Fall Bloom Of Rhododendrons
Dr. Sandra McDonald, Hampton, VA
Fall bloom of rhododendrons has been a problem for nurserymen and rhododendron gardeners for many years. It occurs in species and hybrids of rhododendrons, including evergreen and deciduous azaleas. This phenomenon has occupied my thoughts at various times and I am putting my observations and conclusions forward where I hope they will be tested as a research project for some professor or graduate student.
R. L Ticknor briefly discussed the problem in "Rhododendron News'", February, 1980, published by the Portland Chapter of the American Rhododendron Society, and suggested dry conditions followed by a thorough soaking of the soil, then warm weather to stimulate the plants. Norman Pellett and Bertie Boyce in "'Flower Bud Cold Hardiness" which appeared in American Horticulturist, Oct/Nov 1979, mention that sometimes environmental conditions, perhaps a period of dry weather in late summer overcomes inhibition (dormancy). Henry Skinner, Fred Galle and Daniel Milbocker have mentioned this phenomenon in lectures or in conversation with me.
In field observations at Le-Mac Nurseries, Inc., Hampton, Va., budded 2 year old plants of Rhododendron 'Cunningham's White' were compared to budded 3 year old plants of R. 'Cunningham's White' in beds in the same field. The 2 year old plants did not fall bloom while the 3 year old plants did. A major difference in culture of the 2 beds was the watering. The 2 year old plants received frequent irrigation (usually twice a week) while the 3 year old plants received less frequent irrigation. Temperatures should have been the same in both beds since all plants were in an open field in full sun. Older plants of R. 'Cunningham's White' in a stock ground which has afternoon shade, but was not irrigated at all, bloomed out almost 100% in the fall.
In another area of the nursery, evergreen azalea 'Prize' was grown in 2 situations. These plants were all the same age, potted in the same medium, and fertilized similarly, but one group was under 51% saran shade and the other was in sun. The plants under shade did not fall bloom, but the plants in sun did have occasional buds break dormancy and bloom in the fall. In this case I think it was still dryness causing the fall flowering. Several times in the summer I had noticed the plants in sun were not being kept adequately watered and eventually discovered the automatic watering had broken and the plants in sun were being inadequately hand watered. Since these plants did not have shade they actually needed more water than the shaded plants, but were receiving less with resultant early bud set and fall bloom.
Dry conditions (though not dry enough to kill the plant or completely stop bud set) seem to promote early bud set. These early-set buds can then bloom out after they have had more dry weather to overcome dormancy followed by rainfall or irrigation in conjunction with warm autumn weather to force the flowers.
Larson (4) has done much work with forcing azaleas and has an interesting discussion on breaking azaleas flower bud dormancy with cold treatment. He also reports work of several others who used gibberellic acid to overcome flower bud dormancy.
Schoeneweiss (9) did a study on breaking dormancy of red oak ( Quercus rubra ) seedlings in the greenhouse. In his study GA (gibberellic acid)-lanolin paste on leaf scars broke terminal bud dormancy. In another of his tests, GA-glycerol and pure glycerol also broke dormancy in oak, while untreated plants did not. In all these cases if GA-glycerol or glycerol alone was used, lateral rather than terminal buds grew. Schoenweiss mentions studies by Kemp, Fuller and Davidson (2) and Kramer (3) in which breaking of dormancy in plants is associated with an increase in soluble carbohydrate content. Scheoneweiss devised tests using pure ethylene glycol, pure glycerol, 1 N glucose, 1 N sucrose, 1% GA in pure glycerol, 1% GA in water and also untreated checks, treatments applied December 7. Dormancy breaking from treatments with ethylene glycol, glycerol and GA was consistent; glucose and sucrose seemed to have some effect, but results were erratic. In another test with ethylene glycol, glycerol and GA in glycerol on two species of oak ( Q. palustris and Q. macrocarpa ) and on sweet gum ( Liquidambar styraciflua ), the oaks broke dormancy with all treatments, but the sweetgum only with GA-glycerol.
Schoeneweiss reported that after the first flush of growth following dormancy breaking, test plants became quiescent and did not put on further growth as they would during the summer. This corresponds with my experience with a test lot of deciduous azalea rooted cuttings. The rooted cuttings treated with GA put on one flush of growth then went dormant in early summer until the next year.
Several at least partially interchangeable treatments have been found to overcome bud dormancy and certain type of seed dormancy. Chemical treatment with 2-chloroethanol (called ethylene chlorohydrin in older works) applied in the vapor form breaks dormancy of some fruit trees (8); very short exposure to warm water bath sometimes breaks dormancy; exposure to low temperatures for a minimum number of days to months breaks bud dormancy for many plants, including fruit trees and rhododendrons and azaleas; gibberellins have been found to break dormancy of many deciduous plants and some cold requiring seeds (peach). Gibberellin can partially or completely replace cold storage in acceleration of flowering of well developed azalea buds (1;5). I have personal knowledge of a Virginia Beach, Va. camellia gardener who 'gibbs' some of her rhododendrons as well as her camellias to get fall bloom.
I believe drought also has some effect in breaking some kinds of dormancy as has been casually observed by several others mentioned earlier. Salisbury and Ross (8, on page 693) state "Hydrolytic activities, including breakdown of metabolites in general but particularly the hydrolysis of starch and protein, increase during desiccation. This general increase in rate of metabolic breakdown is probably the most universal characteristic of high water stress." They also state there is a decrease in rate of translocation of solutes. I believe this transformation of starch into sugar and decrease in translocation should lead to a build up in sugar content during drought conditions.
I suspect an important factor that several, if not all the above mentioned dormancy breaking treatments have in common is the conversion of starch to sugar, especially the breakdown of starch by the enzyme a amylase to dextrins and perhaps further to glucose and maltose. a amylase, beta amylase and starch phosphorylase (all enzymes which convert starch to sugar) are very active in germinating seeds high in starch (8). beta amylase is already present in the seeds, but most of the a amylase is produced as germination progresses.
An interesting aside remark mentioned by Salisbury and Ross (8) is that potato tubers stored at temperatures only slightly above freezing, showed an accumulation of reducing sugars such as glucose and fructose, and of sucrose with an accompanying loss of 1 to 5 percent of the starch. Pressey and Shaw (7) who did this work found that the accumulation of reducing sugars results from an accompanying increase in the enzyme invertase. The sugars are reconverted back to starch on re-warming to above 10°C.
Salisbury and Ross discuss the paradox with which we are faced. Low temperature is known to reduce the rate of chemical reactions, but we seem to have an increased production of something at low temperatures. They discuss the scheme proposed by Melchers and Lang in Europe and Gregory and Purvis in England, to solve this problem. The researchers in Europe and England proposed two interacting reactions, one with a fairly low temperature coefficient and the other with a higher temperature coefficient. The product of the first reaction is acted upon by the second reaction. If reaction I progresses at low temperatures more rapidly than reaction II, we can have an accumulation of product at low temperatures. This would seem to be an explanation of how the potato tuber example works, as well as how gibberellin can increase during cold treatment.
Paleg (6) and Harugoru Yomo suggested that a gibberellin (GA3) was the hormone provided by the embryo and that it induced an increase in the content of a amylase (an enzyme which converts starch to sugar) and of proteases in aleurone layers of seeds with embryos removed. Joseph Varner (in Salisbury and Ross, pp. 477-478) proved with radioactive carbon that the increase of alpha amylase is actually due to a greater production of this enzyme in the presence of gibberellic acid, rather than to an activation by this hormone (gibberellic acid) of preexisting enzyme molecules.
The starch to sugar conversion seems to be a common factor in dormancy breaking in several of the methods discussed above. Sugars serve as food for the developing meristematic tissue in seeds and buds. Verifying the drought, sugar and fall bloom relationships in rhododendrons would be an interesting project and perhaps would be a start towards solving the problem.
There is much individual variation in fall blooming of rhododendrons. In my seedling lots of the species R. carolinianum , some individual plants seem to fall bloom regularly and others never do at all, although they are growing side by side within a bed. Fall bloom is probably a quantitative character varying with individual plants within a species, varying with individual hybrids, and even varying from species to species with some species more prone to fall bloom than others.
Most of us are interested in preventing fall bloom in our rhododendrons. Certain inhibitory chemicals such as cycocel and phosphon can delay bloom, but probably would not be practical solutions for the gardener. Adequate water throughout the growing season may help prevent fall bloom in all but the most regular fall blooming rhododendrons. Clear definition of the problem should lead to more precise and more practical solutions.
Some rhododendrons which fall bloom in the area of Hampton, Virginia are:
|Album Elegans||Princess Juliana||R. hippophaeoides (some)|
|Antoon van Welie||Purple Splendour||Balta|
|Confection||Red Cloud||EVERGREEN AZALEAS|
|Cunningham's White||Spring Dawn||Hexe|
|Graf Zepplin||Van Nes Sensation||Dorset|
|Kevin||Evening Glow||Golden Oriole|
|Kentucky Cardinal||R. carolinianum (some)||R. luteum (some)|
|Mist Maiden||R. dauricum (some)||Peachy Keen|
1. Boodley, J. W. and J. W. Mastalerz, 1959, The use of gibberellic acid to force azaleas without a cold
temperature treatment, Proc. Amer. Soc. Hort. Sci., 74:68-85.
2. Kemp, H. T., R. G. Fuller, and R. S. Davidson, 1957, Gibberellic acid, Agri. Chem., 12(4):30-31.
3. Kramer, P. J., 1934, Methods of breaking dormancy in certain forest trees, J. For., 3 2:734-741.
4. Larson, Roy A., 1975, Continuous production of flowering azaleas, pp. 72-77 in: Anton Kofranek and Roy
Larson eds. Growing azaleas commercially. Univ. of Cal., Div. of Ag. Sci. 108 p.
5. Martin, L. W., S. C. Wiggans and R. N. Payne, 1960, The use of gibberellic acid to break flower bud
dormancy, Proc. Amer. Soc. Hort. Sci., 76:590-593.
6. Paleg, L. G., 1965, Physiological effects of gibberellins, Ann. Rev. of Plant Physiol., 16:291-322.
7. Pressy, R. and R. Shaw, 1966, Effect of temperature on invertase, invertase inhibitors, and sugars in potato
tubers, Plant Physiol., 41:1657-1661.
8. Salisbury, Frank B. and Cleon Ross, 1969, Plant Physiology, Wadsworth Pub. Co., Ca. 747p. (pp. 244-
246; 459-460; 475-478; 570-571; 576-577; 693-694).
9. Schoeneweiss, Donald F., 1963, Methods for breaking dormancy of oak seedlings in the greenhouse, Proc.
Amer. Soc. Hort. Sci., 83:819-824.