Winter Killing of the Flower Buds of Azaleas
By C. W. Culpepper, Arlington, Va.
Cold injury to the flower buds of azaleas in winter may not be immediately apparent. Unlike the killing of partially or fully opened flowers by late spring frosts, which may be obvious to anyone, winter injury may be overlooked unless the plants are examined closely.
Experienced azalea growers are, of course, aware of the fact that the unopened flower buds may be killed at any time by sudden or severe cold periods without the plants being otherwise injured. If the evergreen leaves are not injured, it usually does not occur to the homeowner to look for winter injury to the flower buds. In the spring, however, he may be puzzled when his plants fail to bloom and have cause to wonder what he did or did not do that resulted in the failure to flower. An instance of this character occurred during the winter of 1951-52 in the Washington, D.C., area where bud killing was rather prevalent.
Varieties of azaleas differ greatly, of course, in the ability of the woody parts of the plants, the foliage, and flower buds to endure cold. This report gives briefly the behavior during the winter of 1951-52 of the writer's planting in Arlington.
Character Of The Planting
The planting is located a few miles from the Potomac River on the opposite side from Washington, D.C., in an incompletely built up section of residences and apartment structures. It is distributed over about one acre of wooded land consisting of Virginia scrub pine mixed with such hardwoods as oak, wild cherry, dogwood, blackgum, and various other trees. Near the center of the area is a plot 3 or 4 times the size of a tennis court from which the trees have been removed.
Most of the azaleas planted in this area were not protected by overhead branches, but were protected somewhat by nearness to the trees. A considerable part of the planting in other plots was beneath overhead forest cover, and is subsequently referred to as shaded. Some of the varieties were planted only in the open while others were only in the shade. Many were planted both in the open and in the shade.
The azalea plants were of various sizes and ages, some 12 to 18 inches tall, others 8 or 9 feet tall, ages ranging from 4 to 12 years. The current year's growth on all plants seemed about equally mature regardless of size or age.
The topography of the area is slightly rolling, but the azalea planting is not on the highest or lowest ground. Consequently the air drainage, while not highly unfavorable is not ideal.
The soil is a sticky, grayish clay and very acid. Adequate under drainage has been provided. Generous amounts of rotted leaves and other organic matter, but no sand or other similar corrective material has been added. The entire planting has been mulched every year with leaves (principally oak) sufficiently deep to prevent the growth of weeds. This helped to conserve moisture and otherwise provide satisfactory growing conditions. Fertilizer has been added from time to time as seemed needed. Soon after flowering in the spring of 1951, the planting received a moderate application of ammonium sulphate at the rate of about 300 pounds per acre in order to keep the plants in a good healthy state of growth.
TABLE 1. Official maximum and minimum temperature of the Washington airport station for the months of November and December 1951 and minimum temperatures observed in the azalea planting. November Temperatures December Temperatures Day of the month Airport
°F °F °F °F °F 1 51 41 63 38 2 42 40 64 31 3 42 31 60 38 4 52 29 63 49 5 45 31 28 63 56 6 47 32 69 50 7 67 43 74 54 8 46 34 65 58 9 52 34 70 45 10 63 38 45 36 11 65 37 46 35 12 58 36 40 31 13 60 47 33 26 14 73 55 33 25 22 15 70 54 45 19 16 16 74 48 19 12 10 17 50 34 30 10 5 18 41 29 38 26 22 19 36 27 25 31 24 20 20 38 27 23 38 23 18 21 40 25 19 61 37 22 50 31 26 40 26 23 67 45 45 26 24 60 39 50 32 25 39 30 39 31 26 55 33 46 31 27 43 31 2 24 28 42 25 35 20 29 50 31 49 34 30 57 32 51 47
A daily record of the temperature in the immediate vicinity of the azaleas was not kept. However, minimum temperatures were recorded at critical periods. The nearest Weather Bureau Station is at the Washington Air Port, which is located 6 or 7 miles away from the azalea planting. Minimum temperatures at the air port will vary from 2 to 6 degrees higher than those at the azalea planting. Table 1 gives the daily maximum and minimum temperatures at the Washington Airport for the months of November and December 1951.
No very severe cold weather occurred until November 19-22. During this period the temperature went down to 25°F. at the air port and to 19° among the azaleas.
The lowest temperatures of the entire winter occurred during the period of December 13 to 20. On the morning of December 17 the temperature registered 10°F. at the air port and 5° among the azaleas. There was a very strong cold north-west wind on the 14th and the 15th that also had a drying effect.
Such low temperatures do not occur frequently in this area so early in the season. No very severe cold periods occurred during the winter after December 13 to 20 and there was no evidence that further injury occurred to the azaleas after this period.
Amount Of Cold Injury
Immediately after these severe cold periods neither the current year's growth nor the older woody parts of the plants appeared to be injured to any appreciable extent. Even the foliage seemed to be little harmed.
It did not occur to the writer to examine the flower buds closely until the very last days of December. At this time, by chance, the appearance of a plant of one variety caused the writer to suspect some injury to the flower buds. Splitting them open, at once, made it apparent that some of the buds were dead. Further examination here and there over the plantings revealed considerable injury.
It seemed apparent from the degree of darkening of the primordial flower pans at this time that some of the buds of certain varieties were killed earlier than others. A considerable part of the killing therefore probably occurred during the period of November 19-21. However, much further killing probably occurred during the December 13-20 period. Accurate estimates of the injury could not be made at this time because of the time necessary to dissect sufficient buds for the necessary data. It was necessary to open the buds and note the condition of the stamens and the ovary. If they were brown or black, the flower buds were killed. Most of the buds appeared entirely normal from the outside.
Difference In Varieties
Estimates of the amount of bud killing had to be delayed until the plants were in flower in the spring. The percentage percentage of killed buds could then be readily determined.
TABLE 2. The amount of winter killing of azalea blossom buds. Amount of Killing Variety Type or Group Shade Percent Open Percent Amoena Obtusum 0 0 Amoena Coccinea Obtusum 0 0 Anne Chenee Pericat Hybrid 50 Aphrodite Glendale 0 0 Apple Blossom Kurume 0 100 Benigiri Kurume 0 0 Beni-Kirishima Macrantha 100 100 Bethoven Vuyk Hybrid __ 100 Betty Kaempferi Hybrid 0 0 Billy Gable Hybrid 0 0 Bridesmaid Kurume 20 100 Cagaribi Kurume 0 0 Carminita splendens Obtusum 98 100 Caroline Gable Gable Hybrid 0 0 Cattleya Kurume 0 __ Cheerfullness Kurume 0 0 Chinook Gable Hybrid 0 0 Christmas Cheer Kurume 0 __ Coral Bells Kurume 60 100 Corsage Gable Hybrid __ 10 Daphne Salmon Indian 10 __ Daybreak Kurume 0 0 Dayspring Glendale 0 10 Edna Gable 0 0 Emilie Rosseau Pericat __ 20 Ethelyn Gable __ 100 Flame Kurume 90 100 Formosa Indian 100 100 Fortune Pericat Hybrid 0 100 Fujinishiki Macrantha 100 100 George L. Tabor Indian 95 100 Glacier Glendale 5 5 Gumpo Macrantha 90 100 Hakatashiro Macrantha 95 __ Hampton Beauty Pericat 95 _ Hazel Dawson Dawson Hybrid __ 95 Hexe Sander Hybrid 0 0 Hino-Crimson Obtusum 0 0 Hinodegiri Kurume 75 100 Illusion Glendale __ 100 Indica Alba Mucronatum 90 100 Izayoi Macrantha 100 __ James Gable Gable 0 0 Joya Glendale 25 100 Jindai Chugai Hybrid 100 100 J. T. Lovett Macrantha 100 100 Kagaribi Kurume 0 0 Koch-No-Moi Kurume 100 100 Louise Gable Gable 0 0 Mary Dalton Gable __ 90 Maxwell Ryukyu Hybrid 20 __ Minuett Glendale 0 0 Morning Glow Kurume 0 __ Mme. Pericat Pericat 100 __ Mrs. L. C. Fischer Bob and Adkins __ 98 Picador Glendale __ 5 Pink Pearl Kurume 80 __ President Clay Indian __ 100 Rosebud Gable 0 0 Rose Greeley Gable 0 0 Rosette Glendale 0 0 Rythm Pericat 0 __ Salmonea Macrantha 90 __ Sakuragata Macrantha 90 100 Schubert Vuyk Hybrid __ 0 Sekedera (Indica Magnifica) Mucronatum 10 100 Sherwoodi Kurume 0 0 Sherwood Red Sherwood Hybrid 75 __ Sho-Qua Macrantha 95 __ Springtime Gable 98 __ Snow Kurume 50 0 Swan Gable 0 0 Sweet Briar Kurume 0 0 Sweetheart Supreme Pericat 100 100 Torch Kaempferi 0 0 Yaeshojo Kurume 98 100 Vitata Fortunei Indian 0 __
Differences in bud killing among the varieties were found to be very striking (Table 2). There was some killing in nearly all groups, classes, or varieties of the evergreen or semi-evergreen types, but some groups were more severely injured than others. Varieties of the Macrantha and the Indica groups were very severely injured. The Pericat hybrids were also severely injured, although some of these in shade came through with little injury. The Gable and the Glen Dale relatively little injured. The Kurumes were somewhat intermediate and variable as to location.
Generally the semi-evergreen types were less injured than the completely evergreen varieties, but there were exceptions in nearly all groups. As an example the variety, 'Spring Time', an early blooming Gable hybrid, nearly deciduous, was almost totally killed. 'Glacier', a fully evergreen type, suffered only a small amount of injury. 'Vitata Fortuneii' and 'Daphne Salmon' in the Indian group were not badly injured, nor was 'Rhythm' in the Pericat group. Only two varieties of Kampferi hybrids are here recorded, neither of which were injured but certain other unidentified varieties of the group were partly or totally killed.
In summary, it may be noted that of the varieties of the larger groups, the Gables had 3 out of 12, the Glen Dales 2 out of 8, the Kurumes 9 out of 19, the Pericats 7 out of 8, the Indians 4 out of 5, and the Macranthas 9 out of 9 with more than 10 per cent of their flower buds killed.
The difference due to position, whether in the shade or in the open, was also sometimes striking. Generally there was less killing in plants growing under the protection of overhead branches of trees than was the case of plants grown in the open. However, in the case of 'Snow', the reverse was true. Some varieties suffered no injury in either open or shaded positions while others were killed in both positions.
There are many other factors than shade or sun that may influence the degree of injury by cold. The question arises as to what effect the fertilization with ammonium sulphate may have had. This question cannot be answered in this case as all but a very few plants received the fertilizer at the same rate in so far as was possible by hand application. It is believed that the bud-killing would have been nearly the same had no fertilizer been used. The fertilizer was applied early and the new growth was well matured early in the fall. The plants were not unusually succulent, and the fact that the woody parts of the plants suffered no injury would indicate that the nitrogen supply was not excessive. The same reasoning may be applied to the use of the mulch.
In general, neither the age nor size of the plants seemed to make any difference in the amount of injury. With a number of varieties there were plants of several sizes often the large and small growing side by side. One might expect this result as all the plants of a particular variety seemed equally mature or dormant in early November just before the cold periods. Since the vegetative parts of the old and young plants appeared equally mature, the flower buds must have been equally mature also. However, it is likely that the flower buds of some varieties were further developed than those of others but no specific observations were made.
In travels connected with everyday duties, the writer noted that the bud killing was pretty general in the suburban areas of Washington. In the closely built sections of the city there appeared to be very little injury. Generally, the injury seemed greater in the writer's planting than was the case in other nearby areas, probably due to a greater proportion of tender types. The plantings of the average homeowner consists principally of the hardier Kurume types. However, in general, one could note that growers that gave the plants the best care suffered the most.
It seems surprising that the late blooming Macrantha group considered to be very hardy, suffered more bud killing than most others. One would conclude that its flower buds are inherently more tender to cold than the others. This may be due in part to the state of development of the flower buds at the approach of winter. Being late flowering, one might suspect that the Macrantha flower buds set late and hence the flower structures were very underdeveloped and fragile. They would therefore be easily torn apart by the formation of ice crystals. The flowers also could have been very embryonic in character and not conditioned for such low temperatures. This also might be true with respect to different varieties in the same group. The writer is aware of no work with azaleas that would settle this point.
It is suspected that the very severe injury in all groups during this season was caused by the earliness of the cold as well as to its severity. The primordial flower structures and the protective mechanism against the cold had not time to condition the cells for the winter.
Since weather conditions are highly variable, there are, of course, many other combinations of circumstances that may cause injury in azaleas. The most common combination is a very warm period during January and February followed by a sudden cold period. During the warm period the buds start the growth and expansion that occurs previous to flowering and thus lose their resistance to cold. This results in their being killed when the cold period occurs. It should be noted that behavior of the varieties under each set of conditions might be different. Observations over a period of several years would therefore be necessary to determine the susceptibility to bud killing. The results of the writer's observations suggest that more attention should be given to the evaluation of the varieties with regard to this injury.
Injury To Deciduous Types
No injury occurred to stems or flowers of any of the deciduous types grown by the writer. These include the species nudiflorum, mollis, vaseyi, arborescens, calendulaceum, alabamense, canescens, atlanticum, viscosum, austrinum, prunifolium, serrulatum, japonicum, bakeri, and a few varieties of the Ghent, Knaphill, and, Exbury hybrids. Even azaleas from a south Georgia nursery that arrived and were planted outside in the open a few days before the cold period of December 13-20 suffered no injury. These included the species austrinum, alabamense, canescens, prunifolium and serrulatum. Most of these had flower buds that opened perfectly normally at the expected blooming time the following spring. It was concern for these plants and the examination of them that resulted in early discovering that injury had occurred to certain evergreen types along side of which the recently acquired deciduous types were planted.