Hybridizing Rhododendrons For Plant Habit
Reprinted from Spring 1971 University of Washington, Arboretum Bulletin
In 1967, Mr. Smith, Aurora, Oregon, received the Gold Medal, highest award of the American Rhododendron Society, as well as in his own Chapter, Portland. His riverside, woodland garden is one of the finest rhododendron gardens in the northwest. He has generously shared both his knowledge and the pollen from his choice plants. He started hybridizing in 1951 and the results of his labor have been bringing him growing recognition during recent years.
My main effort in hybridizing rhododendrons has been to produce semi-dwarfs with good foliage and a sturdy plant habit. One reason for this objective is that more small plants can be grown in a given space. A group of one hundred seedlings can be taken from the flat and spaced six inches apart, thus taking up a space of twenty-five square feet. These may be left for two years. Then at least that half which has poorer foliage can be eliminated. The remaining better plants can be rearranged in the same area and perhaps left until they bloom by doing some more weeding out. Thus a selection may be made by the use of twenty-five square feet. If the cross produced a better than average group of plants, more space would be required, which should not be objectionable.
Another good reason for breeding for sturdy semi-dwarfs in the elepidote group is that there are so few of them, as compared to the medium to large growing ones. It would appear to me that there is much more room for improvement here than in working for larger blooms.
A sturdy plant will hold the blooms upright in the rain, which is more than can be said of a great proportion of those we now have. The chances of breaking down in heavy snow are remote, which is not the case with large growing rhododendrons.
Another big plus for the sturdy plant is that it will not, after ten or fifteen years, bend over and leave a hollow space in the middle, as do so many of the medium to faster growers.
To produce good foliage should be a primary goal of the hybridizer. It is to be seen for twelve months as compared to two to four weeks for the bloom. I like to see the leaves have uniform color without spots. Some spottiness is caused by sucking insects and some, they say, by soil deficiencies. I have never been able to overcome the light shading along the veins of the leaves by soil amendments. A sure way to prevent irregular shades of green in the leaves may be to select parents without these defects. Also, it would appear that some species and hybrids are more susceptible to foliage injury by insects than others.
A leaf which is re-curved or turned down around the edges will often keep its shape better than one not re-curved. A plant with glossy leaves is especially attractive in wet weather in the winter when the garden is in the least attractive season.
Another characteristic that I look for when selecting a parent is the number of years that the leaves will stay on the plant. This ability to retain the leaves varies from a little over a year to as much as eleven years. A minimum of two sets of leaves is desirable, and in a cross, a plant with three sets will be more attractive than the one with two sets, other qualities being equal.
Indumentum may not be valued highly by a person who observes rhododendrons chiefly in the blooming stage, but the attractiveness of the new growths on an indumented plant may be as great and last over a longer period of time than will the bloom. Unfortunately, in order to retain an attractive indumentum, at least one parent has to be abundantly indumented, and the other needs at least a trace, as has R. calophytum. This generally recessive characteristic can be quite a problem in an attempt to produce a yellow flowered indumented hybrid.
Since the retention of viability in pollen is no longer difficult, two prospective parents with widely separated blooming dates may be crossed. If the earlier blooming one produced no pollen, a mating can often be consummated by holding pollen from the later blooming plant over winter and using the earlier bloomer as the seed bearing parent. If pollen cannot be released by contact with the pistil, success may sometimes be attained by jarring the anther, thus causing the pollen to flow out.
It is my belief that breeding for size of flower has been overdone in many cases. Emphasis should be placed on the shape of the corollas and their placement in the truss, and such matters as substance and pureness of color.
The ability to last longer than average should be an important goal in breeding. This goal is probably overlooked more often than sought after. A truss on the show bench picked fresh that morning may look fine and take a ribbon, but how long will it last in the garden?
Below is a list of characteristics discussed above, which I think desirable to breed for. Included are some species or hybrids which might, hopefully, help to attain the desired goal.
For plant sturdiness: yakushimanum, 'Noyo Chief', calophytum.
For dwarfing effect: yakushimanum, williamsianum, dichroanthum, bainbridgeanum.
For good foliage: yakushimanum, williamsianum, 'Noyo Chief', wardii, strigillosum, campylocarpum.
For retention of foliage: yakushimanum
For glossy leaves: 'Noyo Chief', wardii.
For indumentum: yakushimanum, bureavii, macabeanum, rex, fictolacteum.
For beauty of new growth: yakushimanum, bureavii, fictolacteum, macabeanum, selected strigillosum forms.
For long lasting blooms: 'Crest', dichroanthum.
Hybridizing rhododendrons is, as has been said, the only hobby in which you can spend five to fifteen years in anticipation and have but one day of frustration.