Adaptability of Evergreen Rhododendrons to the Great Plains as Influenced by Landscape Exposure
John C. Pair
Horticultural Research Center
Kansas State University, Wichita, Kansas
Review of Literature
According to a recent survey, rhododendrons continue to be among the favorite evergreen shrubs in America (Schafer, 1989), having surpassed conifers in sales in the nursery industry many years ago, according to USDA (Leach, 1959). The most favorable climatic regions for the cultivation of rhododendrons continue to be the Pacific Northwest and Middle Atlantic states, but test gardens and private individuals have indicated success in various other regions such as Ohio (Ford, 1982) and Oklahoma (Miller, 1977). Much has been written about low temperature injury, winter desiccation, and effects of rapid temperature drop on Rhododendron (Pellett and Holt, 1988; Sakai et. al., 1986), but George Ring (1981) and others have suggested that more information is needed on heat tolerance of the genus. In recent years, attempts have been made to breed plants with combinations of heat tolerance and cold hardiness required in regions of wide temperature ranges. Species such as R. degronianum ssp. heptamerum ( R. metternichii Slebd and Zucc.) from Japan (Arisumi et al., 1985) and R. yakushimanum Nakai or combinations of R. catawbiense Michx. x R. yakushimanum have been suggested (Leach, personal correspondence, 1987) as possible sources of heat tolerance.
Ultimately, a test for heat tolerance has to involve resistance to Phytophthora , particularly P. cinnamomi but also cactorum and other species involved in the root rot syndrome (Hoitink and Schmitthenner, 1975).
Materials and Methods
After numerous suggestions by members of the American Rhododendron Society, nurserymen, and growers of rhododendrons, a selection of species and cultivars was obtained for trial in Kansas. The test site was the Kansas State University Horticulture Research Center at Wichita. Its climate is characterized by dry, cold winters (usually -5° to -10°F) and often hot, dry summers in which temperatures can exceed 100°F. Two distinct tests were designed in this experiment: (1) a screening of cultivar performance under the most favorable conditions of semishade, wind protection, and soil modification, and (2) a specific comparison of large-leaved*** rhododendrons with lepidote types around structures previously used for evaluating heat tolerance of blue holly cultivars (Pair and Still, 1982). Previous tests with the PJM group, a lepidote type, indicated good performance in a site with north exposure receiving considerable exposure to summer sun and winter winds (unpublished data).
Three single plant replications of 38 cultivars were established in the spring of 1987 under the partial shade of honey locust trees bordered by a row of pine trees on the south, which provided excellent winter shade. Soil preparation began with the addition of rotted wood chips, approximately one third by volume plus sulfur granules at the rate of 3 lbs. per 100 sq. ft. to a Canadian sandy loam soil to form a raised bed approximately 8-10 in. above grade. Prior to planting, sphagnum peat was incorporated at 5 cu. ft. per 100 sq. ft., and a pine bark mulch was applied following planting. Plants were initially watered by overhead irrigation for establishment, but a drip irrigation system was added later.
Plants supplied from various wholesale growers ranged from 6-inch liners to plants in 3-gal. containers but were arranged in a block design of six rows to accommodate various sizes in uniform rows with plants on 2.5-ft. centers. A winter windscreen was erected to protect the planting during the first two winters. Fortunately, good snow cover during the coldest portions of winter also protected the smallest plants from desiccation. Wind protection was not supplied during 1988-89, and no appreciable snow accumulation occurred.
Plantings to compare two lepidote types with two large-leaf rhododendrons began in the spring of 1988 around cross-shaped structures constructed to simulate typical residential dwellings. Buildings were erected to provide exposures of north, south, east, and west as well as inside and outside corners of southeast, northeast, northwest, and southwest (Fig. 1). Soil preparation was similar to that in Experiment 1. Cultivars chosen were 'Roseum Elegans' and 'Nova Zembla' to represent standard, large-leaf types, and 'PJM Victor' and 'Molly Fordham'* to represent lepidotes.
Replacements were made in the spring of 1989 for any losses sustained from transplanting or from Phytophthora root rot occurring in the first year. Leaf temperatures were monitored by thermocouples attached to a strip chart recorder as in previous holly studies (Pair & Still, 1982). Moisture was supplied uniformly through drip irrigation, and leaf water potential was measured periodically by means of a pressure bomb (Waring and Cleary, 1967) to detect plant stress from temperature variations among exposures.
|Figure 1. Structure for evaluating heat tolerance of rhododendrons.|
Results and Discussion
Experiment 1. Excellent survival of most cultivars occurred in the shade of honey locusts in the well-drained, amended soil. Losses were attributed principally to invasion of Phytophthora root rot, especially in 'PJM Compact'*. Several other cultivars, especially large-leaf types, also sustained stem dieback from cankers. Plant condition and flowering of 25 of the original 38 cultivars planted were recorded over four years (Table 1). Best survival and plant growth occurred for 'Aglo'*, 'Anna H. Hall', 'Belle Heller', 'Catawbiense Album', 'Chionoides', 'Lodestar', 'Olga Mezitt', PJM group, 'PJM Elite', 'Roseum Pink'*, 'Vernus', and 'Windbeam'
Although large-leaf types grew and set abundant flower buds, not all buds opened well because of desiccation very typical of the winter injury that can occur in our continental climate. However, 'Lodestar', 'Nova Zembla', and 'Roseum Pink'* flowered most consistently. 'Belle Heller' and a few others flowered prematurely in the fall, if warm weather returned after the first killing frost. Rhododendron yakushimanum and hybrids, although heat tolerant, were a bit too tender for the severe winter of 1989-90. Low temperatures reached -18"F and -19°F on December 22 and 23, 1989, respectively.
Lepidote types such as 'Aglo'*, 'Olga Mezitt', and 'Windbeam' performed quite well and bloomed profusely. This agrees with recent work by Hawke (1991) at the Chicago Botanic Garden, where much success with lepidotes such as 'Windbeam' occurred in full sun with temperatures ranging from -26°F to 103°F.
|Table 1. Survival, condition, and flowering of Rhododendron cultivars 1|
|No. Surv.||Plant condition 2||Flowering 2||Begin Flower Date|
|Anna H. Hall||3/3||7.7||5.5||5.7||6.0||0||1||0||0||0.3||5-1|
|Boule de Neige||3/1||8.7||4.3||6.0||4.0||8.0||0||0||0||2.1||5-11|
|yaku x Vulcan||3/2||8.1||4.3||died||-||0||0||-||-||0|
|1 Planted spring, 1986, in amended soil under partial shade, survivors out of possible 3 or 4 planted.|
|2 Rated on scale of 0-9 w/9 = best condition, and most flowers.|
Because several cultivars survived and flowered under the partial shade of honey locusts, a more severe test was performed to evaluate heat tolerance in a range of conditions from full sun to shade in sites typical of residential structures. Many plants on the south, southwest, and other hot exposures were lost in 1988 during an unusually hot summer. Even after all losses were replaced in 1989, the hotter locations, where temperatures reached near 118°F (48°C) against the buildings, proved too much for these shrubs. Many survived the cooler locations of north, northeast, and even northwest which was quite sunny in late afternoon.
Figure 2. Flowering of 'PJM Victor' (left) after -18F
compared to less hardy 'Roseum Elegans' (right).
Best survival and flowering in all locations occurred with 'PJM Victor' (31 out of 36 possible) followed closely by 'Roseum Elegans' with 27 alive after two years, although the latter did not flower well (Fig. 2). More dependable flowering occurred on 'PJM Victor' and 'Molly Fordham'* during 1990 and 1991 (Table 2). 'Molly Fordham'* grew somewhat slower and appeared to be more susceptible to
than 'PJM Victor'. Flowering was best in cooler sites of north, northeast, and northwest exposures, as with most other cultivars. On west and southwest exposures and even southeast, which proved very hot in the mornings, both 'PJM Victor' and 'Molly Fordham'* flowered (although sparsely); whereas, 'Nova Zembla' and 'Roseum Elegans' frequently set flower buds but did not bloom.
Pressure bomb measurements, which indicated amount of drought stress, were not greatly different among exposures, although the most negative readings (greatest stress expressed as Leafy in Table 2) tended to be on the west and south. As expected, best overall performance occurred in the more sheltered north, northeast and northwest exposures.
|Table 2. Leaf water potential, plant condition and flowering of rhododendron cultivars as affected by landscape exposure 1 .|
|Plant condition 3||Flowering 3|
|(out of 9)||6-12-89||9-23-89||4-2-90||'90||'91|
|1 Average of 3 replications each under 3 exposures for a total of 9 observations each were grouped together to obtain averages.|
|Symbols: NZ= 'Nova Zembla'; PJM = 'PJM Victor'; MF = 'Molly Fordham'* and RE = 'Roseum Elegans'.|
|2 Leaf water potential (ψ) determined by a pressure bomb to measure moisture tension as affected by exposure (less negative numbers indicate less moisture stress).|
|3 Rated on a scale of 0 to 9 w/ 0 = dead and 9 = best condition and most flowering.|
Although the harsh, continental climate of Kansas is quite inhospitable to rhododendrons, these experiments confirmed that several cultivars were tolerant once proper modifications were made. Certain lepidote types obviously survived and flowered better than traditional, large-leaf rhododendrons, although Phytophthora root rot is a serious threat under high temperatures. New hybrids, which include such heat-tolerant species as R. minus var. carolinianum Carolinianum Group Rehd., R. minus var. chapmanii A. Gray, R. dauricum L., R. minus var. minus Michx., or others of similar parentage, offer new hope to growers where climates are not as conducive to traditional, large-leaf rhododendrons.
This research has been supported by a grant from the American Rhododendron Society. Lepidotes were supplied by Weston Nurseries, Hopkinton, Mass., and large leaf types by Greenleaf Nurseries, Park Hill, Okla.
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* Name is unregistered but not in conflict with a registered name.
** Registered name is 'Sammetglut'.
*** "Large-leaf' refers to the larger leaf of the standard rhododendron.