Journal ARS Article
Vol. 47: No. 3: Year 1993
Deciduous Azaleas and Elepidote Rhododendrons of the
Mark P. Widrlechner
Richard A. Larson
But wild azalea and rhododendron populations in the Midwest have received little notice. Azaleas and rhododendrons are generally not common in the region and can easily be overlooked. One of the most ambitious collectors of native azaleas and rhododendrons, Henry Skinner (1955), skipped over Midwestern sites during his travels to areas in which these plants are more abundant.
Three azalea species, the roseshell azalea (Rhododendron prinophyllum [Small] Millais), the pinxterbloom azalea (Rhododendron periclymenoides [Michx.] Shinners), and the flame azalea (Rhododendron calendulaceum [Michx.] Torrey), and the rosebay rhododendron (Rhododendron maximum) reach the northwest edge of their native range in isolated populations stretching from the Ohio River near Wheeling, West Virginia, to south-central Missouri. At such sites, January mean temperatures range from 30 to 34°F, about the same as those found from New York City south to Wilmington, Delaware; but in July, mean temperatures are similar to those farther south, such as Washington, DC and Winston-Salem, North Carolina (75 to 78°F). Although azaleas and rhododendrons from Midwestern states should be better adapted to climatic extremes than those populations found father east, they are not normally available to hybridizers or nurseries.
In 1989, we received a grant from the Research Foundation of the American Rhododendron Society to investigate native Midwestern species, with the goal of collecting pollen and seed samples for the ARS Pollen Bank and Seed Exchange, and also to analyze soils to see if we could locate plants thriving on sites with soils less acidic than typical.
During 1990 and 1991, with the help and advice of many foresters and botanists, we were able to visit many sites in Ohio, Missouri, and Illinois. These trips led to the collection of pollen from 16 populations in Ohio, 11 in Missouri, and 3 in Illinois. A similar number of seed collections was made, including all four species native to the region. In addition, 33 soil samples were taken for analysis of pH, phosphorous, potassium, and organic matter. We will summarize our experiences in three sections: Rhododendron prinophyllum in Missouri and Illinois; deciduous azaleas in Ohio; and Rhododendron maximum in Ohio.
Rhododendron prinophyllum in Missouri and Illinois
In Missouri and Illinois, Rhododendron prinophyllum is usually found on acidic, rocky soils in the understory of oak and oak-shortleaf pine forests. Common companion plants include small red maples (Acer rubrum L.), sour gum (Nyssa sylvatica Marsh.), flowering dogwood (Cornus florida L.), farkleberry (Vaccinium arboreum Marsh.), and the low-bush blueberry, Vaccinium vacillans Torrey. The azaleas were often growing on north or west-facing slopes and generally did not flower well except along the edges of trails or where the forest was thin.
Flowering characteristics (time of bloom, flower color, and flower size) were rather uniform among the 14 populations we visited in the spring. Throughout this area, the peak flowering occurred between the 10th and the 17th of May. Petal colors only included shades of pink (Royal Horticultural Society [RHS] Color Classes 55, 56, 62, and 65). Tube colors were even less variable (RHS Color Classes 55 and 62, with most 55A).
We turned up no true whites, although at least one was described from Sainte Genevieve County, Missouri (Steyermark, 1963) and we heard rumors of others. The palest flowers were found in Ripley County, Missouri (RHS 56C) and the most intensely colored flowers came from a variable population at the Spiva Azalea Park east of Fredericktown in Madison County, Missouri (RHS 65C to 65A). Nearly all the flowers lacked petal blotches. Such blotches are not typical of Rhododendron prinophyllum, but we did find some plants with faint orange petal blotches at the Spiva Azalea Park. Among all populations, flower diameter, measured with petals spread, varied from 4.75 to 6.0 cm with most between 5.0 and 5.5 cm.
We found one of the most attractive azaleas of the trip in Oregon County, Missouri, in a unit of the Mark Twain National Forest. The plant was very floriferous with clear pink flowers. It stood about a meter tall with a spreading form. A single-plant collection was made for pollen, distributed in the ARS Pollen Bank as Oregon 1A.
Soil samples were hard to make because of the abundance of chert, sandstone, or quartz at almost all sites. Every soil sample was quite acidic with pH ranging between 3.96 and 5.80. The samples were low in phosphorous (3 to 20 ppm), low to moderate in potassium (32 to 123 ppm), and wildly variable for organic matter (3.1 to 56.8%). Azaleas growing on soils low in organic matter generally had roots scattered among the rocks or in cracks, while the high organic matter samples came from small pockets above and around rocks where leaves and other organic matter could accumulate and decompose.
In mid-October, we returned to collect seeds. We collected seeds from 10 of the sites we visited earlier and 2 additional sites in the Mark Twain National Forest in Missouri. Pods were ripening but few had already split open.
Some of Missouri's more easily observed and attractive azalea populations occur in Hawn State Park, in Sainte Genevieve County, on bluffs just across Pickle Creek south of the campground, in Pickle Spring Natural Area, southwest of Hawn State Park in Sainte Genevieve County, and at the Spiva Azalea Park on State Highway 72 east of Fredericktown in Madison County.
Deciduous Azaleas in Ohio
Together we visited another population of Rhododendron calendulaceum in June 1991. It is Ohio's northernmost population, located on private land near Sugar Grove, Fairfield County, and also one of the state's largest populations. We were too late to observe flowering, but the owner kindly showed us the population, in addition to photos of both yellow and orange blossoms. Seeds were collected from this site in late Sept. 1991.
Rhododendron prinophyllum and R. periclymenoides both had light pink flowers and the Pike County populations of R. calendulaceum included a range of yellow to orange flowers. Most plants were growing on north to northwest-facing slopes, often in soils with a pronounced surface layer of sand. Ohio azaleas generally grow on somewhat less rocky soils than do these from Illinois and Missouri; but, with two exceptions, Ohio soil test results were similar to those for sites farther west.
At all but two sites in Scioto County, azaleas were growing on well-drained, acidic soils (pH 4.33 to 5.69), very low in phosphorous (1 to 5 ppm), low to moderate in potassium (44 to 64 ppm), and low to moderately high in organic matter (1.8 to 8.6%). It is important to note, however, that R. prinophyllum was growing at two locations in Scioto County on atypical soils with no signs of chlorosis. At these sites, the soils were neutral (pH 6.86 to 7.13), very low in phosphorous (2 to 3 ppm), moderate in potassium (57 to 72 ppm), and moderate to moderately high in organic matter (3.0 to 8.5%). It may be that limestone fill from nearby roads has modified these soils, but such influences were not obvious. These two populations were made available through both the ARS Pollen Bank and Seed Exchange (as Nile 1 and 1 Union) and we would be very interested to learn if these azaleas are especially well adapted to higher pH soils.
Rhododendron maximum in Ohio
Rhododendron maximum, unlike the azaleas, grows on fairly moist sites with hemlock (Tsuga Canadensis [L.] Carr.), sweet birch (Betula lenta L.), sugar maple (Acer saccharum Marsh.), and beech (Fagus grandifolia Ehrh.). In most populations, a large majority of plants did not flower either because of their small size or excess shade.
For nearly all populations, flowering characteristics were quite uniform. Throughout the Ohio populations, peak flowering occurred during the last two weeks of June. Petal colors included only shades of light pink (RHS Color Classes 56 and 73), fading to nearly white, except for the Jackson County population (distributed as Byer) which showed considerable variability for flower color including a few plants with a more intense pink color (petals RHS 74D, bud 72C). Most flowers also had a yellow-green blotch (RHS 150A). Floral diameter ranged from 3.0 to 5.0 cm.
Plants were growing on a mix of sand and organic matter, usually over sandstone or along sandstone cliffs. The soils were very acidic (pH 3.55 to 4.84), very low in phosphorous (1 to 5 ppm), except for the Byer population which had a reading of 104 ppm, low to moderate in potassium (28 to 94 ppm), and moderate to high in organic matter (4.5 to 16.4%). We suspect that excess organic matter is needed for the establishment of seedlings of this plant on sandy soils.
In Ohio, Rhododendron maximum often grows on beautiful, but nearly inaccessible sites, in ravines or otherwise difficult terrain. The most easily accessible population of this plant in Ohio can be observed along the road leading to the Wahkeena State Memorial in Fairfield County near Sugar Grove.
A complete list of collection sites and soil test results is available on request from the senior author.
There are many individuals and organizations that kindly helped us with this project. Our thanks go to all who helped, including Ray Smith and John Taylor of the US Forest Service in Illinois, Jody Eberly, Gary Houf, B. Erik Morse and James Murrell of the US Forest Service in Missouri, James H. Wilson of the Missouri Department of Conservation, Joe Manick and Paul W. Nelson of the Missouri Department of Natural Resources, Douglas Ladd of the Missouri Field Office of The Nature Conservancy, Allison Cusick, Charlie Foster and Rick Queen of the Ohio Department of Natural Resources, Paul Carringer of Ohio Camp Fire Girls, and Bill Pearson for identifying sites and arranging permission, to the curators of the herbaria at Iowa State University, Southern Illinois University, Southwest Missouri State University, and the University of Missouri for providing collection information, and the Research Foundation of the American Rhododendron Society for financial support.
1. Boge, D. The Ross-Boge Rhododendron
macrophyllum expedition. J. Amer. Rhod. Soc. 40:62-65; 1986.