JARS v53n4 - Let's Talk Hybridizing: Hybridizing for Dwarfness

Let's Talk Hybridizing: Hybridizing for Dwarfness
Allan and Shirley Anderson
Franklin Lakes, New Jersey

Introduction to New Feature
"Let's Talk Hybridizing" is a new feature provided by the Journal as a forum for rhododendron hybridizers to share their experiences with others. Much information may be obtained from growing seedlings and studying the results of specific crosses. The hybridizer speculates about the reasons for the results observed but is reluctant to state them in a formal way. The very complexity of the gene pool makes it impossible to draw rigorously correct conclusions about any observation. However, reporting results and drawing tentative conclusions can be very helpful to others who may find it useful to their own progress.

Hybridizers should feel free to submit contributions about inheritance in rhododendrons whether it be about color, flower size, hardiness, bloom time, or other features of interest. These may be:

1) A short article, 800-1,000 words. One or two slides may be submitted to illustrate a conclusion.

2) A paragraph or two to pose a question that would invite discussion in a future issue.

3) Commentary about a previous article. This might add information or state a contrary opinion or experience.

Healthy debate should bring to light much information that normally remains unpublished and therefore unavailable to the beginning hybridizer as well as the experienced one.

It has been frequently stated that the landscape around modern houses would benefit from the use of smaller rhododendrons. Most commercial hybrids grow to mature heights that hide the windows of small houses or become rather large trees among other shrubbery. Careful and regular pruning might keep such plants in scale, but this is rarely done by the average homeowner.

Some rhododendron hybridizers are interested in breeding for dwarfness which usually means that they wish to produce slow growing plants with short internodes and branching plant habit. Dwarf plants are those that remain under 3 feet (1 m) for many years. These may be too slow to grow quickly into commercially salable size. However, they are a choice asset for a small garden.

We have been conditioned to believe that rhododendron genetics is complex and that most of the interesting physical features are controlled by so many interdependent genes that simple Mendelian dominance cannot be assumed. This is clearly true in many cases. It seems, however, that dwarfness due to the short annual growth of some rhododendrons is in fact a simple dominant trait that may be predicted, at least in a few crosses.

(R. yakushimanum, Exbury x R. maximum)
( R. yakushimanum , Exbury x R. maximum ), a cross by Allan and Shirley Anderson.
Photo by Allan and Shirley Anderson

When we crossed Rhododendron aureum with R. maximum the resulting seedlings were small, rather sprawly plants that resembled the R. aureum parent. We noted that Weldon Delp had similar results when he crossed R. aureum with a R. maximum from a different source. When he crossed these hybrids with larger plants both as seed and as pollen parents many of the seedlings were also small R. aureum -like plants. Some of the crosses that involved yellow-flowered parents produced particularly charming, small yellow-flowered offspring. Unfortunately nearly all of these proved difficult to grow in our hot Northeast summers. These observations suggest that dwarfness and poor heat tolerance may be linked, at least in R. aureum progeny. It is clear that many forms of R. yakushimanum are small enough to be called dwarfs with much better plant habit than R. aureum . When these are crossed with R. maximum we note that seedlings resemble R. yakushimanum in size but without the indumentum. Another hybrid introduced as 'Ghost' by W. Delp is a cross of [( R. maximum x R. yakushimanum ) x R. maximum , white form]. This plant is a low, mounded plant with pure white flowers and no larger than its R. yakushimanum grandparent. It is in fact a rather tidy dwarf plant said to be extremely cold hardy.

R. 'Ghost'
'Ghost' [( R. maximum x R. yakushimanum ) x R. maximum ],
hybridized by Weldon Delp.
Photo by Allan and Shirley Anderson

Of course there are many R. yakushimanum crosses that are not dwarf. Perhaps parents other than Rhododendron maximum or larger forms of R. yakushimanum may account for the larger "yak" hybrids that are so widely available. The dwarf dominance we note with ( R. maximum x R. yakushimanum ) crosses suggests that we have a means to produce dwarf plants with exceptional adaptability to cold winters and hot summers. One of our crosses of ( R. yakushimanum , Exbury x R. maximum ) produced a dwarf seedling with good plant habit and tidy, ivory-colored flowers.

hese results lead us to speculate about the possible use of other dwarf species as a source of dwarfness and good plant habit. This could include some forms of Rhododendron brachycarpum and its subspecies fauriei which are quite low-growing. Our experience with R. brachycarpum (as ssp. tigerstedtii ) has invariably produced seedlings with dark green, glossy foliage on well-branched but fairly large plants. Might dwarf seedlings result if one of the smaller forms were used? This is a cross we intend to try.

Another possibility to consider is the use of dwarf plants from the subsection Taliensia . Rhododendron pronum and R. proteoides are notably slow to flower and difficult to grow in the Northeast. However, when R. proteoides is crossed with R. yakushimanum the resulting plants have low, mounded habit similar to R. proteoides but bloom quite young. They also seem more adaptable to growing in the Northeast. Further testing is necessary.

Rhododendron maximum is an especially tolerant plant to cold, to heat, and to drought. If crossed with R. proteoides would the wonderful dwarf habit of R. proteoides prevail in a hybrid tolerant to the Northeast climate yet bloom as a young plant? Much interesting work remains to be done.

Allan and Shirley Anderson are members of the Tappan Zee Chapter.