A Half Century of Rhododendrons at Secrest Arboretum
John E. Ford, Curator, Secrest Arboretum
Ohio Agricultural Research and Development Center
Reprinted with permission from 1974 Turf & Landscape Research,
Research Summary 79, Ohio Agricultural Research & Development Center, Wooster, Ohio
Rhododendrons have been planted in the Secrest Arboretum for 57 years and on the OARDC campus for 53 years. The first plantings of rhododendrons were made in 1917 when about 200 Rosebay Rhododendrons (Rhododendron maximum) were under planted in forest plantations in 11 different locations. These plants have held their own in competition with other vegetation. Although they bloom every year and set seed, no self-seeded plants have been found.
The next plantings were made in 1921, 1922, and 1930 when Smooth Azalea (Rhododendron arborescens) and a number of the Ironclad types of rhododendrons were used. Many of the 1921 to 1966 plantings were made as foundation plantings around the buildings on the OARDC campus. Since 1966, many rhododendrons and azaleas have been set out in the Rhododendron Display Garden. At present, there are 170 different species, varieties, hybrids, or cultivars.
R. maximum at Seacrest Arboretum
To date, rhododendrons which have grown for 40 years or more are the two species Rosebay rhododendron (Rhododendron maximum) and Carolina rhododendron (Rhododendron carolinianum). The Ironclad rhododendrons are white catawba rhododendron (Rhododendron catawbiense album), 'English Roseum', and 'Purpureum Elegans'.
The Smooth or Sweet Azalea (Rhododendron arborescens) survived for 45 years until a volunteer Black Walnut (Juglans nigra) grew up through the planting. All but one of the azaleas died and the surviving plant is in very poor condition, even though the walnut has been removed.
Although the early plantings of Rosebay rhododendron were successfully grown in natural settings, attempts to grow some of the newer hybrid rhododendrons in natural looking plantings were not too successful. In many cases, these hybrids could not successfully compete with other vegetation and were crowded out. A number of rhododendrons were smothered out by ferns, especially Sensitive Fern (Onoclea sensibilis). Sensitive Fern in the Rhododendron Display Garden is very aggressive. Hybrid rhododendrons set out in beds where all competing vegetation was controlled have had much better survival.
Depth of roots is very important in planting rhododendrons, since they are very shallow rooted. Many of the first plantings of rhododendrons in the Display Garden were set in holes which had been augered into the ground. Settling of the soil and wash-in of soil from around the planting holes soon had some of the root systems buried under 3 to 4 inches of earth. Many of the first plantings had to he lifted and reset so that the shallow root systems were near the top of the ground level. A number of plants died before they were reset.
In six different locations where rhododendrons or azaleas have been planted under Black Walnut, they have either died or grown so poorly they are barely alive. In some instances when rhododendrons have been transplanted from these sites, they have started to recover. Many rhododendrons have been killed during the winter when they were on excessively wet, poorly drained sites or exposed to severe winds. The same cultivars on well drained sites protected from the winds came through the winter in perfect condition. Some rhododendrons were planted on sites which appeared to be well drained. They grew until 1972 when a wet year left these sites in a continual wet condition. Plants in these areas deteriorated rapidly.
Size of plant can also have a hearing on survival. Rhododendrons of the same clone were set side by side in the nursery and were out planted in the Arboretum. Many plants less than 1 foot high were either killed or partially killed the first winter, while adjacent larger plants 1 to 2 feet tall showed no signs of winter injury. Established plants will also often survive climatic extremes, while recently planted shrubs may be killed.
Survival rate of some rhododendrons planted on the south and west sides of buildings has not been as high as the same cultivars set on the north and east sides. Wind is often an important factor. Many rhododendrons set out on windy sites haven't survived or grown as well as the same kind of plants on sites protected from severe winds. This can be noticeable at times at the corners of buildings. One north foundation planting of the azalea 'Fireball' has plants at the northeast and northwest corners of the building only half the height after 4 years as azaleas growing near the middle of the building where there was less exposure.
The following rhododendrons and azaleas have all been grown in the Secrest Arboretum or on the OARDC Campus for 10 years or more. All have survived temperatures to - 20 F. with no winter kill unless noted otherwise. Most of the rhododendrons are the so-called Ironclads and have been on the market for more than 100 years. Many are capable of surviving colder weather than they have been exposed to at Wooster.
- Rhododendron 'America': Grown for 14 years. Fair bloom mid-May to early June. Flowers dark red.
- Rhododendron arborescens: Grown for 53 years. Fair bloom late May. Flowers white and fragrant.
- Rhododendron 'Caractacus': Grown for 18 years. Fair bloom late May to early June. Flowers purplish red.
- Rhododendron carolinianum: Grown for 44 years. Excellent bloom every year, early to mid-May. Flowers pink.
- Rhododendron catawbiense album: Grown for 44 years. Excellent bloom every year late May. Flowers white.
- Rhododendron 'Cunningham's White': Grown for 16 years. Blooms late April to early May. Often has scattered bloom in fall. Flower buds often partially killed at -5 F. All flower buds killed at -12 F. Shrub not damaged and has survived to -20 F. Good foliage plant but flowers not dependable. Flowers white.
- Rhododendron 'English Roseum': Grown for 44 years. Good bloom late May to early June. Flowers rose colored.
- Rhododendron 'Fireball' (deciduous azalea): Grown for 14 years. Good bloom every year late May to early June. Flowers scarlet.
- Rhododendron 'Lee's Dark Purple': Grown for 22 years. Good bloom mid-May. Flowers deep purple.
- Rhododendron 'Louise Gable' (azalea): Grown for 14 years. Excellent bloom late May. Flower buds all winter killed at -10 F. Shrub has survived to -20 F. Flowers pink.
- Rhododendron maximum: Grown for 57 years. Fair bloom every year late June to early July. Flowers white, partially hidden by new growth.
- Rhododendron 'Mrs. Charles S. Sargent': Grown 16 years. Good bloom late May to early June. Flowers rosy pink.
- Rhododendron 'Parsons Grandiflorum': Grown 13 years. Fair bloom late May. Flowers purplish-rose.
- Rhododendron 'Purpureum Elegans': Grown 44 years. Good bloom late May. Flowers deep purple.
- Rhododendron 'Purpureum Grandiflorum': Grown 13 years. Good bloom late May to early June. Flowers medium purple.
- Rhododendron 'Roseum Elegans': Grown 13 years. Good bloom every year late May to early June. Flowers rose lilac.
- Rhododendron 'Roseum Superbum': Grown 13 years. Fair bloom late May. Flowers purplish-rose.