Some Notes on Rhododendron Maximum As A Parent
by R. L. Ticknor
North Willamette Experiment Station
Oregon State University, Aurora, Oregon
Rhododendron maximum, which grows in the coldest areas of the evergreen native rhododendron range, would appear to offer possibilities for cold tolerant plants. Other reasons for considering this species as a parent are the absence of blue color in the blooms and the late blooming habit. Only six hybrids of R. maximum are listed in the R. H. S. stud book, although many other people must have tried this species as a parent.
During 1957 while working in Massachusetts, I crossed R. maximum x 'Romany Chai', a late blooming orange red. This same year the late Mr. Walter Hunnewell of Wellesley, Mass., made a cross of Ostbo's Yellow #3 x R. maximum. Only five plants of each cross were moved to Oregon in 1959, the remaining seedlings being given to Weston Nurseries of Hopkinton, Mass. Growth has been more rapid in the milder climate of Western Oregon, but some plants of both crosses are growing satisfactorily at Weston Nurseries.
Apricot flowering plants blooming in May are the result of Mr. Hunnewell's cross. Red flowering plants blooming in late June or early July are the result of my cross. The red is not the orange red of 'Romany Chai' but one with some blue in it. One of the five plants of R. maximum x 'Romany Chai' in Oregon has bloomed white and appears to be the result of apogamy. Apogamy is the development of a seed without a sexual union occurring. The resulting seedling resembles the seed parent without a trace of the pollen parent.
Plants resembling a R. maximum hybrid occur in several old Massachusetts gardens. These plants, possibly 'Wellesleyanum' - R. maximum x R. catawbiense, bloom between the seasons of these two species with pink flowers. While rather large growing, this plant does lengthen the blooming season in cold areas.
All the hybrids observed to date have been tall growing plants with comparatively small trusses. Possibly when some lower growing species are crossed with R. maximum we will see some more compact hybrids. Even more improvement in flower color and plant habit will likely occur if advance generation hybrids are raised. A limited amount of seed of R. maximum x 'Romany Chai' crossed with a sister seedling will be available through the A.R.S. seed exchange this year.