Breeding Azaleodendrons for Commercial Growing
Dr. Robert L. Ticknor
Some of the earliest recorded rhododendron hybrids (1804) were azaleodendrons. These plants, called 'Azaleoides', are thought to be crosses of Rhododendron ponticum x periclymenoides (Rhododendron nudiflorum). Most of the recorded azaleodendrons are crosses between elepidote rhododendrons and azaleas of the Luteum series. An exception is the Hardijzer group, which are the lepidote Rhododendron racemosum crossed with Obtusum series azaleas.
Although many azaleodendrons were hybridized in the 19th century, only a very few are presently grown. Fragrance and different flower colors in evergreen plants are the advantages of azaleodendrons, but these characteristics are often coupled with poor foliage and growth habit. Poor foliage is not surprising since one of the parents is deciduous, and many of the characteristics tend to be intermediate between the parents.
Developing procedures so that young rhododendrons can be used as seasonal pot plants has been one of our research goals. Among the 180 or so cultivars which have been screened for this purpose was one azaleodendron. This plant appeared in a field of 5000 plants from a cross of Rhododendron occidentale x a Mollis azalea raised by an Oregon grower. Obviously pollen from a rhododendron was also present when the cross was made. This plant has orchid-pink flowers, is fragrant, and the foliage is mildew-resistant.
This azaleodendron was included in the 1976 screening trials reported in Table I. The plants started as cuttings during the summer of 1975 were larger than any of the rhododendrons in the trial and had more flowers than any rhododendron cultivar except 'Vulcan'. While the flower color limits the commercial possibilities because of rapid growth rate and good flower bud formation.
Table I. Average Plant Size and Average Number of Vegetative and Flowering Shoots on a Container Grown Azaleodendron as Influenced by Nitrogen Source in the Potting Mix-1976 N. Source N/cu yd
Nitroform 2 16.8 16.2 17.1 3.2 83 36 Osmocote 18-6-12 2 18.1 17.0 12.1 6.9 94 35
My first azaleodendron cross was done in 1975 between Rhododendron occidentale and Rhododendron 'Purple Splendour'. Most of the seedlings of that cross were susceptible to mildew and discarded. 'Purple Splendour' is one of the most mildew-susceptible rhododendron cultivars, and Rhododendron occidentale is also susceptible, so for disease-resistance this was not a logical cross. However, 10 or so plants had a flower color like 'Blue Peter' or Blue Ensign', and 8 had dark blotches like 'Purple Splendour'. Of the other plants, one had a green and one an orange blotch. Cuttings of the orange, green, and one black-blotched plants were taken in 1978. The growth results of these plants are shown in Table II. All the selections have grown rapidly, and all plants set flower buds, but the plant with the orange blotch has the best fragrance and heaviest flower bud formation and will be increased for further testing.
Table II. Average Plant Size and Average Number of Vegetative and Flowering Shoots of
Three Selections of R. occidentale X R. 'Purple Splendour' - 1979
Orange 16.8 13.8 15.2 10.8 100 6 Black 17.2 15.2 20.2 3.4 100 5 Green 19.7 14.8 14.7 5.7 100 6
With the apparent success of my first azaleodendron cross, several crosses were attempted in 1976. Twelve crosses were done on one plant of Rhododendron prinophyllum (Rhododendron roseum in a planting of 41 at the North Willamette Experiment Station. Large seed capsules were formed with pollen from 'Chevalier Felix de Sauvage', 'F.C. Puddle', 'Olympic Lady' and 'Olympic Lady' x 'Lackamas Spice'. However seedlings grew only from the 'Olympic Lady' crosses. Unfortunately all the resulting seedlings dropped their leaves in the fall and were discarded. The success ratio with Rhododendron occidentale was better, as four of seven attempted crosses yielded seedlings with azaleodendrons resulting from all crosses.
A fragrant red and the fragrant yellow 'Altaclarense' - only 'Altaclarense' x 'Lackamas Spice' yielded seed which produced deciduous seedlings.
In 1978 a student, Clyde Thomas from George Fox college, interested in a hybridizing project for a biology class to determine whether there was variation between plants of the same species to set seed and to produce azaleodendrons, crossed 36 plants of Rhododendron prinophyllum with pollen from two seedlings from the Rhododendron 'Olympic Lady' x 'Lackamas Spice' cross. Blue plastic labels were used to identify crosses made with one of the plants, and pink labels the other plant, saving time writing information on labels. Twelve set seed with pollen from the blue-labeled plant. Six of these plants yielded only azaleas, while a total of 7 azaleodendron seedlings resulted from the other 6 plants. The pink-labeled plant proved to be a better pollen plant with 19 plants setting seed. Azaleodendrons are growing from 11 of these crosses, with a total of 60 plants. Similar results have occurred with Rhododendron occidentale where 8 different selections have been pollinated with rhododendron pollen, but only 4 have yielded seed. The seedlings from one of these plants have proved to be all azaleas. A plant in my yard has been the best parent for azaleodendrons which I have identified to date, but even there the success ratio is not high. Five of 24 crosses in 1978 and 6 of 40 in 1979 set seed capsules.
With the low ratio of successful crosses, why keep trying? One good feature is that Rhododendron occidentale in my yard does not set seed unless I pollinate the plant so no time is wasted in emasculation. By the end of the first growing season, a large number of plants can be eliminated because of foliage characteristics, so only a limited area is needed to grow the plants on to flowering when most of the rest of the plants can be eliminated. The dream is to have the fragrance of Rhododendron occidentale or Rhododendron prinophyllum on a plant with brilliant yellow or red flowers and good evergreen leaves that grows rapidly and sets flower buds at a young age and is disease-resistant.