding" desirable?
Practices in feeding of rhododendrons vary greatly. Some do no feeding at all, or resort to it only when there is definite evidence of need. Others provide a light feeding for the winter, particularly if the plants are growing in a light mixture. Still others feed when the plants are in full bloom and continue periodically until the end of August or September. (Wertheim)
We do no feeding of plants in the Rhododendron Dell. Instead, we use about half-and-half pine needles and peat, and throw in a few handfuls of sulphur, just to be sure of staying safely on the acid side. In contrast with these plants grown in the open, rhododendrons raised in containers at home are fed. (Hudson)
What materials are used in feeding?
A handful of superphosphate and cottonseed meal is added to good, light humus in containers. (Osequeda)
Fish emulsion on foliage gives good results. Also, fish emulsion and urea have been reported to be in common use by azalea growers. (Coe)
Rapid-Gro is used on nursery rhododendrons once a year, in March or April, at the rate of 1 pound to 33 gallons of water, plus 1 ounce of iron chelates. The solution is sprayed onto the plants and later washed off onto the ground but griersonianum hybrids are avoided with this treatment. (Druecker)
Good results have been obtained from use of a dilute solution of Milorganite and ammonium sulphate, at the rate of one-half teaspoon per gallon. (Sumner)
What is the relation of sunshine to growth and flowering?
Though R. 'Fragrantissimum' and some others will flower well in shade, the ideal seems to be to give a rhododendron as much sun as it can stand without burning. Of course this requires experience with individual species and varieties in one's own locality. There might be a rough generalization that small-leaved species, such as in the Triflorum or Lapponicum series, take the sun better than large-leafed forms. (Sumner)
Two characteristics commonly attend excessive shading, namely the plants tend to grow straggly and droopy and flower buds do not set so well. At my place, R. johnstoneanum, 'Rainbow', and 'Unknown Warrior' showed these effects when shaded too much. (Long)
Even in Orinda heat, the small-leaved rhododendrons take the sun well, whereas in the shade of oaks the plants incline to die. Light shade on a north exposure is good for rhododendrons in our vicinity and under these conditions the flowers do not bleach badly. (Pearce)
What causes bud dropping and what can be done about it?
Smog has been suspected of being a factor in bud dropping but every year R. 'Jean Marie de Montague' drops buds in Golden Gate Park though no smog exists there. (Hudson)
The die-back fungus (Phytophthora) limits flowering and growth considerably. Feeding in August appears to help control this disease. (Druecker)
Experts now recognize much of the bud dropping as caused by fungi and it has been found to be controllable by fungicide spray. A personal experiment with the fungicide Orthorix on a Camellia that had always dropped its buds previously resulted in good blooming. (Osegueda)