QBARS - v31n4 The Potentials of Rhododendron smirnowii as a Parent for Hardy Hybrids

The Potentials of Rhododendron smirnowii as a Parent for Hardy Hydrids
August E. Kehr and George W. Ring, III
Respectively, Staff Scientist, National Program Staff, U.S. Department
of Agriculture, ARS, Beltsville, MD and Highway Research
Engineer, Department of Transportation, Washington, DC

If we consider hardiness ratings, we get the impression that Rhododendron smirnowii is a very hardy rhododendron species. It rates H2 in the American system and H4 in the British system. From these ratings, it would be expected this species would be a good parent for developing hardy hybrids and hybrid derivatives.
Our results do not substantiate the hardiness ratings of R. smirnowii . Although we have grown several forms, none has flowered normally in the spring. All have blasted many of their buds during the winter, even in winters that had minimum temperatures of 12° F.
Some years ago, we obtained a form from New Jersey and were assured by the nurseryman that it was the hardiest form he had ever selected. Despite this assurance, the New Jersey form has not been bud hardy in Maryland.
Some of the hybrids with R. smirnowii tend to flower in the fall for us. Perhaps the lack of winter hardiness is associated with this inclination of delayed dormancy in the fall. For example, one of us (AEK) obtained raspberries which were recommended for production in New York State, but when grown in the milder climate of Maryland, were not winter hardy because they did not become dormant in the fall. Apparently the flower buds of certain so-called R. smirnowii react in this manner and do not "harden up" here for winter. To a lesser degree, at least one selection of R. yakushimanum has had buds winter killed for the same reason.
Before the species R. smirnowii is further used in hybridizing, the hybridizer should be encouraged to select a form 1) bud-hardier than most other forms, and 2) plant-hardier than other forms, although the former is by far more important. We feel that clonal forms of R. smirnowii having the desired bud hardiness exist, even though most of those forms now grown do not. As an example of clonal hardiness, Dave Leach indicates that R. ungernii is as hardy or hardier than R. smirnowii , but the form of R. ungernii we have obtained from the West Coast is hopelessly plant tender in this area (0 to + 5° F.). In contrast, George Miller of Hanover, PA, has a form of R. smirnowii which is very hardy at that location.
Rather than discourage other hybridizers from using R. smirnowii , we believe that a concerted effort should be made to determine why some rhododendrons, such as R. smirnowii , tend to fall bloom. We should then try to either reinforce this tendency in order to obtain larger flower display in the fall, or try to counteract this tendency, perhaps by using as the other parent a species or hybrid highly stable as to blooming time. We have noticed that some hybrids and species tend to vary two weeks or more in the time they bloom from year to year, while others, such as the hybrid 'Boule de Neige', bloom on almost the same day year after year. We suspect that the blooming of some rhododendrons, especially those that force well, is perhaps partially triggered by temperature, while those with more consistent blooming times are perhaps triggered by increasing length of daylight hours. However, our understanding of the reasons for the different responses is incomplete.
At any rate, R. smirnowii does have very desirable characteristics as a parent of hybrids. These include superior foliage, as witnessed by the R. yakushimanum x R. smirnowii cross, and ability of the flowers of R. smirnowii hybrids to assume the character and color of the other parent. The latter is exemplified in many of Halfdan Lem's R. smirnowii hybrids. In more northern climates, outstanding hybrids of R. smirnowii and R. fortunei have been developed. To capitalize on these desirable traits, future hybridization with the species R. smirnowii is likely to be successful when there is a more complete understanding of the blooming "trigger" in rhododendrons. In the meantime, to obtain spring-blooming hybrids of R. smirnowii suitable for the Washington, DC, area, we suggest that the hybridizer select and use only clonal forms of R. smirnowii which have been tested and found to be bud hardy here.