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Journal American Rhododendron Society

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Volume 56, Number 3
Summer 2002

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An Exploration and Study of White Rock Mountain, WV For Unusual Forms of Rhododendron Species
Parker L. Little
Beaverdam, Virginia

Summary
Extensive areas of Rhododendron species were found covering sections of White Rock Mountain in West Virginia. On a western slope of the mountain, Rhododendron calendulaceum, R. maximum and R. prinophyllum displayed unusual corolla and leaf diversity.

Analysis
Three Rhododendron species found on White Rock Mountain in southern West Virginia displayed corolla variations. The variation is probably the result of the small size of the mountain limiting genetic migration (geologic-genetic isolation), microclimates, relict populations, limited reproduction (self-pollination and limited seed dispersal), hybridization, and natural variation of the species.

Five Rhododendron species, R. calendulaceum (Michx.) Torrey, R. catawbiense (Michx.), R. maximum L., R. periclymenoides (Michx.) Shinners, and R. prinophyllum (Small) Millais were found on or near White Mountain. Additionally, three other Rhododendron species were found in the region. Rhododendron arborescens (Pursh) Torrey and R. viscosum (L.) Torrey were found near Peters Mountain to the south, and R. canescens (Michx.) Sweet was found in Botetourt County, Virginia.

Criteria for selecting specimens of Rhododendron species for this study were based upon preference for specimens with atypical form, size and color of corollas and leaves. Selections of R. calendulaceum, R. maximum and R. prinophyllum were chosen for registration and propagation.

North end of White Rock Mountain, WV
North end of White Rock Mountain, viewed from Dickson Plantation along Howard Creek.
Photo by Parker L. Little

White Rock Mountain
There are no published accounts of botanical exploration for Rhododendron species and variants on White Rock Mountain in West Virginia, excepting one by this writer (1). Thomas Jefferson was said to have visited caverns at the base of the mountain in 1791 (2). Edgar T. Wherry described Phlox buckleyi from the northern base of the mountain in 1929 (3). Henry T. Skinner traveled close to this mountain in 1951 in search for native species near the New River (4). Mary G. Henry collected specimens of Gaylussacia brachycera near the southern end of the mountain range in 1961 (5).

White Rock Mountain range is about 10 miles (16.1 km) in length and 2.75 miles (4 km) wide with shale barrens to the east and a karst plateau to the west and south. The northern base of White Rock Mountain is adjacent to the Greenbrier River at 1,680 feet (512.4 m.) above sea level. A larger mountain range, Greenbrier Mountain, is adjacent to the north. The southern end of the White Rock Mountain range is an area of disorganized ridges rising to 3,250 feet (991.3 m) above sea level. The adjoining karst plateau averages about 2,200 feet (671 m) above sea level.

Rock formations on White Rock Mountain are sedimentary with folded and horizontal layers of conglomerate, sandstone, shale, and siltstone. Soil pH varies from 5.1 to 6.2 at selected Rhododendron populations. Soil types at selected rhododendron sites were rocky clay loam, sandy loam, dry shale with a slight layer of leaf mold, relict mudflows with mixed rock types with deep layers of leaf mold, and relict mudflows with boulders without leaf mold. (Data was based upon the author's analysis).

Unlike other known populations with unusual forms of Rhododendron species from higher elevations in the Southern Appalachian Mountains (4, 34, 35), the Rhododendron species of White Rock Mountain are found at lower elevations with a warmer and drier climate. Temperature extremes on the adjacent karst plateau range from -27°F (-32.7°C) to 101°F (38.3°C ) near the town of Union. The mountain receives about 41 inches (104.14 cm) of precipitation a year. The heaviest amounts of precipitation occur in late winter and in summer. About 3.8 inches (9.7 cm) of liquid precipitation occurs in March. The month of July receives the most rainfall with an average of 4.4 inches (11.2 cm). Almost every morning in summer heavy fog and mist occurs below 2,400 feet (720 m) in elevation. Tornadoes are not uncommon near the southern end of the mountain. (Data from local newspaper accounts, local residents, and the author's observations).

U.S. Geological Survey Map of White
Rock Mountain, WV
Figure 1. U.S. Geological Survey Map of White Rock Mountain, WV, 1891.

Rhododendron calendulaceum
Rhododendron calendulaceum was admired and appropriately named Sky Paint Flower by the Eastern North American Indian tribes and was possibly known to earlier Cro-Magnon Man during the Ice Age in the Virginias more than 17,000 years ago (6, 7, 38). William Bartram described the Fiery or Flame Azalea ( Rhododendron calendulaceum Michx.) found near the Broad River of Georgia in 1776 with corolla colors of red, yellow, orange and cream (8).

Bartram's descriptions of the variations in color are similar to the color types of Rhododendron calendulaceum found on White Rock Mountain in southern West Virginia. On this mountain, the corollas of R. calendulaceum have colors of orange-red, orange, yellow, yellow-pink, pink, purplish-pink, and yellow-white. White throated corolla forms were found above 2,800 ft. (854 m) elevation. Scarlet corolla types were first to flower below 2,800 ft. (854 m) in elevation. Yellow, orange and pink corolla forms flowered one week after the scarlet forms (the early flowering scarlet forms may have a evolutionary association with the arrival of ruby throated hummingbird pollinators, since they are attracted to red flowering plants). Numerous atypical corolla forms were observed and recorded. Corolla sizes varied from 1.24 inches (3.1 cm) in diameter (coll. # WRHTB-DIPL2400); to 3.0 inches (7.5 cm) in diameter (coll. #WRN2400). Corollas varied in form with frilled, wall-edged, elongated and twisted lobes. Corollas of Rhododendron calendulaceum normally have five lobes; one specimen had five to seven corolla lobes. Calyx sizes varied from 0.0 cm. to 1.32 inches (3.3 cm) in length. The largest truss size found was 3.6 inches (9.0 cm) tall and 5.2 inches (13.0 cm) wide with eight corollas from one bud (coll. # WRN2400). From samples collected, the number of flowers per truss from a bud varied from five to twelve. Winter buds varied in color from yellow-green to brown to purple. The maximum number of flowers in a truss was nineteen, originating from four terminal buds. Peak bloom dates are May 10 to May 30 with one yellow specimen flowering as late as June 18.

R. calendulaceum, white with gold blotch    R. calendulaceum with frilled margins, white-yellow
R. calendulaceum corolla lobes white with light pink blush. Gold blotch over most of upper
lobe. Growing on exposed relict mudflow with boulders. May 20. Coll. # WR2450W.
Photo by Parker L. Little
   R. calendulaceum corolla with frilled lobe margins, white-yellow throat,
pink lobes. May 28. Coll. # WR3A.
Photo by Parker L. Little
 
R. calendulaceum, yellow, yellow-gold blotch over
entire upper lobe.    R. calendulaceum with light yellow and green
variegated leaves.
R. calendulaceum; corollas intense yellow, yellow-gold blotch over entire upper lobe.
Corolla lobes short, broad, wavy; upper lobe frilled. Growing with
R. periclymenoides
and R. prinophyllum. May 24 to June 15. Coll. # WRHT2900.
Photo by Parker L. Little
   R. calendulaceum; rare form with light yellow and green
variegated leaves. Coll. # WRHTB2450.
Photo by Parker L. Little

Leaves varied in color from light yellow and green variegated to glossy dark green. Leaf sizes varied from 2.48 inches (6.2 cm) to 3.24 inches (8.1 cm) in length. Shrub sizes of Rhododendron calendulaceum in flower varied from one foot (30.5 cm) tall to 15 feet (4.57 m) tall. One shrub with scarlet flowers was found with a trunk diameter of 2.48 inches (6.2 cm) at three feet above shrub base. Colonies of Rhododendron calendulaceum typically cover areas to one acre in size, with individual plants growing alone nearby. Rhododendron calendulaceum was found only on the western side of White Rock Mountain, most frequently on dry, rocky, exposed sites on steep southwest to northwest-facing slopes. Rhododendron calendulaceum was found growing on well-drained dry or moist soil with pH of 4.7 pH. Selections were made from south and west-facing slopes and ridges, north slopes and along streams (see Table 3).

Table 3. Named Selections of Rhododendron Species of White Rock Mountain, West Virginia
Registered selections of Rhododendron calendulaceum by this writer from White Rock Mountain,
West Virginia; (JARS 51: 52-53; 1997):
Rhododendron calendulaceum 'Little's Blushing Giant' coll. # WR2450X
Rhododendron calendulaceum 'Little's Enchanting Dawn' coll. # WRHT3100
Rhododendron calendulaceum 'Little's Fiery Star' coll. # WR2400C
Rhododendron calendulaceum'Little's Giant Salmon' coll. # WR2450N
Rhododendron calendulaceum'Little's Misty Blush' coll. # WR2450W
Rhododendron calendulaceum 'Little's Mountain Gold' coll. # WR2300BR (MAC Blue Ribbon. June 1, 1996)
Rhododendron calendulaceum 'Little's Mountain Melody' coll. # WR2400B
Rhododendron calendulaceum 'Little's Mountain Passion' coll. # WR2350E (MAC Blue Ribbon. June 1, 1996)
Rhododendron calendulaceum 'Little's Pink Glow' coll. # WR2450BR (MAC Blue Ribbon. May 16,1998)
Rhododendron calendulaceum 'Little's Pink Gold' coll. # WR2450A
Rhododendron calendulaceum 'Little's Salmon Blush' coll. # WR2100B
Rhododendron calendulaceum 'Little's Sunburst' coll. # WR2400D
Rhododendron calendulaceum 'Little's Sun Mist' coll. # WR2350C
 
Selections of Rhododendron calendulaceum; registered Feb. 8, 2002:
Rhododendron calendulaceum 'Little's Enchanting Eve' coll . # WR2400BR (PVC Blue Ribbon. May 1997)
Rhododendron calendulaceum 'Little's Fancy Twist' coll. # WRHTB2350
Rhododendron calendulaceum 'Little's Fire Glow' coll. # WRHT3B
Rhododendron calendulaceum 'Little's Gold Giant' coll. # WR2100A
Rhododendron calendulaceum 'Little's Mountain Giant' coll. # WRN2400
Rhododendron calendulaceum 'Little's Mountain Sunlight' coll. # WRHT2900
Rhododendron calendulaceum 'Little's Orange Crush' coll. # WRSW2850
Rhododendron calendulaceum 'Little's Orange Ridge' coll. # WRSW285BR (MAC Blue Ribbon. May 16, 1998)
Rhododendron calendulaceum 'Little's Tangerine Giant' coll. # WRXER2900
Rhododendron calendulaceum 'Little's Tangerine Luster' coll. # WR2550N
Rhododendron calendulaceum 'Little's Tangerine Pink' coll. # WRHT2850
 
Selections of Rhododendron calendulaceum x R. periclymenoides; registered Feb. 8, 2002:
Rhododendron calendulaceum x R. periclymenoides 'Little's Wavy Pink' coll. # WRXER2900
 
Named selection of Rhododendron prinophyllum; in process of registration:
Rhododendron prinophyllum 'Little's Purple Glow' coll. # WRHTSW2850
 
Selection of Rhododendron maximum, registered Feb. 8, 2002:
Rhododendron maximum 'Little's Enchanting Shells' coll. # WRW90

The most frequently encountered plants found growing with R. calendulaceum were Gaultheria procumbens, Hamamelis virginiana, Quercus coccinea, Q. prinus and Sassafras albidum. Vascular plants associated with Rhododendron calendulaceum are recorded in Table 1.

TABLE 1. Associated Plants Growing with R. calendulaceum.
Acer rubrum Nyssa slyvatica
Amelanchier laevis Oxydendrum arboreum
Anemone virginiana Phlox subulata
Antennaria plantaginifolia Polygala paucifolia
Betula lenta Polystichum achrostichoides
Carex platyphylla Pinus echinata
Carya ovata P. rigida
Castanea dentata P. strobus
Chanaelirium luteum P. virginiana
Comptonia peregrina Pteridium aquilinum
Cornus florida Quercus coccinea
Cypripedium sp. Q. prinus
Dioscorea villosa Rhododendron periclymenoides
Epigaea repens R. prinophyllum
Equisetum sp. R. maximum
Euphorbia cyathophora Rhus toxicodendron
Galax urceolata Rosa sp.
Gaultheria procumbens Rubus allegheniensis
Geranium maculatum Rudbeckia laciniata
Goodyera sp. Sassafras albidum
Hamamelis virginiana Silene caroliniana
Hypoxis sp. Smilax rotundifolia
Iris verna Tovara virginiana
Kalmia latifolia Uvularia caroliniana
Lindera benzoin Vaccinium caesium
Lycopodium porophilum Viburnum acerifolium
Magnolia acuminata Viola sp.
Menziesia pilosa (typical and decumbent forms) Zizia aurea

Rhododendron calendulaceum, R. periclymenoides and R. prinophyllum occur together only at 2,800 feet (854 m) above sea level. One hybrid between R. calendulaceum and R. periclymenoides was found with small light pink corollas and a gold blotch, growing on a dry shale slope. There were no seedlings near this specimen. The wide variation in corolla colors of R. calendulaceum is usually due to the natural variation of the species (9, 10). Interspecific hybridization with R. periclymenoides or R. prinophylum is a likely cause for the purplish-pink to pink corollas of R. calendulaceum. The pink selections of R. calendulaceum reported herein have corollas that are mostly smaller than the other selections of the species on the mountain. Bright pink corollas of R. calendulaceum generally suggest hybridity with R. periclymenoides (11). Seed pods of the purplish-pink types did not expand to release seeds. Although the seeds were fertile, reproduction of these hybrids may be limited. Orange-pink corolla selections of R. calendulaceum appear to be hybrids with R. prinophyllum. Most of these selections were moderately fragrant.

Ruby-throated hummingbirds and swallowtail butterfly species were frequent visitors to the opened corollas of Rhododendron calendulaceum.

Rhododendron prinophyllum
Rhododendron prinophyllum occurs on White Rock Mountain above 2,750 ft. (839 m.) in elevation. Two separate populations of R. prinophyllum were found on west facing slopes growing with R. calendulaceum, R. periclymenoides and mature Quercus coccinea. Peak bloom date for the R. prinophyllum was May 15. Corollas were distinctive in form and color for each population. One population ("A") had light pink corollas with elongated, obtuse lobe ends. Seven to ten corollas were found in a cluster or truss. It appears that R. prinophyllum in group ("A") is a distinct variety of the species or possibly a hybrid population. The second population ("B") had purplish pink corollas with acute lobe ends. All specimens of R. prinophyllum were fragrant. Collection # WRHTSW2850 with purplish pink corollas was selected for propagation and registration from population ("B").

The two populations of Rhododendron prinophyllum were small. Seven mature specimens were in group ("A") and the other group ("B") contained three mature specimens. Since there were no specimens of R. prinophyllum found below 2,750 ft. (839 m), it appears that R. prinophyllum is a post-glacial relict species that is limited to the highest ridges with cooler temperatures.

ARS Research Foundation logo

Rhododendron maximum
Rhododendron maximum corolla colors vary from white, to rose-pink, to purple and even a rare red form (12, 13, 14). An orange-yellow form reported by local mountaineers in West Virginia remains unsubstantiated (15). Corolla dorsal lobe spots vary from green to yellow to orange (12, 13). On White Rock Mountain the corolla colors vary from white, to deep pink and light purple. Spots on the dorsal lobe vary in color from deep yellow, to yellow-brown, to yellow-green and dark green. Most spots are shaped like minute horseshoes. Atypical corolla forms are found in an area between 2,100 feet (640.5 m) and 2,750 feet (839 m) in elevation. In comparing corolla variations of a related species, R. macrophyllum in California, Oregon and Washington varies in color from white to deep pink to maroon; however, data for variations in corolla form is lacking (27).

Rhododendron maximum on White Rock Mountain is in flower from May 20 to July 25 and rarely in early October, with peak flowering from late June to early July. Corollas varied in size from 1.7 inches (4.3 cm) wide (coll. # WRW80A), to 2.4 inches (6.0 cm) wide (coll. # WR2250D). The largest truss was 4.4 inches (11.0 cm) tall and 4.6 inches (11.5 cm) wide with 19 corollas in a truss from one terminal bud (coll. # WR2250D). Also, a shrub with a double truss and small corollas was found, formed from two terminal buds (coll. # WRW80P). Corollas were also found with notched lobes, extra lobes (up to 10), acute and cuspidate lobe ends, elongated lobes, twisted lobes, and separate or slightly united lobes. Some corollas had orange pink fan-like stigmas 0.2 inches (0.5 cm) wide (coll. # WRW80A), while others had pinkish-purple glands on the outside of the corolla lobes (coll. # WRW80A).

The evergreen leaves of Rhododendron maximum varied in size from six inches (15.0 cm) to 13.3 inches (33.2 cm) long, with new leaves covered with a white scurfy layer. One shrub had young leaves with red-purple glands raised above the dense white scurfy layer (coll. # WRW90). The largest specimen of R. maximum found on White Rock Mountain was 23 feet (7 m) tall and 25 feet (7.6 m) wide. In the Blue Ridge mountains of South Carolina, R. maximum reaches tree size growing to 40 feet (12.2 m) tall and 22 feet (6.71 m) wide (16, 26).

Rhododendron maximum occurs on the east and west sides of White Rock Mountain. Rhododendron maximum is absent on the north end of the mountain. On the east side of the mountain R. maximum is found near streams and on steep north facing shale slopes below 2,400 feet (732 m) in elevation. On the west side of the mountain R. maximum is more abundant. There, the species occurs near streams, on northern, western and occasionally on southern slopes. On the southern slopes, scattered shrubs are found in sunny, dry open woods up to 2,700 feet (823.8 m) in elevation. These plants may have xeromorphic properties (17). Elsewhere, R. maximum is found on well-drained, moist wooded slopes and streamsides with soil acidity of 6.2 pH. Some populations of R. maximum on White Rock Mountain are isolated from other populations by up to one mile (1.6 km) to three miles (4.8 km) apart. Extensive colonies over one mile in length occur along a few streams. Rhododendron maximum is the predominant plant species along these streams. Mountaineers call these dense thickets of R. maximum "laurel hells."

On the western slopes , the most common species found with Rhododendron maximum were Hamamelis virginiana, Kalmia latifolia, Oxydendrum arborem and Quercus coccinea. On the northwest slopes Betula lenta, Ostrya virginiana and Oxydendrum arborum typically occurred with R. maximum. Along the streams on the west side of the mountain, the most common plants found with R. maximum were Betula lenta, Hamamelis virginiana, Quercus alba and Tsuga canadensis. On the east side of the mountain on northeast slopes, plants usually found with R. maximum were Betula lenta, Pinus strobus, and Tsuga canadensis. On the west side of White Rock Mountain, Rhododendron maximum was the most common vascular plant found growing on some west-southwest to north slopes and along some of the north and west flowing streams. On the east side of the mountain, R. maximum was the most common vascular plant on some northeast slopes and along east flowing streams. (For associated plants growing with R. maximum, see Table 2.)

Pollinators of Rhododendron maximum were ruby-throated hummingbirds, bumblebees, bee species, and longhorned borer species.

TABLE 2. Associated Plants Growing with R. maximum.
Acer rubrum Medeola virginiana
A. penslvanicum Menziesia pilosa (typical and decumbent forms)
A. saccharum Mitchella repens
Adiantum pedatum Nyssa sylvatica
Aesculus octandra Osmunda claytonia
Amianthium muscaetoxicum Osmunda cinnamomea
Anemone virginiana Ostrya virginiana
Aristolochia macrophylla Oxydendrum arborem
Aralia nudicaulis Panax quinquefolium
Asarum virginica Parthenocissus quinquefolia
Betula lenta Pinus echinata
Carex ssp. P. rigida
Carya ovata P. pungens
Castanea dentata P. virginiana
Cimicifuga americana P. strobus
Clintonia borealis Platanus occidentalis
Comptonia peregrina Polygala paucifolia
Convallaria montana Polypodium virginianum
Corydalis sempervirens Polystichum acrostichoides
Diphasiastrum digitatum (Lycopodium digitatum) Quercus alba
Dryopteris marginalis Q. coccinea
Epigaea repens Q. prinus
Equisetum scirpoides Q. ilicifolia
E. hyemale Rhododendron calendulaceum
Eupatorium rugosum R. prinophyllum x R. calendulaceum
Fagus grandifolia Rhus typhina
Gaultheria procumbens Robinia pseudo-acacia
Galax aphylla Rudbeckia laciniata
Hamamelis virginiana Sassafras albidium
Hepatica americana Smilax herbacea
Heuchera americana Tilia ssp.
Hydrangea arborescens Tovara virginiana
Ilex montana Tsuga canadensis
Iris verna Vaccinium caesium
Juglans cinera Vaccinium ssp.
Kalmia latifolia Ilex opaca
Liriodendron tulipifera Gaylussacia brachycera
Lycopodium lucidulum Physocarpus opulifolius
Magnolia acuminata  
Matteuccia struthiopteris  

Variation in Rhododendron maximum
RHODODENDRON MAXIMUM VAR. ASHLEYI RADFORD, AHLES & BELL: In two locations in the western part of North Carolina Rhododendron maximum has corollas about 1.2 inches (3.0 cm.) wide with separate or slightly united petals that were first described as R. ashleyi by Coker (28). Radford, Ahles, and Bell determined that this variant was best treated as a botanical variety of R. maximum, var. ashleyi (24). These unusual corolla forms resemble specimens found on White Rock Mountain in West Virginia. However, the selections from North Carolina have leaves and corollas that are much smaller than the selections found on White Rock Mountain. The two known sites of var. ashleyi (Ashe and Macon counties, North Carolina) are separated by a distance of about 140 miles (225.3 km) with the nearest North Carolina site about 100 miles (160.9 km) south of White Rock Mountain. Given such a disjunct range, one can expect leaf and corolla differences as observed.

RHODODENDRON MAXIMUM VAR. LEACHII HARKNESS: Rhododendron maximum var. leachii has only been documented from Greenbrier County, West Virginia (29, 36). This variety may also occur in Pocahontas County, West Virginia (15). Two specimens (# WRNSS1; # WRNSS2) were found on White Rock Mountain; these had the typically twisted and wavy leaves of R. maximum var. leachii; however, the corollas were typical for the species and did not resemble the more widely open cup shape corollas of the selection photographed by Vernimb (30). In comparison with typical R. maximum, var. leachii has a more dwarf habit and smaller leaves with deeply waved margins and down-curved tips (13).

RHODODENDRON MAXIMUM FORMA PURPUREUM (PURSH) FERN: This variety has typical corolla shapes that vary in shades of purple, with green spotted upper lobes (13). Three selections were found on White Rock Mountain: collections # WR80B; # WRNS2250 were purple; # WR2250A was light purple, in flower on July 13. Some seedlings of collection # WRNS2250 display red stem buds.

RHODODENDRON MAXIMUM FORMA ALBUM (PURSH) FERN: White forms of Rhododendron maximum are uncommon on White Rock Mountain, however, selections # WR2250XA, # WRNS2200, and # WR2175A have atypical widely open corollas with yellow-gold blotch spots. Since these selections have atypical corolla morphology, it might be worthwhile to describe them as a new form rather than include them with f. album.

R. prinophyllum with purple-pink corollas
and red-purple filaments.    R. maximum with wavy, narrow, pointed,
very elongated corolla lobes.
R. prinophyllum, a very fragrant selection with purple-pink corollas and
red-purple filaments. May 15. Coll. # WRHTSW2850.
Photo by Parker L. Little
   R. maximum with wavy, narrow, pointed, very elongated corolla lobes. Corollas
5.8 cm wide, truss 11.5 cm wide. July 10. Layered specimen
collected. Coll. # WR2250B.
Photo by Parker L. Little
 
R. maximum with broad acute cupspidate
corolla lobe ends.    R. maximum with 4.3 cm wide corollas,
broad stigmas, purple edged lobe margins.
R. maximum with broad acute cuspidate corolla lobe ends. Corollas 5.7 cm wide,
truss 13.6 cm tall, 20 corollas in truss. Indumentum of young leaf surface covered
with purple-red glands above a dense white scurf layer, becoming black.
July 7. Coll. # WRW90.
Photo by Parker L. Little
   R. maximum with 4.3 cm wide corollas, broad stigmas, purple edged lobe margins.
July 1. Coll. # WRW80A.
Photo by Parker L. Little
 
R. maximum var. purpureum corollas with
purple lobe margins.    R. maximum var. ashleyi
R. maximum var. purpureum corollas with purple lobe margins,
outside upper lobe purple, buds red-purple. June 25. Coll. # WRNS2250.
Photo by Parker L. Little
   R. maximum var. ashleyi with separate upper corolla lobes. Locally common.
June 30. Coll. site # WRHT2040.
Photo by Parker L. Little

Evidence of Non-hybridity in Rhododendron maximum
The Rhododendron maximum selections displayed no indications of hybridization with R. catawbiense. Rhododendron catawbiense plants were found at the base of the mountain, but these were an introduced roadside planting made by a local resident around 1980, from materials collected 22 miles (35.4 km) to the northeast at Meadow Creek Mountain. A rare hybrid population of R. catawbiense and R. maximum exists on Meadow Creek Mountain. The only naturally occurring R. catawbiense plant was found four miles to the east of the mountain on the southeast slope of a shale barren hill. All the R. catawbiense plants at the base of the mountain were too young to have been parents of the observed R. maximum plants. Also, self-pollination (autogamy) of R. maximum tends to occur before the corolla is completely open, making cross-pollination with R. catawbiense a difficult and a rare occurrence (22, 23). The purplish selections of R. maximum had the typical corolla form for the species and did not resemble the corollas of R. catawbiense.

Refuge Area for Rhododendron maximum
Rhododendron maximum populations in Virginia are found over widely scattered areas from 20 feet (6.1 m) above sea level on the coastal plain to 5,000 feet (1525 m) in elevation on Haw Orchard Mountain and White Top Mountain (18). In West Virginia, R. maximum is more prevalent than in Virginia, except on lower elevations where it is absent. On White Rock Mountain, R. maximum is found growing on the lower slopes and along streams in an area between 1,800 feet (549 m) and 2,800 feet (854 m) above sea level. On this mountain, there is an area of moderate temperatures between 2,300 feet (701.5 m) to 2,600 feet (793 m) in elevation that escapes late spring frosts and freezes in April, May and June. As an optimal area with a longer growing season, this zone of moderate temperatures may act as a refuge area during significant climatic changes. The zone of moderate temperatures would likely move up and down mountain slopes or be reduced in area during significant climatic changes. The warmer southern slopes and hollows below 2,600 feet (793 m) in elevation could also provide protection to Rhododendron maximum during repeated glacial cycles. The zone of moderate temperatures gives evidence as a refuge, since numerous relict and endemic plant species exist in the local area (32). (The last glacial phase began about 70,000 years ago and ended about 10,000 years ago. The next glacial phase for the area is expected in 8,000 to 9,000 years.) (31).

In the last Pleistocene glacial phase, altitudinal temperature zones probably dropped only 2,000 feet (610 m) in elevation (19, 20). The greatest glacial ice sheet advancement reached 175 miles (281.6 m) to the northwest of White Rock Mountain 18,000 to 21,000 years ago (21). This distance from the ice sheet would have allowed R. maximum to remain in protected mountain areas below the 3,000 ft. (915 m.) elevation as calculated from the present 5,000 ft. (1,525 m) elevation limit of R. maximum. The upper elevation limit for R. maximum is probably the highest elevation that would allow for sufficient heat-unit accumulation for its survival and may also be the species low winter temperature limit. Rhododendron maximum and other species in the Ponticum subsection are bud-hardy to -30°C (-22°F) (37). Cold tolerance of R. maximum is exhibited in the present range of the species that extends into the cold areas of western Maine and the White Mountains of New Hampshire (39).

White Rock Mountain and the adjacent mountains are a refugium area with numerous restricted endemics and relict plant species The very rare Iliamna coreii is an endemic species found only on Peters Mountain, adjacent to White Rock Mountain to the south. Endemic species found on White Rock Mountain were Convolvulus purshianus, Phlox buckleyi and Solidago harrisii. Other endemic species found nearby were Allium oxyphilum, Clematis albicoma, Eriogonum alleni, Magnolia fraseri, Rhododendron catawbiense and Saxifraga leucanthemifolia (32, 33). Relict species Cymophyllus fraseri, Cypripedium reginea, Galax aphylla, Lycopodium porophilum, Menziesia pilosa, Magnolia fraseri, Rhododendron catawbiense and Thuja occidentalis were found growing on or within a few miles of White Rock Mountain. (A post-glacial retreat refugium for Rhododendron prinophyllum occurs above 2,750 feet or 839 meters on White Rock Mountain).

The presence of numerous restricted endemic plant species on and near White Rock Mountain indicates that significant climatic changes have made little impact upon the vegetation of the area over hundreds of thousands of years. Plant species in the area either remain in their habitat, or "migrate" up or down mountain slopes, or around slopes during repeated glacial cycles.

Mutations, Isolation and Natural Variation of Rhododendron maximum
The small size of White Rock Mountain with adjacent karst plateaus may enhance the chances of genetic isolation of Rhododendron maximum from the main gene pool of the region. Rhododendron maximum selections differed from the norm with larger corollas, extra corolla lobes, more narrow petals, incised lobes, acute petal ends, more widely open corollas, enlarged stigmas, purple corolla glands, and corollas with separate petals. In comparing other nearby populations, typical corolla forms of the species were found at three selected sites to the north of the mountain. Rhododendron maximum at site "A" at North Caldwell, site "B" at Dutch Hollow Road, and site "C" at Blue Bend on Anthony Creek all had typical corolla forms. (Site "D" at Meadow Creek Mountain contained rare hybrids between R. catawbiense and R. maximum).

Isolation of genetic material in a small or localized population would favor the expression of recessive mutations through inbreeding resulting in unique characteristics, since outside influences from the main gene pool would be limited. Self-pollination (autogamy) in Rhododendron maximum may also help to slow the transmission of genetic material by pollinators from one plant to another. However, seeds of R. maximum may be dispersed by animals (on wet feet or fur) to allow for some genetic migration into or out of a population. Most of the unusual variations of Rhododendrn maximum were found between 2,100 feet (640.5 m) and 2,750 feet (839 m) in elevation on White Rock Mountain. About 5% of the randomly collected seedlings from this area displayed unusual corolla variations. Natural variation of the species is displayed with the forms of R. maximum var. ashleyi, var. leachii, var. purpureum and var. album found on the mountain. These varieties of R. maximum including the numerous local variants found, indicate that the species gene pool in the area has more random variation than is typical of other populations of the species over its widespread range. In comparison, a genetic study of R. macrophyllum by Denton reveals that native populations of R. macrophyllum have unexpectedly high genetic diversity (25). Intraspecific variation along with isolation due to the small size of the mountain with localized populations and microclimates, may be the primary cause for the unusual corolla forms and mutations of R. maximum on White Rock Mountain. These new forms or mutations may become advantageous or continue to exist within a small isolated population over time. A novel mutation will become advantageous over the genetic type if it has a selective edge. (25). However, most mutations that appear typically become neutral or disadvantageous within a large population.

Conclusion
The numerous selections of Rhododendron species from White Rock Mountain displayed unusual diversity for a small area, since all the selections were found within a one square mile (1.609 kilometer square) area. Some of the R. calendulaceum corollas were much larger than normal, while others had much extended calyxes. Corollas were also found with short wide lobes, frilled lobes, elongated and twisted lobes, wall edged lobes, and extra lobes. The R. maximum corollas varied from the norm with larger corollas, notched lobes, extra lobes, acute and cuspidate lobe ends. Also, corollas with elongated and twisted lobes, separate or slightly united lobes, and corollas with wide fan-like stigmas were found. Shrubs of R. prinophyllum in one population varied from the norm with corollas having elongated, obtuse lobe ends.

These variations of the Rhododendron species may be due to partial isolation of relict populations and recent populations that have developed new forms. These new forms may or may not become advantageous or widespread throughout the species range over time.

Rhododendron maximum from White Rock Mountain exhibits no evidence of hybridization with R. catawbiense; therefore the unusual corolla forms of R. maximum indicate that some of the numerous variations in the species are due to slow inbreeding (autogamy) within partially isolated populations. Variants were often found growing adjacent to each other, while surrounding shrubs of R. maximum were typical. This localized concentration of variants would indicate that at least one extreme variant was responsible for contributing to the array of variations displayed in the offsprings.

In order to determine if some of the variations of Rhododendron maximum on White Rock Mountain are new varieties or local variants, the selections need to be compared, matched and categorized by their corolla form, size, and blotch color to other selections throughout the species entire range.

Propagation Notes

Propagation of Rhododendron calendulaceum was made by selecting cuttings of new stems about one month after flowering (June 25). Cuttings were taken in early morning and placed in plastic bags and transported to the nursery in coolers with ice to keep cuttings cool. The ice layer was placed in the bottom of the cooler below a layer a newspaper.

Lower leaves of cuttings were cut off upon arrival at the nursery. The stem base was inserted into either a liquid Dip'N Grow rooting concentrate hormone at 10x to 15x dilution rate, or RooTone, a dry rooting hormone. Cuttings were then labeled and inserted in plastic cups with drainage holes into a 50/50 mix of moist perlite and well-decayed pine bark mulch. Six cuttings were placed in the container. Cuttings were then enclosed with clear plastic bags and placed in shade for several months.

Plastic was removed after foliage drop in early winter. Rooted cuttings were left in the containers buried to the top of the container in mulch or sand until spring. Before new growth appeared individual plants were placed into separate containers with 50/50 perlite and well decayed pine bark mulch and watered. Containers of new plants were placed in half shade. No fertilizer was applied to cuttings or rootings.

Rooting success varied for each selected specimen. Scarlet and deep orange corolla forms were very difficult to root, while the yellow to white corolla forms established roots easily.

Rhododendron maximum selections were propagated by layering on site, placing lower branches into the humus layer. Branches were weighted down with sticks or small rocks. Branches were lightly scratched or slightly broken to form a callous layer for roots to grow. Rooted branches were left for several years until well rooted. Wire mesh was placed over layered branches to prevent browsing by deer.

References

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  2. Organ Cave Brochure. Organ Cave, Inc., 417 Masters Road, Ronceverte, WV. 24970.
  3. Wherry, E. T. A long lost Phlox. Journal of the Washington Academy of Sciences. 20: 25-28; 1930.
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  16. National Register of Big Trees. 1981. The American Forestry Association. 8701 Old Kings Road, Jacksonville, FL 32219.
  17. Czekalski, M. Rhododendrons of the former Soviet Union. JARS 52: 86; 1998.
  18. Herbarium, Longwood College, Farmville, Virginia. Selected herbarium field notes on specimen sheets of Rhododendron maximum in Virginia.
  19. Braun, E. L. Plant distribution in relation to the glacial boundary. The Ohio Journal of Science. 51 (3): 144-145; 1951.
  20. Braun, E. L. The phytogeography of unglaciated eastern United States and its interpretation. The Botanical Review. (The N. Y. Botanical Garden). 21 (6): 304 - 367; 1955.
  21. Thornbury, W. Principles of Geomorphology. 388-390, 512, 515. John Wiley & Sons, Inc., New York; 1969.
  22. Goodrich, R. H., and Ring, G. W. Getting the max out of R. maximum or "Apomixis , Anyone?". Amer. Rhododendron Soc. Quart. Bul. 26: 263-264; 1972.
  23. Ring, G. Society News. JARS. 52: 34-35; 1998.
  24. Radford, A. E., Ahles, H. E., Bell, C. R. Manual of the Vascular Flora of the Carolinas. 797-798; 1968.
  25. Hall, B. D. DNA changes accompanying the evolution of plant species. JARS. 52: 9-12; 1998.
  26. Petrides G. A. A Field Guide to Trees and Shrubs. 286-287; Houghton Mifflin Co., Boston, MA; 1972.
  27. Smith, Britt. Rhododendron macrophyllum varies, too! JARS 26: 240-241, 250; 1972.
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  29. Leach, D. G. Rhododendrons of the World. p. 192. Charles Scribners Sons, New York; 1961.
  30. Rhododendron maximum var. leachii Harkness. Photo by Bryan Vernimb. JARS 33: 1: 35; 1979.
  31. Calos, Katherine. Exhibit explores ice age. Richmond Times-Dispatch. (Newspaper, Richmond, Virginia). Section J p. 1, J p. 3. Dec. 20, 1998.
  32. Castanea 17: 116; 1952.
  33. Braun, Lucy E. Some relationships of the flora of the Cumberland Plateau and Cumberland Mountains in Kentucky. Rhodora. 39: 193-208; June 1937.
  34. Cantrell, H. Furman. Native azaleas: Wayah Bald. Amer. Rhododendron Soc. Quart. Bul. 34: 198-200; 1980.
  35. McLellan, George K.; McDonald, Sandra. Magic on the mountain (an azalea heaven on Gregory Bald). JARS 50: 90-94; 1996.
  36. Voss, Donald H. Validation of botanical names: Rhododendron brachycarpum ssp. tigerstedtii and Rhododendron maximum var. leachii. JARS 55: 157-158; 2001.
  37. Sakai, A., L. Fuchigami and C. J. Weiser. Cold hardiness in the genus Rhododendron. J. Amer. Soc. Hort. Sci. 111 (2): 273-280; 1986.
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  39. Jorgensen, Neil. How growing rhododendrons in New England builds character. JARS 53: 152-154; 1999.

Acknowledgments

I would like to thank The American Rhododendron Society members who contributed to the ARS Research Foundation for their financial support that enabled this research project and to thank the Research Committee of the American Rhododendron Society for selecting this research project. Also, I would like to thank the registrars Dr. Alan C. Leslie of the Royal Horticultural Society and Mrs. Jay W. Murray for the registration of the numerous Rhododendron species selections from White Rock Mountain, West Virginia. I wish to thank Longwood College for allowing me to study the collection of Rhododendron maximum specimens in their Herbarium.

Parker Lewis Little has explored White Rock Mountain, West Virginia, for native Rhododendron species since 1980. He is the owner of Piping Tree Gardens & Nursery, 13171 Scotchtown Road, Beaverdam, VA 23015. He also has a BLA degree from VPI & SU, Blacksburg, Virginia. Selections of registered Rhododendrons species from White Rock Mountain can be seen at The National Native Azalea Repository, University of North Carolina, Asheville, North Carolina, and Prague Botanical Garden V Troji, Czech Republic. The R. maximum selections are planned for display at The Rhododendron Species Botanical Garden, Federal Way, Washington in a few years.


Volume 56, Number 3
Summer 2002

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