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Journal American Rhododendron Society

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Dr. Glen Jamieson ars.editor@gmail.com


Volume 58, Number 1
Winter 2004

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The Jenkins Arboretum - The Jewel in Our Back Yard
Harold Sweetman
Jenkins Arboretum
Devon, Pennsylvania

        Introduction and Brief History
Over the past twenty-eight years Jenkins Arboretum has distinguished itself as a premier garden with its comprehensive and diverse permanent plant collections emphasizing azaleas and rhododendrons. The Philadelphia area has a rich heritage in horticulture going all the way back to early Quaker influences. There are over thirty public gardens, college and university arboreta, and large private estate gardens in the Philadelphia area. One of the most important contributions public gardens like Jenkins Arboretum can provide is to maintain and display permanent plant collections. Surprisingly, in spite of the general popularity of rhododendrons and azaleas among the gardening community, there are few public gardens specializing specifically in this plant group. Consequently, the general population is unaware of the wealth and diversity in the genus Rhododendron. Two other important factors have contributed to this lack of familiarity about rhododendrons. First, residential gardens have generally diminished in size with continued development and second, shrub borders are not as popular as in the past. Although huge rhododendrons and large deciduous azaleas are magnificent in any landscape, there is just not adequate room to truly showcase this plant group in most contemporary gardens. Permanent displays provide not just beauty but a rich educational experience allowing the public to discover many wonderful hybrids and species that are not generally available. The nursery industry will likely never produce more than a small number of potentially wonderful garden-worthy plants. The American Rhododendron Society and the Azalea Society of America, along with their local chapters, promote this genus with lectures and plant sales; however, there are few public gardens especially dedicated to this plant group. This makes Jenkins Arboretum exceptional.
        Jenkins Arboretum is located twenty miles northeast of Philadelphia and remains a lush, verdant isle in the midst of the highly developed suburbs. Although the views from the Arboretum out over the Great Valley are primarily of Valley Forge National Historic Park, a gaze to the right reveals one of the largest shopping malls in America. In the nighttime of winter when the deciduous trees open the view, the King of Prussia Mall and megalopolis is dramatically exposed as twinkling lights. Several major highways also merge nearby allowing easy accessibility for many visitors.
        During the early part of the twentieth century, this area that is now called the Main Line, was largely rural and became a country retreat in the spring and fall for many affluent Philadelphians. The Main Line refers to the train which allowed easy access to the area and later ensured intense residential and commercial development. Undeveloped open space today is less than 5 percent in some communities which makes the Jenkins Arboretum's forty-six acres such an important asset and provides balance for a healthier community.
        In 1926, B. Pemberton Phillippe, a Pennsylvania Railroad executive, purchased twenty acres and built a large stone house on the property as a wedding present for his only daughter, Elisabeth Phillipe Jenkins. Elisabeth and her husband, Lawrence, gardened near their home but allowed the majority of the property to remain undisturbed woodland. After his wife died, Mr. Jenkins directed in his will that the property be "preserved in perpetuity" in memory of her passion for nature and gardening. The will stated that the property be developed as "a public park, arboretum and wildlife sanctuary for use by the public and responsible organizations engaged in the study and promotion of arboriculture, horticulture and wild life for educational and scientific purposes..." In 1970 a careful study and site analysis was initiated in accordance with Mr. Jenkins' will. Also a bequest in 1972 from Louisa P. Browning of her adjoining property, house and cottage increased the Arboretum's size to forty-six acres. This gift provides on site housing for staff, interns, and educational fellows which is a great benefit to the Arboretum due to 365 days a year visitation from sunrise to sunset.

First Director and Modest Beginnings
Jenkins Arboretum opened to the public on May 26, 1976. Coincidentally, 1976 was the last time there was a convention of the American Rhododendron Society in Philadelphia. Sadly, the Arboretum was a fledgling and would have to wait more than twenty-five years before maturing enough to qualify as a tour destination for rhododendron enthusiasts. The first Arboretum director was my father, Leonard H. Sweetman, and planting started in the fall of 1974. Leonard Sweetman worked with Dr. John Wister at Swarthmore College for a number of years and this experience served him well as he began building an arboretum specializing in rhododendrons. Until his retirement at age 74, Leonard planted over 3,000 azaleas and rhododendrons, gradually expanding the plant collections. In 1986 I was hired as the second director by the Arboretum Trustees. Planting and development continued with dedicated part-time staff and many enthusiastic volunteers. As the Arboretum grew and matured it began to generate increasing community support from individuals and foundations which has contributed to its continued success. The Silver Anniversary Campaign to help build a supporting endowment is nearing completion and will provide resources for increased horticultural staff, education and research activities in the future.

Jenkins Arboretum
Jenkins Arboretum
Photo by Harold Sweetman

Latitude, Climate and Site Location
The newest USDA plant hardiness maps indicate the Arboretum to be in Zone 7 (formerly called 6b) that is characterized as having 0 - 10F (18 to -12C) minimum temperatures. However, experience has shown temperatures to 5F (-20C) every 5-10 years. Philadelphia also has a latitude of 40 degrees north which makes this area ideal for growing a great diversity in the genus Rhododendron. As a general rule, evergreen azaleas dominate gardens further south and large leaf rhododendrons dominate gardens to the north. Jenkins Arboretum is unique in its specialization and ability to represent so much diversity in one location. Also, unlike many regions of the country, rainfall averages about 45 inches per year with the greatest distribution during the growing season. Rainfall of approximately 48 inches per year equivalent to an inch per week would be ideal but occasional severe droughts have periodically knocked back many large leaf rhododendrons. This year a permanent overhead irrigation system was installed with great celebration. The irrigation system has a unique shallow trench design with no damage to tree roots and other vegetation.

Orientation, Microclimate and Plant Hardiness
In addition to zone designations, orientation and microclimate greatly benefit the plant collections. Most of the Arboretum faces north and northeast and this orientation provides ideal winter and summer growing conditions. Northern exposure and woodland conditions allow for the coolest possible summer growing temperatures. In addition, erratic winter conditions, such as unusually warm weather, normally would cause premature vernalization. For example, rhododendrons of the PJM Group in our area often exhibit partial fall blooming on warm southern exposures. However, the PJM Group never blooms in the fall on the Arboretum's northern exposure. Bark splitting, a common winter problem with many evergreen azaleas, is rarely a problem due to the northern orientation. Also, winter hardiness of many hybrids may be increased owing to the more consistent dormancy conditions of the northern exposure. The majority of the Arboretum varies from 8% to 30% slope and this terrain allows excellent air drainage with killing frosts often occurring later in November thus affording plants a long period of time to prepare for winter dormancy.

Soil, Drainage, and Underlying Geology
Every garden and landscape is literally grounded by its soil. "Acid well-drained soil" is the mantra and desire for all growers of ericaceous plants and Jenkins Arboretum is thoroughly grounded. There are large regions of the United States that have soil conditions that are not suitable to culture ericaceous plants. Successfully growing of rhododendrons and azaleas in many regions is limited by alkaline soils, heavy clay soils, aridity and excessive heat. Arboretum soils are Glenelg-Channery silt loam derived from weathered gneiss, granite and mica schist with a pH of 5.5. Staff and volunteers may have a different term for the predominate schist rocks, but it is highly permeable with many shale-like fragments (Channery). When planting, smaller rocks must be returned to the planting hole with the root balls otherwise there would likely be an empty hole. The soils appear to be poor but allow air and water to pass easily, unlike denser clay soils. The Arboretum is definitely "grounded" in acid well-drained soils which has allowed this garden to excel in its chosen specialty.

Jenkins Arboretum
Jenkins Arboretum
Photo by Harold Sweetman

Existing Vegetation Before Development of the Arboretum
Jenkins Arboretum is often referred to as a remnant of the once continuous eastern hardwood forest. At the turn of the last century, the Arboretum was majestically a "chestnut hill". Even today, many small chestnut trees continue to grow throughout the property but rarely reach 6-8 inches in diameter before succumbing to disease. After the departure of the mature chestnuts, the woodland grew into a thick canopy of red, white, black and chestnut oaks along with tulip poplar, black cherry, black birch, black gum and mockernut hickory dominating the existing vegetation. Flowering dogwoods , paw paws and Carpinus, also called ironwood or muscle wood, make up the understory trees. Due to the slow decomposition of the heavy oakleaf litter, stoloniferous shrubs such as maple-leaf viburnum, pinxterbloom azalea (R. periclymenoides), blueberry and deerberry have spread successfully. The large numbers of ericaceous plants, specifically blueberries and pinxterbloom azaleas, find the soil pH of 5.5 optimal for growth, thus spreading over large areas of the site.

Two Proposed Areas of Specialization
I. Naturalistic landscape design gardening with native plants

One of the goals of naturalistic landscape design is to give an impression that nature is doing all the work in the garden. The tranquility and informality in the Arboretum create a meditative mood for visitors, yet the Arboretum is still a garden requiring skilled horticulturists. The Arboretum tries to maintain a careful balance between the native plants and the built landscape. Jenkins Arboretum has always emphasized the flora of eastern North America long before native plants became horticulturally popular. Companion plants, including over 400 species of native ferns and wildflowers, have been planted throughout the Arboretum in order to extend multi-season interest. The ecological perspective derived from these diverse plantings is very important since nuts, fruits and berries help support wildlife. When there is greater plant diversity in an ecosystem, there is often greater overall biological diversity. This includes many animal groups in addition to the more visible birds and small mammals. Deer exclusion fencing is an absolute necessity - end of comment. Most wildflowers are clearly labeled with common and botanical names which enhances the Arboretum's educational potential for all those who visit the garden.

II. Ericaceous or heath family plants with emphasis on rhododendrons and azaleas
Owing to its unique history of not being a converted estate garden, the Jenkins Arboretum was conceived and developed with a specific vision and area of specialization. The entire heath family is too large to be represented fully so a decision was made to specialize in azaleas and rhododendrons. However, there are large collections of Kalmia or mountain laurel, the Pennsylvania state flower, Pieris, Leucothoe, Vaccinium and Enkianthus. Pennsylvania, as part of the Appalachian Mountain chain, has a respectable number of rhododendron species. Native deciduous azalea species are increasing in popularity among knowledgeable gardeners. The Arboretum has many deciduous azalea species and hybrids planted over twenty-five years ago and all eastern North American native species are well represented. Rhodora or Rhododendron canadense is an exception and has not survived south of the Pennsylvania Pocono Plateau.

A lady slipper along the Woodland Walk.
A lady slipper along the Woodland Walk.
Photo by Harold Sweetman

        Careful planning to not homogenize rhododendrons into one large collection has allowed a unique garden feel at Jenkins. Elepidote, lepidote, evergreen and deciduous rhododendron categories are grouped with careful consideration. As a public garden open all year-round, deciduous azaleas look dormant to rhododendron lovers but appear dead to many visitors. Plantings of mostly April-blooming evergreen lepidotes with deciduous azaleas that are later bloomers, make an attractive combination throughout the year. Because of different bloom times, color clashes between many lavender/purple lepidotes and the oranges and yellows of many deciduous azaleas can largely be avoided.
        The Rosebay rhododendron (Rhododendron maximum) is ideally suited as a good doer. This well known species is featured prominently throughout Jenkins Arboretum as background and screen plantings between wandering paths. These important design elements create new discoveries around every bend. Five hundred R. maximum were planted throughout the Arboretum from sources in North Carolina. Surprisingly, species rhododendrons dominate the Main Line suburban landscape west of Philadelphia due in part to the climate and to history. It is generally drought and quite heat tolerant here in the piedmont which is in close proximity to the Atlantic coastal plain. It also is one of the most shade tolerant among all rhododendrons. Although their summer blooming trusses are small and transient, their foliage is attractive all year. The closeness of the Pocono Mountains, which has huge native stands of R. maximum, created a familiarity as well as a renewable natural nursery resource. It was common during the post-WWII development era for vendors to drive through residential neighborhoods with pickup loads of Rosebay rhododendrons selling for a couple of bucks, with a replacement survival guarantee when they returned the following year.

Partnership with the American Rhododendron Society
Local chapters are often active supporters of public gardens in their area and the Valley Forge Chapter has been a commendable example. The Valley Forge Chapter holds their pubic meetings, plants sales and other regular activities at the Arboretum. In order for Jenkins to excel in the depth and breadth of its rhododendron and azalea collections, it needed all the combined resources of the plant societies and their local chapters. In 1992, the Valley Forge Chapter provided a multi-year, $15,000 grant to the Arboretum in order to acquire seed, new hybrids as well as rare old hybrids, landscape-size plants, and cuttings. Included in this initiative was a serious effort to attempt to grow many different rhododendron species to test adaptability. The Valley Forge Chapter also provided grant support for a new irrigation system as well as funding for other public gardens and conservancies. The Azalea Society of America's 10 Oaks Project also allowed the Arboretum to obtain a very large collection of Glenn Dale azalea hybrids.

Scientific Plant Collections
As a botanical garden, Jenkins Arboretum takes meticulous care in keeping plant records. Every permanent woody plant is given an accession number, recorded in a computer database, and then mapped on a 50 ft. by 50 ft. surveyed plot system. With an ever-increasing number of new hybrids and seed-grown species, it is vital that the Arboretum maintain a record of the identity of every plant. The added bonus of labeling is the wonderful educational potential for the public. Hybrid groups, or the hybridizer if known, are routinely engraved on identifying plant labels. Botanical gardens refer to each accessioned plant as a taxa and the Arboretum currently has over 13,000 taxa in its database. As one can imagine, this takes a great deal of attention in order to maintain these museum standards with dynamic living collections.
        Some of the earliest and most respected hybridizers of rhododendrons and azaleas spent their lives creating and selecting many wonderful plants for our regional landscapes and gardens. Joseph Gable, Guy Nearing, Lanny Pride, John Wister, William Rhein, Charlie Herbert and many others have spent their lifetimes hybridizing and selecting many wonderful cultivars. These hybridizers were pioneers in the early history of gardening with rhododendrons and their hybrids have stood the test of time. The Arboretum honors many of these regional hybridizers by maintaining permanent plant collections of their tried and truly adapted cultivars.
        In 1993 I had the opportunity to accompany Kenneth Cox on a botanical expedition to remote mountainous regions of Yunnan Province, China. Rhododendron species seed collection in springtime during peak bloom in China was a rare opportunity. Though not many seeds were collected from plants at blooming time, propagation has been successful and several species have done well here after ten years. The most exciting successes would be hardy forms of Rhododendron decorum, R. racemosum, and R. rubiginosum which are doing quite well.
        When asked about hybridizing, Kenneth Cox, on his last visit to the Arboretum, suggested using "weedy" or the most vigorous growing cultivars and species. Rhododendrons have to be vigorous to perform well at the Arboretum because of native soils and tree root competition. Several elepidote hybrids exhibit remarkable vigor and among them is the Gable hybrid 'Caroline' and 'Maximum Roseum' (synonym for the registered name 'Ponticum Roseum') whose hybrid origin is uncertain. Although East Coast hybrids dominate the plant collections, truly wonderful exceptions such as mature specimens of 'Bob Bovee' excite visitors with exquisitely perfect leaves, peach buds and creamy yellow flowers. Visitors also marvel at the size of many of the North Tisbury R. nakaharae hybrids at Jenkins that have been able to grow without deer and rabbit herbivory. The number of different hybrids and hybridizers are too numerous to mention and a visit can be an eye opening experience into the wonderful world of rhododendrons.

Jenkins Arboretum Welcomes Visitors to Help Fulfill Our Mission
The mission of the Arboretum can be simply stated as follows: The Arboretum is to be preserved as a place of natural beauty for the enjoyment of the public, as a living example of natural systems and the value of open space, and as an asset to the community through the pursuit of excellence in botanical display, research and education.
        Native trees, shrubs, vines, wildflowers, and ferns contribute to the full ecological potential of a large garden like Jenkins. In recognition of their value and under appreciation by the general public, the Arboretum this year initiated its Green Ribbon Native Plant Award program. The goal is to select and promote especially garden-worthy plants.
        The Arboretum continues to distinguish itself as a showcase for the versatile and vibrant genus Rhododendron. By institutional standards, it is still a young garden in active growth and development. Staff and volunteers are always in nursery production with wild collected species and hybrid seed plus cutting propagation in an effort to improve and expand permanent plant collections. The Arboretum is open 365 days a year from dawn to dusk and there is no admission fee. Visitors are welcome throughout the year.

Harold E. Sweetman, Ph.D., is Executive Director of the Jenkins Arboretum. He is a member of the Valley Forge Chapter and chairman of the ARS Research Committee.


Volume 58, Number 1
Winter 2004

DLA Ejournal Home | JARS Home | Table of Contents for this issue | Search JARS and other ejournals