Evergreen Azaleas and Rhododendrons in Southern California, Part I
Carl A. Deul
Laguna Woods, California
When Bill Moynier, Bill Jenkins, Bill Paylen and some others joined to form the basic group of the Southern California Chapter of the ARS, we knew that we not only had to test many rhododendrons but also develop varieties that would do well in our area. Others added to our knowledge, particularly in terms of soil mix. Nuccio's Nurseries tested many varieties and found that they got good results with both azaleas and rhododendrons using coarse peat moss. As high quality coarse peat moss became harder to obtain at one time, the chapter members switched to a mix of equal amounts of fir bark, Perlite and coarse peat moss by volume, i.e., a 1-1-1 mixture. George Klump did a lot of experimenting in this area, and a reference suggested to him that he should substitute redwood bark fibers for the fir bark. Apparently, the redwood bark seems to be toxic to Phytophthora cinnamoni which itself solves a big problem. While Phytophthora cinnamoni is not as virulent in Southern California as it may be elsewhere, the Total Dissolved Solids (TDS) in some domestic water supplies causes root burn and weakens resistance to pathogens.
The plants we found easiest to grow in Southern California were evergreen azaleas, deciduous azaleas, vireyas (the subtropical and largest group of the genus Rhododendron), many of subsection Maddenia as well as the usual elepidote rhododendrons. We can divide the elepidotes into two basic categories: (a) those from an unstable climate area and (b) those plants that do a good job controlling the moisture in their stems and leaves which in turn controls the mineral levels in their leaves. To understand this, one must look at plants of this genus grown in our climate: nothing is a substitute for first hand observation. Next, one has to look at our weather patterns in Southern California and understand the outside influences and how they affect our seasonal climate.
We have a very large heat sink west of us called the Pacific Ocean. It cools our evening temperatures and frees us from the oppressive nighttime heat that people and plants in the South must endure.
Let's look first at plant damage from various environmental factors. When it occurs, leaf burn is the result of excessive daytime heat, low humidity and salt accumulation from our domestic water supplies which we use to water our plants. We have a number of sources for our domestic water. While there are a few wells (notably in Glendale), most of our water comes from run-off from mountain rain and snowfall. One of our chapter members, Fred Renich, lives in Camarillo, an area which receives hot dry winds, the Santa Ana's. The water there is basically alkaline from years of fertilizer use in the surrounding vegetable fields and orchards. Fred solved his water problem by using a reverse osmosis system and completely eliminated difficulties with leaf burn on his rhododendrons.
While azaleas are members of the genus Rhododendron, we do not find leaf burn on evergreen azaleas and seldom on deciduous azaleas. There is a reason for this. Evergreen azaleas are dimorphic (that is, they have two sets of leaves during the year's growing season), while deciduous azaleas are bi-morphic meaning they shed their leaves on an annual basis. If one accepts the fact that plants use this as a method for eliminating excessive minerals from their tissues, one begins to understand the real problems in successfully growing rhododendrons in Southern California. I understand that Ted Van Veen was told by the British that rhododendrons would not grow in Oregon. Eventually, he made them "eat crow." Problems need to be solved to make problems go away.
We found that Phytophthora cinnamoni was not a major problem in Southern California, but we do have more of a problem with Rhyzoctonia, a soil fungus. The root tips of plants in the family Ericaceae are subject to burn from a mineral salt accumulation. Once the root tips burn, Rhyzoctonia and other pathogens gain access to the plant tissues. I lost a couple of container-grown rhododendrons in 1978 and asked the soil and plant labs to do an analysis and report on what caused the plants to expire. The results were Rhyzoctonia, not Phytophthora. Yes, it gets hot in Southern California, too, but the heat does not build up enough for Phytophthora cinnamoni to become a major problem.
A Historical Prospective on Southern California's Domestic Water Supply
The water supply for the metropolitan area once came down from Mono Lake, and the water supply for outlying areas was from wells mixed with the Colorado River. Because the water of the Colorado River had a large amount of dissolved limestone, they tried to soften it with traditional softening, which meant increasing the socium chloride level of the water. Those doctors who had patients with cardiac programs objected and complained that the sodium chloride levels in the water were detrimental to their patients' health. The first step in water improvement was made when the counties changed their water softening technique to remove the sodium chloride from the processed water. More restrictions came into effect as the environmentalists began to use legal means to restrict the amount of water Los Angeles County could take out of Mono Lake. With a shrinking source of water, more conscientious efforts were made to preserve the quality. In southern Orange County, the Santa Ana River used to empty into the Pacific Ocean. The county made a big step when they dug into the area of outflow of the Santa Ana River to pour a concrete barrier to block the salt water intrusion and permit the percolation of the Santa Ana River into the subsurface water table. The Santa Ana River is water that flows off the San Bernardino Mountains in the spring; it is good water. Basically, there is more that has to be done, including desalinization of ocean water. The secondary benefits are the horticultural industry; many ericaceous plants can now be grown successfully here.
A very successful plant that was produced for the nursery industry is X Chitalpa tashkentensis, a polyploid hybrid between Catalpa bignonioides and Chilopsis linearis. Converting the catalpa to a tetraploid allowed it to be receptive to the triploid pollen from the Chilopsis linearis. The taxonomist decided the resulting plant deserved the honor of being named a new species, hence the exotic name of X Chitapla tashkentensis. The plant acquired some remarkable disease resistance to powdery mildew as well as a tolerance to the catalpa sphinx larvae. This cross exemplifies how crossing two species, even species of differing ploidy levels, can bring together attributes of both.
There is a long history of rhododendron hybridizing in England as is well known. A side benefit of this brought about a plant called 'Brocade' (or Brocade Group), claimed to be a cross of 'Vervaeneanum' (an azalea) and R. williamsianum (a rhododendron). Both plants here were diploids. However, R. williamsianum has many characteristics of evergreen azaleas. Interestingly enough, this plant is supposed to be more resistant to root fungi than other rhododendrons.
The USDA's Quest for the Yellow Evergreen Azalea
R.L. Prior and Augie Kehr were involved for many years before the 1980s in trying to produce a yellow evergreen azalea. They employed a Kurume and a yellow Mollis azalea that would produce fertile hybrids which they could use in their quest for a yellow evergreen azalea. The first thing which they discovered was that the carotenoid yellow present in the Mollis azalea did not transfer very well from the pollen parent. While they got fertile hybrids, there was little yellow in the flowers of the seedlings. You might, then, ask why not use the Mollis azaleas as the seed parent? The answer was that they did and got nice yellow flowers. However, no matter how many generations they pushed the crosses, a deciduous plant always resulted. Later, Augie Kehr continued the project with tetraploid azaleas (R. calendulaceum, 'Banka', 'Tahei' and 'Getsutoku'). He still ran into the same problems.
The same rules seem to apply to azaleodendrons. The Hardijzer Group uses the lepidote rhododendron R. racemosum and a Kurume azalea as the other parent. These hybrids produced flowers that were midway between the flowers of R. racemosum flowers and those of Kurume azaleas, the other parent. Since R. racemosum is the seed parent, the plants are evergreen as opposed to being dimorphic. Broughtoni Aurum used an elepidote rhododendron as the seed parent and a Mollis azalea as the pollen parent. The R. maximum was used as a seed parent with a Mollis azalea as the pollen parent. The resulting hybrid has some yellow in the flowers which in this case is intensified by the Mollis pollen parent. This plant also holds its leaves for more than one year.
At this point we may draw some conclusions and establish some rules:
(1) Azaleas have some morphological characteristics tied to the seed parent, e.g., dimorphism or bimorphism (deciduous).
(2) Dimorphism is a desirable characteristic for Southern California, since it is an easy way to control saline levels in plant tissues.
(3) Other than carotenoid yellows, flower colors and fragrance may be passed on through the pollen parent.
(4) If you look at the flower size and trusses of rhododendron hybrids, improvement seems to occur over several generations, and these traits can be passed on by both the seed and pollen parents.
Everything is possible through plant hybridizing. Converting diploids to tetraploids is easier and much safer than before. A group of evergreen dimorphic azaleodendrons (or whatever you want to call them) can be produced with spectacular flower trusses, colors and fragrance to please the Southern California gardener.
Contreras, Ryan, Nelson. Azaleadendrons: investigating parentage, fertility and effects of polyploidy amongst hybrids of deciduous azaleas and evergreen rhododendrons. Masters Thesis, N. Carolina State University. Jones, J.R., and T.G. Ranney, N.P. Lynch, S.L. Krebs. 2007. Ploidy levels and relative genome sizes of diverse species, hybrids and cultivars of Rhododendron. Journ. Ameri. Rhod. Soc. Vol. 61, No. 4: 220-227.
Kehr, A.E. 1977. Azaleadendron breeding. Quarterly Bulletin ARS Vol. 31, No. 4: 226-232.
Kehr, A.E. 1996. Polyploids in rhododendron breeding. Quarterly Bulletin ARS Vol. 50, No. 4: 215-217.
Olsen, R.T.A. Catalpa Taskentensis, the development of an intergeneric hybrid (Catalpa bignoniodes X Chiliopsis linearis), utilizing polyploidy for developing improved nursery crops: restoring fertility in wide hybrids, limiting fertility of invasive species, embryo culture of triploids, pest resistance, and inheritance of ornamental traits. Dissertation submitted to Graduate Faculty of N. Carolina State University.
Carl Deul is a retired aerospace engineer with over 35 years of experience in that field. His major degree work was in electronic engineering with a minor in chemistry. He has been involved in horticultural matters involving rhododendrons and azaleas for just about the same amount of time and has been and continues to be an invaluable resource for the Southern California Chapter.