JARS v42n1 - Flowering Of Flame And Roseshell Azaleas

Flowering Of Flame And Roseshell Azaleas
Norman E. Pellett and Karen Alpert
University of Vermont
Burlington, Vermont

Native deciduous azaleas are prevalent in many uncultivated areas in the eastern United States, but urbanization and accompanying development is reducing the diversity of populations. We collected seed of Rhododendron calendulaceum and R. prinophyllum from the northern regions and higher elevations where the species are endemic. Knowledge of the behavior of these seed sources may help identify the diversity and adaptation of these species. Plant breeders may use this information in choosing desired characteristics.

Materials and Methods:
Seeds of the two species were collected from numerous plants within each natural habitat during the years 1978 to 1980 (Table 1). Seeds were germinated in the greenhouse and grown in outdoor ground beds for two years before planting in a loamy-sand soil at the Vermont Agricultural Experiment Station in South Burlington, Vermont. Each plot contained 24 plants one foot apart within the rows with rows 3 feet apart. Plants were irrigated daily with a trickle irrigation system and fertilized twice annually with ammonium sulfate and 15-15-15 applied approximately May 15 and July 1. No winter protection other than natural snowfall was provided.
Data were taken on the date for start of flowering and length of the flowering period. Flowering was considered to start when the first corolla was open to expose the stamens and to finish when the last corolla dropped or shriveled. The flowering period was probably one-third longer than most azalea enthusiasts would consider that the flowers were attractive. Flower color quality was rated 1 = the lightest color and 5 = the darkest color. The diameter of the top floret in the 3 highest stems was measured at full bloom.

Table 1. Location and elevation for native azaleas used in this study.
Species Location Elevation (ft) Latitude (°N) Longitude (°W)
R. calendulaceum
Kerens WV 2250 3902' 7949'
Dolly Sods WV 4000 3857' 7922'
Mt. Pisgah NC 5500 3525' 8245'
BRP 408 NC 4916 3534' 8334'
Wayah Bald NC 4200 3510' 82°03'
BRP 338 NC 3692 35°50' 82°03'
R. prinophyllum
Wolcott VT 1196 4434' 7225'
Lake Dunmore VT 600 4353' 7304'
Wendall St. For. MA 1100 4234' 7226'
Summit NY 1950 4233' 7432'
Dolly Sods WV 4000 38°57' 79°22'
BRP = milepost number on the Blue Ridge Parkway.


Flowering Dates
R. calendulaceum Plants flowered from 1 to 7 days earlier and for a longer period in 1985 than they did in 1984. Plants from seeds collected in West Virginia generally flowered several days earlier than those collected in North Carolina (Fig 1). Plants from Kerens WV flowered 10 days earlier than plants from BRP 338 NC in 1984 and 13 days earlier in 1985.
The first to flower each year came from Kerens WV and the last to start flowering came from BRP 338 NC. There was a trend toward shorter flowering periods for the West Virginia plants compared to the North Carolina plants. Kerens WV plants had shorter flowering in both 1984 and 1985 than the BRP 338 NC plants. Because of high plant-to-plant variation in duration of flowering, there were not always significant differences between the means of West Virginia and North Carolina sources.

R. calendulaceum first bloom chart
Figure 1. First bloom (A. Days after May 1) and number of days for flowering
(B.) of Rhododendron calendulaceum (Flame Azalea) from 6 seed sources.
Bars of the same year with the same letter on top are not significantly different (.05).

R. prinophyllum Plants from Dolly Sods WV started flowering later than the plants from Vermont, Massachusetts and New York (Fig 2A). The number of days for flowering was highly variable year-to-year and among plant sources during the same year and clear trends were not observed (Fig 2B). As with R. calendulaceum , there was a trend toward earlier and longer flowering in 1985 than in 1984 (Fig 2A and B).

R. prinophyllum first bloom chart
Figure 2. First bloom (A. Days after May 1) and number of days for flowering (B.) of
Rhododendron prinophyllum (Roseshell Azalea) from 5 seed sources.
Bars of the same year with the same letter on top are not significantly different (.05).

Heat units for flowering
The flowering of native azaleas may be related to the accumulation of heat units. The number of heat units are calculated by subtracting a critical base temperature from the average daily temperature. The base temperature is assumed to be that above which flowering is stimulated. Widrlechner et al. 1983, used accumulated heat units for determining the microsporogenesis of hybrid azaleas.
Using accumulated heat units to predict flowering of native azaleas assumes that flowering is only dependent on temperature over a critical base temperature. Also, it assumes that heat units accumulated early in the season during unseasonably warm weather are equally as important to start of flowering as those occurring close to the time of bloom. Other factors such as root zone temperatures, humidity, flower bud moisture content, flower bud temperatures, winter injury and wind may effect flowering time. However, we assume that air temperature has the primary influence on when azaleas commence flowering.
If accumulated heat units are the primary influence on the start of flowering, then the same plants should require the same number of heat units before start of flowering every year. Figure 3A shows the accumulated heat units at base 40° F. for 5 seed sources of Rhododendron prinophyllum . For all seed sources there was a higher number of heat units accumulated before flowering in 1985 than in 1984 (even though flowering time was not necessarily later). Less yearly variation was evident when only the heat units accumulated after May were used. Plants from Dolly Sods VW required more heat units than those from more northerly sources.

R. prinophyllum accumulated heat units to 
start of flowering.
Figure 3. Accumulated heat units to start of flowering (base 40 F.) (A.) and heat units
accumulated after May 1 (B.) for Rhododendron prinophyllum (Roseshell Azalea).

For Rhododendron calendulaceum , a species flowering later, thereby accumulating more heat units, there was less variation between years in heat units until flowering (Fig. 4). Fewer heat units accumulated before flowering of plants of the two West Virginia sources than plants of the North Carolina sources. The plants from BRP 338 NC consistently required more heat units to flowering than the others.

R. calendulaceum
 accumulated heat units to start of flowering.
Figure 4. Accumulated heat units to start of flowering
(base 40 F.) for calendulaceum (Flame Azalea).

Using a base temperature of 50 degrees F. resulted in more year-to-year variation than when 40 degrees F. was used as a base for both R. calendulaceum and R. prinophyllum . If temperature is the primary cause of flowering, then one should be able to predict the flowering time for other locations based on heat units in one location. Table 2 predicts the flowering dates in Albany, New York, and Baltimore, Maryland, based on heat unit accumulation from our 1984 and 1985 data for Burlington, Vermont. We caution that we have no data for Albany and Baltimore and other factors may cause different flowering times.

Table 2. Predicted flowering time in Albany, NY and Baltimore, MD for different seed sources of native azaleas based on accumulated heat units, base 40° F for Burlington VT flowering dates.
1984 1985
Species-location Burl. Albany Bait. Burl. Albany Bait.
R. calendulaceum
Kerens WV 6-4 5-29 5-5 5-29 5-15 4-22
Dolly Sods WV 6-6 6-2 5-8 6-2 5-20 4-27
Mt. Pisgah NC 6-11 6-9 5-19 6-8 5-25 5-1
Wayah Bald NC 6-11 6-9 5-19 6-9 5-26 5-6
338 BRP NC 6-14 6-11 5-22 6-12 5-29 5-6
R. prinophyllum
Lake Dunmore VT 5-26 5-23 4-29 5-23 5-11 4-19
Wendall MA 5-28 5-24 4-30 5-23 5-11 4-19
Summit NY 5-29 5-24 4-30 5-24 5-12 4-20
Wolcott Pond VT 5-31 5-25 5-1 5-24 5-12 4-20
Dolly Sods WV 6-2 5-26 5-4 5-28 5-15 4-22
Range of floral color 
of Flame Azalea collected near 
Nantehala Lake, North Carolina.
Range of floral color of Flame Azalea collected
near Nantehala Lake, North Carolina.
Photo by author

Corolla Characteristics
Corolla diameter for R. calendulaceum was smallest for the plants from West Virginia and largest on the plants from the BRP 338 NC site (Table 3). For R. prinophyllum . the larger corolla diameters were observed on the Dolly Sods WV and Lake Dunmore VT sources with the smallest on the Wendall State Forest MA plants. The pattern of corolla sizes seemed unrelated to latitude or elevation for the latter species.
Darker colors tended to follow the same pattern as corolla sizes for both species (Table 3). When ranked in order of their mean ratings from light pink (lower number) to rosy pink (higher number), R. prinophyllum sources were generally in the same order during 1984 and 1985 ratings as the diameter rankings. Smaller diameter flowers generally corresponded with lighter colors. Yellow in corollas were more associated with smaller corollas for R. calendulaceum than orange or red color.

Table 3. Corolla diameter 1 (cm) and color rating 2 of Rhododendron calendulaceum and R. prinophyllum .
Source R. calendulaceum 1984 R. prinophyllum 1984 R. prinophyllum 1985
Dia. Color Dia. Color Dia. Color
Dolly Sods WV 3.2 2.2
Kerens WV 3.3 3.1
Mt. Pisgah NC 3.5 3.7
BRP 408 NC 3.6 3.4
Wayah Bald NC 3.9 3.7
BRP 338 NC 4.1 4.2
Wendall St. For. MA 2.4 3.4 2.5 2.7
Wolcott Pond VT 2.6 3.1 2.8 2.7
Summit NY 2.7 3.3 2.8 2.5
Lake Dunmore NY 3.0 3.9 2.9 3.2
Dolly Sods WV 3.0 4.2 3.1 3.6
1 Average of 3 florets on all plants. 2 Corolla color rating of 1 to 5 where 1 = lightest color, 5 = darkest color. Mean of all plants.
R. 'Elspeth'
Range of floral color of Roseshell Azalea collected near
Lake Dunmore, Vermont. Background foliage is Striped Maple.
Photo by author

R. calendulaceum Plants from two West Virginia sources flowered earlier with a trend toward shorter periods of flowering than four North Carolina sources. Heat units for flowering in 1985 was within 89% of the values of 1984 for each of the six seed sources. West Virginia sources had smaller flowers and lighter colors (more yellows than reds) than the North Carolina plants. Plants from BRP 338 NC flowered later, had larger flowers and darker red colors than other sources.
R. prinophyllum Plants from Dolly Sods WV flowered later than 4 more northerly sources which weren't consistently different from each other. Accumulated heat units from May 1 to flowering were similar for 1984 and 1985 for all of the plant sources. Corolla diameter was highest for Dolly Sods WV plants and the average color was darker pink than plants from the other sources.
In general, plants from northern sources of both species flowered earlier requiring fewer heat units to start of flowering.

Literature Cited:
Widrlechner, M.P., H.M. Pellett and P.D. Ascher. 1983. The timing of microsporogenesis in deciduous azalea. Amer. Rhod. Soc. Journal 37(2):91-94.

Norman Pellett and Karen Alpert are associated with the Department of Plant and Soil Science, University of Vermont, Burlington, Vermont 05405.