Last updated on August 26, 2024
Each summer, participants in the Isabella Welles Hunnewell Internship Program at the Arnold Arboretum complete a group project in the landscape as the culmination of their training in public garden management. The challenge for 2024 interns was to interweave the distinct and historically relevant stories of the plants and history of the eastern slope of Bussey Hill. Near the summit of the hill the Arboretum’s crown jewel of plant collecting: the Explorers Garden.
The Explorers Garden contains multitudes of plants gathered from expeditions around the world, and the introductions of species never before seen in North America. The ruins of the historic estate of Benjamin Bussey lay just below the garden, occupying a ridge halfway down the hill. In between these sites grow a mixed bag of plantings, including native species of conservation concern in concert with the Center for Plant Conservation (CPC) and a historical lilac hedge, all in dire need of reevaluation.
Interns determined that each of these aspects and their accumulated stories require room to grow. A big part of the solution was removing a large grove of American beech (Fagus grandifolia) trees running through the site to create additional planting areas. Infected with beech leaf disease, the grove was slated for removal in an effort to centralize resources needed to save the most important individuals of this national collection elsewhere in the Arboretum landscape.
PROJECT GOALS
- Transform a grove of diseased beech trees into a new expansion of the Explorers Garden;
- Acknowledge the site’s history as the former location of Benjamin Bussey’s mansion and decide how to incorporate aspects of his complicated history as donor of the Arnold Arboretum property and his acquisition of wealth through trade in goods produced by enslaved people;
- Create planting, landscaping, and construction plans as recommendations for implementation in coming seasons;
- Assess the state of the Center for Plant Conservation (CPC) collections of Diervilla species and devise a plan that is better aligned with the needs of conservation and Arboretum horticulturists.
BEECH LEAF DISEASE
Home to eight species of beech (Fagus spp.) and hundreds of individual specimens, the Arnold Arboretum holds a nationally accredited collection of the genus. Over the past 10 years, however, a deadly combination of pests, disease, and drought has pushed much of the beech collection past the point of no return. Beech leaf disease (BLD), a novel pathogen of the 21st century, is spread via Litylenchus crenatae subsp. mccannii, a subspecies of foliar nematode. The disease can be easily recognized by darkened interveinal banding of leaves caused by cellular damage, beginning in the lower branches. This leads to lessened photosynthetic ability, bud damage or death, and thinned canopies.
Young trees can die after only two to four years of infection, while mature trees can survive up to ten years. There is no known cure for BLD, though soil injections can reduce the severity of symptoms. BLD was first discovered in Ohio in 2012, and has since spread throughout the Midwest and New England.
Much of the collection also struggles with beech bark disease (BBD), an affliction caused by a complex of scale insects and fungal pathogens in the genus Neonectria. Wounds made by scale insects are colonized by fungal pathogens, eventually leading to a perennial canker and red, oval-shaped fruiting fungus. Trees afflicted with BBD are weak to other insects, woodpeckers, fungus, drought, and cold injury. Though the beech collection at the Arnold successfully endured bouts of BBD, the introduction of BLD seriously threatens their prospects of continued survival.
SITE HISTORY & CPC
Benjamin Bussey strategically sited his mansion so it would overlook a view of country and pastureland, including the Blue Hills and Hemlock Hill. Thus, as interns began the designs for their project, they considered how the clearing of the beech grove and new plantings could create viewsheds that would draw visitors into the beauty of the rolling landscape. They also learned through investigating historic landscape maps that this beech grove had existed at least as early as 1938—the year of a catastrophic hurricane—but a select few beeches from the collection were not accessioned until the early 2000s.
Additionally, the Arnold Arboretum partners with the Center for Plant Conservation (CPC) to preserve germplasm for 24 threatened species of native woody plants. The planting bed above the project site contains various CPC plantings, interrupting the flow of the Explorers Garden into the proposed expansion provided by the project site. The majority of the CPC plantings in this bed are mountain-bush honeysuckle (Diervilla rivularis) and southern bush honeysuckle (D. sessilifolia), totaling 36 individual plants. These plantings are difficult to maintain, as individuals must be pruned back regularly from one another to preserve their genetic distinction.
SITE ASSESSMENTS FOR SOIL AND PLANT HEALTH
With the help of Horticulturist Conor Guidarelli, interns took foot-deep soil samples across the project site, and learned how to process soil samples through the University of Massachusetts Extension. Test results indicated that the soil in the former beech grove carries an average pH of 5.1 (strongly acidic), and is severely lacking in potassium, calcium, and magnesium.
Head Arborist John Del Rosso assisted interns in testing a Japanese ash (Fraxinus longicuspis) and a Chinese ash (F. chinensis subsp. rhynchophylla) left standing in the project site for disease and pests. Lab results showed that both trees are infected with anthracnose, a fungal, lesion-causing disease. These results were not surprising, as the site has a history of anthracnose likely caused by a lack of air circulation in the grove. Interns hope that with the removal of the beeches, air circulation will increase and reduce the severity of the disease in nearby plants.
SITEWORK
After finishing site assessment, inters worked with the arborist team to begin removing the beech grove. First they cleared the undergrowth, which was overrun with poison ivy, stinging nettle, Virginia creeper, jewelweed, and pokeweed. Over the course of two weeks—working individually, in pairs, and in teams—they felled smaller trees using chainsaws and handsaws, while arborists removed five larger beech trees using a lift.
After all of the trees slated for removal were felled, the interns learned how to ground the stumps and rake out the detritus. To maintain the integrity of the slope, they plan to place seeding straw and spread no-mow mix, partridge pea, and crimson clover to increase soil fertility and prevent erosion.
FUTURE PLANTINGS
The microclimate that the project site shares with the adjacent Explorers Garden offered interns exciting options for future plantings—and makes possible extending the Explorers Garden itself and its capacity to shelter plants on the margins of hardiness at the Arboretum. Interns recommended that the area should include plants that push hardiness boundaries, house newly-acquired desiderata, and/or serve as an experimental area for species of undetermined hardiness.
To compile planting criteria, interns consulted with many Arboretum staff. Michael Dosmann (Keeper of the Living Collections), Kyle Port (Manager of Plant Records), and Rodney Eason (Director of Horticulture and Landscape) provided horticultural and curatorial perspectives, and Chris Copeland (Assistant Manager of Plant Production) and Sarah Shank (Propagator) at the Dana Greenhouses offered their propagation expertise. With the latter team, interns also reviewed greenhouse inventory to get a sense of potential plantings available in the near future.
Interns envision this space housing species of conservation concern that may be cold-sensitive, thin-barked trees prone to injury in severe winters, as well as species with summer interest as spring-blooming species currently dominate the area. Adding plants with winter interest would also benefit the site seasonally. Most of the suggested plants are limited to shrubs and smaller trees in order to preserve the viewsheds and the perspective of the slope. They also considered plants that will tolerate full sun conditions and create a nice line of continuity from existing plants in the Explorers Garden.
This article was originally published on the Arnold Arboretum’s website, and has been republished here with the arboretum’s permission.
