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Volume 58, Number 1
Winter 2004

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Cold Hardiness of Robin Hill, Holly Springs and Back Acres Hybrid Azalea Selections
Gary J. Keever, Charles H. Gilliam, John W. Olive, John T. Owen, and Victor E. Jackson
Department of Horticulture, Alabama Agricultural Experiment Station
Auburn University, Alabama

Synopsis
Mild fall temperatures followed by a sudden temperature drop below freezing frequently lead to injury of evergreen azalea cultivars. However, freezing tolerance of non-acclimated azaleas appears to vary widely among cultivars within the Robin Hill, Holly Springs and Back Acres hybrid groups. Knowledge of how these cultivars respond to rapid drops in temperature following mild temperatures should aid in the selection of cultivars for large-scale nursery production in the southern U.S. where these climatic conditions are common.

Introduction
As a harbinger of spring, azaleas have enjoyed a traditional importance in the South afforded few other plants. However, due to a short bloom period and despite desirable attributes such as form and foliage texture at other times, growers have experienced a declining market for and price of azaleas common in the nursery trade (thereafter, 'standards'). The problem is exacerbated by the limited number of cultivars commonly grown and sold by the nursery industry. For example, in an unpublished survey of several Mobile, Alabama, growers, fewer than 20 cultivars were identified as comprising the majority of sales, and of these cultivars, most were either Kurume or Southern Indian selections.
        Because of concerns about the long-term health of this major crop, we initiated a large scale azalea evaluation in 1999. The overall goal of this project is to select and promote azalea cultivars that show superior performance under the conditions of rapid growth induced in large-scale commercial nursery production and subsequent landscape plantings. Our current focus is limited to evergreen azaleas. Our approach has been to collect cuttings representing specific hybrid groups not widely available in the nursery industry and to propagate and grow plants under production conditions in south Alabama for one or two years. Following production, plants are field-planted in central Alabama and evaluated for landscape performance. Response to environmental conditions in the winter following fall planting may be influenced by recent production conditions (abundant fertilizer and water) and not be indicative of how the same cultivars would respond after becoming established. A final component of the evaluation will be to promote superior selections through a marketing program.
        In a natural environment, shortening days and decreasing temperatures in fall trigger the cessation of growth, leading to cold acclimation or winter hardiness in woody plants of most temperate zone species (4, 6). Winter injury to evergreen species may occur from desiccation (2), low temperatures (1, 5), or rapid drops in temperature (7). Injury to evergreen azaleas from rapid temperature drops are particularly common in the southern U.S. in the fall before plants have fully cold acclimated, especially when grown under luxurious conditions of abundant fertilizer and water common in nursery production. Furthermore, cultivars of Rhododendron vary in their rate of cold acclimation resulting in differences in injury when exposed to low fall temperatures (3). The specific focus of this paper is to examine cold hardiness differences, as expressed by two types of injury symptoms, among cultivars within hybrid groups grown under commercial production conditions when exposed to mild fall temperatures followed by sudden temperature drops.

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Material and Methods
SCOPE OF THE PROJECT. Efficient utilization of limited resources has forced us to implement our evaluation in phases. Annually, since 1999 we have targeted specific hybrid groups for introduction into our study. These groups include Back Acres, Robin Hill, Holly Springs, and Huang in 1999; Satsuki and Southern Indian in 2000; and Harris, Gartrell, and the 1983 U.S.D.A. Kurume hybrid introductions from Japan in 2001. The focus of this paper is to report results related to cold hardiness of cultivars in the hybrid groups propagated in 1999. Hence, the remainder of the paper will exclude reference to groups subsequently introduced into the study.

PRODUCTION. Between May and July 1999, basal ends of semi-hardwood, terminal cuttings, 3.8-12.7cm (1.5-5 in.) in length, were placed in 2,500 ppm indole butyric acid using the potassium salt of IBA for 5 seconds, stuck into 72-cell flats containing unamended peat: perlite: vermiculite (1:1:1, by vol.), and rooted under intermittent mist in a glass greenhouse in Mobile, Alabama (USDA Hardiness Zone 8b, latitude 30041'N). Rooted cuttings were transplanted between July and October 1999 into 36-cell flats (18 cuttings/cultivar) of a 3 pine bark: 1 peat substrate amended per m3 (yd3) with 8.3 kg (14 lb.) of Osmocote 15-9-12 Plus (12-14 month formulation + minors), 3.6 kg (6 lb.) dolomitic limestone, and 1.2 kg (2 lb.) gypsum. This growing medium approximates mixtures typically used in large-scale nursery production. Plants were grown over winter in a polyethylene greenhouse with a heat set point of 15.6 C (60 F). In April and May 2000, 15 plants per cultivar were transplanted into 7.6 liter (2 gal.) pots containing the same amended substrate and placed outdoors in full sun under overhead irrigation. No preventive insect or disease control measures were taken; however, outbreaks of lace bugs and mites were treated as needed.
        On November 28, 2000, 57 cultivars of Robin Hill, 51 cultivars of Holly Springs, and 27 cultivars of Back Acres azaleas were shipped from the production area to the Piedmont Substation in Camp Hill, Alabama (USDA Hardiness Zone 7b, latitude 32048'N) for planting. In addition, four industry standard cultivars, 'Kirin' (syn. 'Coral Bells') (Kurume), 'Formosum' (Southern Indian), 'Hinodegiri' (Kurume), and 'Snow' (Kurume), grown under the same conditions were shipped. The Huang azaleas were judged too small for planting and were maintained in production for an additional year.

LANDSCAPE. The 16-acre planting site consists of 35-year old planted pines enclosed by an 8-foot deer fence. Beds were prepared by incorporating finely ground pine bark into a Gwinnett sandy loam soil (1:1, by vol.) and tilling with a 3-point lift tiller with an attached bed shaper which formed beds 20 cm (8 in.) high, 1.2 m (4 ft.) wide and 3.7m (12. ft.) apart. On December 13, 2000, azaleas were planted on 1.5 m (5 ft.) centers, using a completely randomized design within hybrid groups and 4 replications of 2 plants each. Plants were irrigated with two micro jet spray emitters (Antelco microjet blue) per plant which at 15 psi, delivered 33.7 l (8.9 gal.) hour. Plants normally were irrigated for 30 to 45 minutes as needed. On January 16, 2001, percent injury was estimated by two individuals and averaged, and the type (wilt and/or burn and location [top half and/or bottom half] of foliar injury, and presence and location [top and/or bottom half]) of bark splitting were determined.
        Table values are the means of all plant replications of each cultivar. Means for type and location of injury less than 1.5 indicate injury was generally restricted to a single type or location, whereas higher value indicate multiple symptoms and locations. Percent, type, and locations of injury were again determined on March 8, 2001, as well as the number of dead shoots. Statistical comparisons among cultivars within a hybrid group and standard azaleas were made using a Fisher's protected LSD (P=0.05). Means within columns that differ by more than the LSD value were considered statistically different (the probability is 5% or less that the observed variation among means could occur by chance).

Results and Discussion
In November 2000, average high and low temperatures in Mobile were 19.6C (67.3F) and 8.2C (46.7F), respectively, with temperatures greater than or equal to 26.7C (80F) on 6 days and temperatures below freezing occurring on only 3 nights. These mild fall temperatures are typical in the Lower South and frequently inhibit cold acclimation of azaleas. In contrast, average high and low temperatures in Camp Hill between December 1, 2000 and January 3, 2001 were 8.9C (48F) and -6.6C (19.9F), respectively, with low temperatures of -14.4 to -12.2C (6 to 10F) occurring on 7 nights and -12.2 to -9.4C (10 to 15F) on 6 nights.
        In general, most cultivars were consistent in their injury percentage and ratings. However, injury percentage and ratings of some cultivars varied greatly, possibly due to differences in location, individual plant growth differences, or both. While location appeared to have a definite effect on cold injury, a pattern was not easily discerned. In contrast, plant growth stage had a major and easily discernable role in cold injury. Generally, first or spring growth, which was older and more hardened, was much less injured than second or summer/fall growth, which was younger and more tender. First growth injury was generally expressed as foliar burn, while second growth injury was expressed predominantly as wilt and secondarily as foliar burn. Cultivars having more second growth tended to have a higher injury percentage.

Robin Hill and other Gartrell Hybrids. In January, foliar injury ranged from 3% to 87% with a mean of 23%. Injury to 10 of the 57 cultivars was greater than or equal to 40%, being as high as 84% in 'George Harding' and 87% in T15-9 (Table 1). In contrast, 15 cultivars exhibited less than or equal to 10% injury and two cultivars, 'Cherie' and 'Mrs. Villars', had only 3% injury. In general, injury was less in the standard cultivars, ranging from 5% to 48%, with a mean of 18% and injury greater than or equal to 40% in one cultivar and less than or equal to 10% in two cultivars. By March foliar injury to Robin Hill hybrids was much less, ranging from 1% to 47% and having a mean of 12%. Injury in only two cultivars, 'Robin Dale' (47%) and T29-9 (41%) was greater than or equal to 40%, while injury in 32 cultivars was less than or equal to 10%.
        Most cultivars exhibited a single foliar symptom of low temperature injury, either wilting or foliar necrosis (burn), in January (type mean of 1.3). In the majority of cultivars, these symptoms were present in either the upper or lower half of the canopy, but not both (location mean of 1.2). There was no bark split on 42% of the Robin Hill cultivars or on any of the standard cultivars, while 81% of the cultivars had bark split ratings of less than or equal to 0.5 indicating bark split was primarily in the upper canopy; the majority of plants of most cultivars had no bark split, or both. Bark split in the upper canopy is less serious than that of the lower mark trunk and usually not fatal. By March, most cultivars (93%) exhibited a single symptom , foliar burn, in the majority of cases, in a single location. The number of dead shoots generally was low (less than or equal to 5/plant) and appeared to be associated with higher bark split ratings. 'Robin Dale' and T29-9 had the most dead shoots, 15 and 9, respectively, which is consistent with the high percentage of foliar injury in these cultivars in January and March.

Holly Springs. In January, foliar injury ranged from 1% to 77% with a mean of 21%. Injury to eight of the 51 cultivars was greater than or equal to 40% and exceeded 70% in 'Cotton Candy' (Vines)* (72%), 'Neptune' (Vines)* (77%) and 'Virginia Vines' (72%). In contrast, injury was less than or equal to 10% in 20 cultivars and less than or equal to 2% in 86-11-7, 86-33-1, 'First Love' and 'Shenandoah'(Vines)*. In March, a similar range in foliar injury remained, from 1% to 74% with a mean of 17%. Injury exceeded 40% in four cultivars, 85-32-7 (46%,), 'Cotton Candy' (Vines)* (51%), 'Morgan Anne' (48%), and 'Virginia Vines' (74%), but was less than or equal to 10% in 24 cultivars, suggesting tremendous differences in cold hardiness within Holly Springs hybrids.
        As with the Robin Hill azaleas, most Holly Spring cultivars exhibited a single type of injury (mean of 1.2 in January and 1.1 in March ), foliar burn, in a single location (mean of 1.2 in January and 1.1 in March), with location variable among cultivars. Eighteen cultivars exhibited no bark split, while 30 cultivars had bark split ratings of less than or equal to 0.5, suggesting bark split was primarily in the upper canopy where it typically is not fatal. Bark split ratings were highest for 85-32-7, 'Cotton Candy' (Vines)*, and 'Virginia Vines', three cultivars with greater than or equal to 40% foliar injury at both sampling dates The number of dead shoots was generally low, ranging from 0 to 10. Eight cultivars had greater than or equal to 6 dead shoots with 85-32-7 (10) and 'Purple Magic' (8) having the most.

Back Acres. As with the two previous hybrid groups, foliar injury to the Back Acres hybrids varied widely among cultivars, from 4% to 75% in January and from 1% to 82% in March; mean percent injury for the two dates was 20.4% and 21.6%, respectively (Table 3). Four and five of the 27 cultivars had greater than or equal to 40% injury on the two sampling dates, 197-49-1, 54-31-4, 'Fred Lee'*, 'Painted Tips', and 'Ivan Anderson' (March only), while 11 and 10 cultivars had less than or equal to 10% injury. The lowest injury percentage in March was to 31-54-1 (4%), 'Debonaire' (3%), 'Largesse' (4%), 'Target' (4%), and 'White Jade' (1%). At both sampling dates, injury to most cultivars was of a single type (January: 93% of cultivars with rating of less than or equal to 1.5 (mean 1.2); March: 100% of cultivars less than or equal to 1.5 (mean 1.1) and location (January: 1.2; March:1.2). Bark split was present on 85% of the cultivars with rating less than or equal to 1.2 on 197-49-1, 222-49-1, 40-57-11, 'Fred Lee'*, 'Heirloom', 'Ivan Anderson', 'Mala-guena', and 'Misty Plum'. In contrast, bark split ratings were 0 for 255-70, 'Keepsake', 'Spring Bonnet', and 'White Jade'. The number of dead shoots ranged from 0 to 12 and were highest in 197-49-1 and 'Painted Tips'. There was a general positive correlation between foliar injury percentage and the number of dead shoots, except in 54-314 with 82% foliar injury in March but no dead shoots.

Table 1. Low temperature injury to cultivars of Robin Hill and other Gartrell hybrid azaleas.
  January 16, 2001   March 8, 2001
Cultivarz % Typey Locationx Bark Splitw   % Type Location Dead Shoots (No.)
'Antoine' 9 1.0 1.3 0.0   13 1.0 1.1 1
'Betty Anne Voss' 35 1.7 1.3 1.0   25 1.0 1.1 5
'Blue Tip' 21 1.4 1.7 0.6   14 1.0 1.1 5
'Chanson' 14 1.1 1.0 0.1   7 1.0 1.1 1
'Cherie'v 3 1.0 1.0 0.0   3 1.0 1.0 0
'Conversation Piece' 19 1.3 1.3 0.0   17 1.0 1.0 5
'Corry' 24 1.7 1.5 0.6   17 1.0 1.0 6
'Dorothy Hayden' 10 1.0 1.1 0.4   5 1.0 1.0 3
'Dorothy Rees' 18 1.7 1.0 0.0 6 1.0 1.1 0
'Eliza Scott' 27 1.7 1.5 0.5   19 1.0 1.3 5
'George Harding' 84 1.9 1.7 0.0   24 1.0 1.3 5
'Glamora' 37 1.5 1.3 0.0   18 1.4 1.5 0
'Gresham' 8 1.0 1.0 0.1   2 1.0 1.0 1
'Greta' 7 1.3 1.0 0.0   4 1.0 1.0 1
'Gwenda' 12 1.1 1.0 0.1   3 1.0 1.0 1
'Hilda Niblett' 18 1.3 1.0 0.5   7 1.0 1.1 1
'Jeanne Weeks' 12 1.0 1.0 0.0   7 1.0 1.3 2
'La Belle Hlne' 34 1.3 1.4 0.3   15 1.0 1.3 2
'Lady Louise' 44 1.3 1.0 0.5 4 1.0 1.0 1
'Lady Robin' 26 1.4 1.1 0.0   6 1.0 1.0 0
'Laura Morland' 49 1.4 1.3 0.4   27 1.0 1.4 5
'Madame Mab Chalon' 49 2.0 1.3 0.8   20 1.0 1.1 6
'Maria Derby' 11 1.0 1.1 0.0   4 1.0 1.1 0
'Maxine West' 25 1.3 1.0 0.3   9 1.0 1.5 1
'Mrs Emil Hager' 55 1.8 1.0 1.8   20 1.4 1.3 5
'Mrs Villars' 3 1.3 1.0 0.0   2 1.0 1.0 0
'Nancy of Robinhill' 13 1.1 1.1 0.0   2 1.0 1.2 0
'Olga Niblett' 38 1.4 1.0 0.0   14 1.0 1.0 4
'Peg Hugger' 40 1.5 1.4 0.3   15 1.0 1.0 3
'Redmond' 8 1.1 1.1 0.0   6 1.0 1.0 1
'Red Tip' 32 1.3 1.0 1.3   21 1.0 1.3 6
'Richie' 12 1.0 1.0 0.0   2 1.0 1.0 0
'Robin Dale' 54 1.5 1.7 0.5   47 1.3 1.7 15
'Robin Hill Elsa' 14 1.0 1.3 0.4   10 1.0 1.0 4
'Robin Hill Gillie' 32 1.7 1.1 0.1   12 1.0 1.1 3
'Robin Hill Wendy' 7 1.1 1.1 0.4   3 1.0 1.0 1
'Roddy' 7 1.0 1.0 0.0   3 1.0 1.0 0
'Sara Holden' 11 1.0 1.0 0.0   1 1.0 1.0 0
'Sir Robert' 10 1.3 1.0 0.5   4 1.0 1.0 1
'Spink' 19 1.3 1.0 0.1   5 1.0 1.0 1
T14-9 Gartrell Hybrid 9 1.4 1.1 0.0   4 1.0 1.0 1
T15-9 Gartrell Hybrid 87 1.9 1.1 0.9   38 1.0 1.4 5
T16-4 Gartrell Hybrid 8 1.3 1.0 1.0   6 1.0 1.0 3
T19-7 Gartrell Hybrid 47 1.0 1.9 0.3   21 1.0 1.1 4
T22-9 Gartrell Hybrid 12 1.7 1.0 0.2   7 1.0 1.0 2
T29-9 Gartrell Hybrid 43 1.7 1.5 1.5   41 1.0 1.4 9
T3-4 Gartrell Hybrid 20 1.3 1.0 0.0   3 1.0 1.0 0
T37-7 Gartrell Hybrid 22 1.3 1.3 1.3   18 1.0 1.0 5
'Turk's Cap' 16 1.4 1.0 0.1   15 1.1 1.0 1
V1-2 Gartrell Hybrid 26 1.7 1.0 0.0   29 1.0 1.0 1
V14-1 Gartrell Hybrid 16 1.4 1.0 0.8   15 1.0 1.0 3
V3-2 Gartrell Hybrid 18 1.3 1.4 0.4   5 1.0 1.0 2
'Watchet' 12 1.4 1.1 0.0   5 1.0 1.2 0
'Welmet' 6 1.4 1.0 0.0   6 1.0 1.0 1
'White Hart' 19 1.4 1.1 0.5   10 1.0 1.0 4
'Whitehead' 8 1.0 1.0 0.0   6 1.0 1.0 2
'White Moon' 9 1.1 1.0 0.0   3 1.0 1.0 1
                   
Standards                  
'Kirin' ('Coral Bells') 6 1.4 1.0 0.0   3 1.0 1.0 1
'Formosum' 13 1.1 1.0 0.0   10 1.0 1.0 0
'Hinodegiri' 5 1.0 1.0 0.0   1 1.0 1.0 1
'Snow' 48 1.3 1.4 0.0   33 1.0 1.0 7
                   
LSDu 16 0.3 0.4 1.2   12 0.1 0.3 3
                   
Table 1 Key:
  z Listing includes named Robin Hill hybrids and unnamed hybrids by Robert Gartrell.
  y Type: 1 = wilt or burn; 2 = both wilt and burn.
  x Location: 1 = top or bottom half; 2 = both top and bottom halves.
  w Bark split: 0=none; 1=upper half of canopy; 2=lower half of canopy; 3=both upper and lower halves.
  v Gartrell Hybrid V12-4; not included by Gartrell in his Robin Hill selections.
  u Mean separation by Fisher's protected LS (P=0.05).
                   
Table 2. Low temperature injury to cultivars of Holly Springs azaleas.
  January 16, 2001   March 8, 2001
Cultivarz % Typey Locationx Bark Splitw   % Type Location Dead Shoots (No.)
81-26 (N.R.) 17 1.0 1.0 0.8   8 1.0 1.0 1
81-33 (N.R.) 2 1.3 1.3 0.9   4 1.0 1.0 1
84-23-22 (N.R.) 16 1.0 1.0 0.0   11 1.0 1.0 4
85-32-7 (N.R.) 53 1.3 1.4 2.0   46 1.0 1.7 10
86-11-7 (N.R.) 2 1.0 1.0 0.0   1 1.0 1.0 0
86-33-1 (N.R.) 1 1.0 1.0 0.0   9 1.0 1.0 1
'Amaretto' (N.R.C.) 38 1.0 1.0 0.0   3 1.0 1.0 0
'Astronaut' (N.R.C.) 8 1.1 1.3 0.8   7 1.0 1.0 1
'Blue Ice' 27 1.3 1.3 0.5   31 1.9 1.7 2
'Brocade Pillow' 7 1.1 1.1 0.0 8 1.5 1.1 6
'Carolyn' 23 1.4 1.3 0.9   23 1.0 1.0 6
'Cotton Candy' (N.R.C.) 72 1.7 1.8 1.9   51 1.7 2.0 6
'Crimson Queen' (N.R.C.) 14 1.0 1.0 0.3   18 1.1 1.0 1
'Curtain Call' 22 1.7 1.5 0.6   12 1.0 1.0  2
'Dawn Elizabeth' 7 1.0 1.1 0.0   9 1.0 1.0 1
'Dolly' (N.R.C.) 16 1.0 1.0 0.5   7 1.0 1.0 1
'First Love' (N.R.C.) 2 1.0 1.0 0.0   1 1.0 1.0 1
'Gemini' (N.R.C.) 11 1.0 1.3 1.1   14 1.0 1.0 4
'Homecoming' 8 1.0 1.0 0.8   7 1.3 1.0 1
'Irish Cream' (N.R.C.) 7 1.3 1.5 0.0   9 1.0 1.0 1
'Jeffery Alan' 11 1.3 1.1 0.3   18 1.3 1.0 2
'Jessica's Jewel' 10 1.0 1.0 0.3   2 1.0 1.0 3
'Jill McDowell' 17 1.1 1.2 1.4   18 1.0 1.0 6
'Jim Baggette' 22 1.1 1.1 0.8   11 1.0 1.0 2
'June Jasmine' 19 1.1 1.4 1.0   20 1.0 1.0 6
'Kaleidoscope' (N.R.C.) 9 1.0 1.0 0.3 21 1.3 1.0 1
'Lady Godiva' (N.R.C.) 32 1.1 1.4 0.0   12 1.0 1.1 1
'Monticello' 9 1.4 1.1 1.0   8 1.7 1.0 2
'Morgan Anne' 56 1.4 1.4 1.0   48 1.0 1.7 2
'Nana' (N.R.) 6 1.0 1.3 0.3 4 2.0 1.0 1
'Nancy Stipa' 7 1.0 1.3 0.0   2 1.0 1.0 1
'Neptune' (N.R.C.) 77 1.4 1.7 0.0   33 1.3 1.3 4
'Nocturnal Bliss' 23 1.3 1.1 0.3   23 1.5 1.5 3
'Nymph' (N.R.C.) 14 1.0 1.0 0.5   3 1.0 1.0 4
'Peggy Vines' 8 1.1 1.0 0.3   13 1.3 1.3 3
'Peppermint Stick' (N.R.C.) 41 1.3 1.5 0.8   37 1.0 1.0 5
'Pink Parfait' (N.R.C.) 12 1.0 1.0 0.3   9 1.0 1.0 0
'Pink Persuasion' 5 1.0 1.1 0.0   3 1.0 1.0 3
'Purple Magic' 65 1.7 1.9 0.1   39 1.0 1.0 8
'Raspberry Rapture' 27 1.1 1.0 0.0   31 1.4 1.4 4
'Ron Vines Junior' 45 1.7 2.7 0.8   35 1.0 1.4 6
'Sabra' 27 1.3 1.3 0.0   18 1.7 1.0 1
'Scarlet Princess' 12 1.1 1.1 0.0   9 1.0 1.0 4
'Sensuous' 24 1.0 1.0 0.0   21 1.0 1.2 0
'Shamrock' (N.R.C.) 28 1.3 1.3 0.8   36 1.0 1.4 3
'Shanghai Suzie' 10 1.0 1.0 0.7   5 1.0 1.0 4
'Shenandoah' (N.R.C.) 2 1.0 1.0 0.0   3 1.0 1.0 1
'Stormy' 9 1.0 1.0 0.0   13 1.0 1.3 0
'Tropicanna' (N.R.C.) 8 1.0 1.0 0.9   6 1.0 1.0 2
'Virginia Vines' 72 1.3 1.5 2.0   74 2.0 1.7 4
'Whipped Cream' (N.R.C.) 22 1.0 1.0 0.8   9 1.3 1.0 4
                   
Standards                  
'Kirin' ('Coral Bells') 6 1.0 1.3 0.0   3 1.0 1.0 1
'Formosum' 14 1.0 1.2 0.0   10 1.0 1.0 0
'Hinodegiri' 5 1.0 1.0 0.0   1 1.0 1.0 1
 'Snow' 48 1.3 1.3 0.0   33 1.0 1.1 7
                   
LSDv 15 0.3 0.4 1.4   12 0.3 0.3 3
                   
Table 2 Key:
  z Listing includes named and unnamed Holly Springs hybrids.
  y Type: 1 = wilt or burn; 2 = both wilt and burn.
  x Location: 1 = top or bottom half; 2 = both top and bottom halves.
  w Bark split: 0=none; 1=upper half of canopy; 2=lower half of canopy; 3=both upper and lower halves.
  v Mean separation by Fisher's protected LS (P=0.05).
  (N.R.) Name not registered.
  (N.R.C.) Name not registered and in conflict with registered name.
 
0.31.0 11.0
Table 3. Low temperature injury to cultivars of Back Acres azaleas.
  January 16, 2001   March 8, 2001
Cultivarz % Typey Locationx Bark Splitw   % Type Location Dead Shoots (No.)
197-49-1 (N.R.) 51 1.1 1.4 1.9   53 1.1 1.6 12
222-49-1 (N.R.) 15 1.3 1.1 1.3   16 1.1 1.0 2
250-49-3 (N.R.) 9 1.0 1.2 0.5   17 1.1 1.0 2
255-70 (N.R.) 7 1.9 1.0 0.0   13 1.3 1.0 1
31-54-1 (N.R.) 5 1.0 1.3 0.3   4 1.0 1.0 0
40-57-11 (N.R.) 24 1.1 1.5 1.2   31 1.1 1.3 4
54-31-4 (N.R.) 75 1.1 1.5 0.3   82 1.5 1.6 0
'Apricot Honey' 20 1.8 1.3 0.3   19 1.0 1.0 6
'Debonaire' 7 1.0 1.0 0.5   3 1.0 1.0 1
'Dream' X 'Gunrei' 23 1.1 1.1 0.3   13 1.1 1.0 1
'Fred Lee' (N.R.) 52 1.5 1.3 1.5   59 1.0 1.5 7
'Garnet Royal' 20 1.0 1.3 0.4   23 1.0 1.1 4
'Heirloom' 34 1.3 1.4 1.5   34 1.0 1.4 5
'Ivan Anderson' 36 1.3 1.1 1.3   40 1.0 1.5 8
'Keepsake' 14 1.0 1.3 0.0   13 1.0 1.0 0
'Largesse' 5 1.0 1.1 0.8   4 1.0 1.0 0
'Malaguena' 8 1.0 1.2 1.3   9 1.0 1.3 2
'Merrymaker' 15 1.1 1.0 1.3   18 1.0 1.1 2
'Misty Plum' 7 1.0 1.0 1.5   18 1.0 1.0 1
'Painted Tips' 43 1.4 1.4 1.0   48 1.0 1.3 12
'Red Slippers' 33 1.1 1.3 0.9   28 1.0 1.2 6
'Saint James' 11 1.0 1.0 0.5   9 1.0 1.0 2
'Spring Bonnet' 11 1.3 1.5 0.0   9 1.0 1.0 2
'Stormcloud' 9 1.0 1.1 0.3   10 1.0 1.1 1
'Target' 7 1.0 1.1 0.1   4 1.0 1.0 0
'White Jade' 6 1.0 1.3 0.0   1 1.0 1.0 0
                   
Standards                  
'Kirin' ('Coral Bells') 6 1.0 1.0 0.0   3 1.0 1.0 1
'Formosum' 13 1.0 1.5 0.0   10 1.0 1.0 1
'Hinodegiri' 5 1.0 1.0 0.0   1 1.0 1.0 1
'Snow' 48 1.0 1.0 0.0   33 1.0 1.5 1
                   
LSDw 18 0.2 0.4 1.4 15 0.2 0.4 4
                   
Table 3 Key:
  z Listing includes named and unnamed Back Acres hybrids.
  y Type: 1 = wilt or burn; 2 = both wilt and burn.
  x Location: 1 = top or bottom half; 2 = both top and bottom halves.
  w Bark split: 0=none; 1=upper half of canopy; 2=lower half of canopy; 3=both upper and lower halves.
  v Mean separation by Fisher's protected LS (P=0.05).
  (N.R.) Name not registered. 
                   

Conclusion
Mild fall and early winter temperatures followed by rapid drops in temperature are common in the southern U.S. and often result in injury or death of evergreen azaleas. Injury under these conditions may be exacerbated by nursery production practices of high fertilizer and water inputs. Under similar conditions, injury varied widely among cultivars in the hybrid groups Robin Hill, Holly Springs and Back Acres suggesting differences in rate of acclimation or overall cold hardiness. While injury to some cultivars was severe, only two plants had died by the end of the 2001 growing season, apparently from phytophthora root rot. This study provides information that may be useful in the selection of cultivars from these three hybrid groups for large-scale nursery production in areas where widely fluctuating fall or early winter temperatures are common.

Acknowledgements
The authors appreciate the invaluable support provided by Maarten van der Giessen, Van Der Giessen Nursery, Semmes, Alabama, Mr. and Mrs. Charles S. Hanners, Azalea Trace, Huntington, Maryland, and the American Rhododendron Society.

Literature Cited
1. Dennis, Jr., F.G., G.P. Lumis, and C.R. Olien. 1972. Comparative freezing patterns in stems of cherry and azalea. Plant Physiol. 50:527-530.
2. Havis, J.R. 1965. Desiccation as a factor in winter injury of rhododendron. Proc. Amer. Soc. Hort. Sci. 86:764-769.
3. Holt, M.A. and N.E. Pellett. 1981. Cold hardiness of leaf and stem organs of rhododendron cultivars. J. Amer. Soc. Hort. Sci. 106(5):608-612.
4. Levitt, J. 1980. Responses of Plants to Environmental Stresses. Academic Press, New York.
5. Lumis, G.P. and R.A. Mecklenburg. 1974. Freezing patterns in twigs of evergreen azalea. J. Amer. Soc. Hort. Sci. 99:564-567.
6. Weiser, C.J. 1970. Cold Resistance and Injury in Woody Plants. Science 169:1269-1278.
7. White, W.C. and C.J. Weiser. 1964. The relation of tissue desiccation, extreme cold, and rapid temperature fluctuations to winter injury of American arborvitae. Proc. Amer. Soc. Hort. Sci. 85:554-563.

*Name is not registered.

Gary J. Keever and Charles H. Gilliam are professors in the Department of Horticulture, Auburn University, Auburn, AL 36849. John W. Olive is superintendent of the Ornamental Horticulture Substation, 411 North McGregor Ave., Mobile, AL 36689. John T. Owen and Victor E. Jackson are superintendents at the Piedmont Substation, P. O. Box 368, Camp Hill, AL 36850.


Volume 58, Number 1
Winter 2004

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