By Jonathan Kavalier
Big changes are underway in the garden, and while only the astute observer is likely to notice, our native animals will certainly benefit.
Gardeners at Dumbarton Oaks are embarking on a multiyear phased effort to manage invasive plants both planted and indicated in designs by Beatrix Farrand, as well as those that have shown up uninvited. The goal is to encourage the growth of native plants that support local animals, reduce the need for insecticides, and improve plant health. In the spring, the team completed a significant phase of this project along the east end of the garden.
This work will not only reduce garden maintenance in the long term but also raise the level of ecosystem services—benefits imparted to animals, such as food and habitat—of the entire landscape. The project encourages the growth of plants that have evolved closely alongside native fauna and are thereby better prepared to exist symbiotically. As Doug Tallamy expresses in Bringing Nature Home, native plants (particularly woody plants) provide exponentially more benefit for native insects because their lifecycle events, such as flowering and fruiting, are naturally timed with the emergence of these insects. Fruit, pollen, and nectar from these plants are more nutritious for these insects owing to the fact that the plants and insects have evolved in the same ecosystems for millennia.
Such evolutionary symbiosis helps reduce the reliance on chemical interventions such as insecticides and fungicides, which in turn fosters a healthier soil microbiome, increasing nutrient cycling and other factors that aid in overall plant health.
For the purposes of this project, we are focusing on “invasiveness” rather than a plant’s native status. For example, poison ivy and Virginia creeper are native plants, but both spread rampantly and are undesirable in a garden setting. We define an invasive plant as one that spreads vigorously, either vegetatively or through seed dispersal, and displaces other plants by outcompeting them.
We also weigh each plant’s importance in the garden from a design perspective. Virginia creeper has its place if carefully managed—for example, the front of the Fellowship House. Wisteria is an invasive plant that spreads vegetatively with an incredible rate of growth, but also reseeds readily. Because wisteria is of central importance in many of Farrand’s designs, we have decided to keep it, but are particularly vigilant in removing it from unwanted locations and pruning off seedpods before they ripen. Meanwhile, privet spreads rampantly by seed and is not a central component in any Farrand designs. We can easily replace it with a native shrub such as buttonbush or arrowwood viburnum, both of which have similar form, bloom time, and color.
Preparation for the project began in 2018, when summer intern Joan Chen completed the report “Reimagining Farrand.” It mapped all invasive plants in the garden and explored several case studies to determine the best treatment strategies—be they removal or management to prevent spread. We implemented two of the case studies. At Urn Terrace, we removed English ivy and replaced it with boxwood varieties that are resistant to boxwood blight. At Lovers’ Lane Pool, we replaced invasive honeysuckle with winter jasmine.
After evaluating the success of these projects and identifying suitable plants with which to replace invasives, gardeners are looking ahead to the next phases of this long-term improvement. This spring, the team restored a buffer planting along the east end of the garden. Running parallel to Mélisande’s Allée, from Camelia Circle to the Lovers’ Lane Pool, the buffer provides privacy at the edge of the garden that abuts Montrose Park, which sees a lot of activity. We removed invasive plants such as honeysuckle, porcelain berry, poison ivy, tree of heaven, and privet to allow planting space without clearing the entire area. Then, in keeping with Farrand’s design intent for this area, native shrubs—a mix of evergreen and deciduous including spicebush, buttonbush, sweetspire, holly, and native virburnum—were planted behind the Allée. They provide a thick screen while adding spring interest. Once these plants have established themselves, we will remove the remaining invasives, allowing this change to occur with minimal disturbance to the intimacy and privacy of the garden.
Jonathan Kavalier is director of gardens and grounds. Photos by Jonathan Kavalier and Elizabeth Muñoz Huber, postgraduate digital media fellow.