JARS v36n2 - Notes on Fall Flowering

Notes on Fall Flowering
W. A. Letcher, Placerville, CA

In November 1975 I moved approximately 300 rhododendrons from a cool mountain climate to a warmer one in the foothills of central California. Since then I have been surprised to find that some of them bloom, at least partially, in the fall. In over 15 years of growing them at Mount Shasta, only one plant ('Elizabeth') had ever fall-bloomed. My experiences may add a little to the rather meager knowledge that has been made available to growers and hybridizers on the subject.

My results since 1976:
Bloomed every fall:
'Queen of Sheba' (Lem)
('Fabia' x R. smirnowii )
Bloomed 4 out of 6 yr:
R. carolinianum
Bloomed 3 out of 6 yr:
('C.P. Raffill' x R. smirnowii )
'Rose Elf'
( R. metternichii x ?)
R. lapponicum
(as R. parvifolium f. albiflorum )
R. dauricum (some)
Bloomed 2 out of 6 yr:
R. japonicum var pentamerum
(as R. degronianum )
Bloomed one year only:
R. wardii var puralbum
(as R. puralbum)
R. impeditum
'Boule de Neige'
'Red Cloud'
'John Wister'
( R. caucasicum x R. discolor )
(a sister seedling of Lems' 'Queen of Sheba')
Never fall bloomed:
Two sister seedlings of the cross
('Fabia' x R. smirnowii )
('C. P. Raffill' x R. fortunei )
('Dido' x R. smirnowii )
'Janet Blair'
R. metternichii (several forms)

It is of considerable interest to note that 'John Wister' has flowered once in the fall, while 'Janet Blair' growing beside it has never done so! Although the two plants do appear identical when in bloom, the above might indicate that 'Janet Blair' and 'John Wister' are, indeed, separate clones and not a single clone with two names.
The summer of 1981 was an especially warm one, with no early fall cold spell followed by warmth, and with only negligible rainfall between June 15 and September 15. In spite of the absence of cool temperatures followed by warmth, and without any change in moisture supply, more plants bloomed this fall than ever before in my garden. Daily recorded temperatures were as follows:

DATES Max. Min. Avg. Max. Min. Avg.
6/15 - 7/15 99° 85° 92.6° 65° 47° 56.5°
7/15 - 8/15 106° 88° 95.6° 64° 47° 56.2°
8/15 - 9/15 103° 84° 92.8° 64° 47°
High temperatures were above 100°F. on 7 occasions, and were above 95°F. on 35 days during the period.

All plants are grown in shade which varies from about 50% to 100%, and all watering is by regularly scheduled overhead sprinkling. Rainfall during the period totaled less than inch.
Dr. Sandra McDonald, in her paper in the Summer 1981 issue of the Quarterly Bulletin "Fall Bloom of Rhododendrons", stated that she had noted much variation in fall blooming of rhododendrons. This variation included both individual plants and individual species. My results would support that conclusion. It seems clear that many factors are of influence, including heredity, in producing fall flowering, but my recent experience indicates that changes in moisture or temperatures may not be required in the presence of a long, warm summer season.