The U.S. Capitol Grounds and Arboretum: an iconic landscape with a holistic approach to pest management

Abstract

The care and maintenance of managed urban greenspaces require integrated pest management (IPM) strategies that are best suited for their local conditions. The evolution of IPM for the public grounds of the U.S. Capitol and Congress provides a unique perspective into the distinct challenges in developing and implementing an overall plant healthcare program for the nation’s historical landscape. The Capitol Grounds and Arboretum (CGA) staff of the Architect of the Capitol are the stewards of this urban greenspace, located in the center of Washington D.C., and present an overview of their historical, current, and prospective IPM developments for urban landscape pest control. With the adoption of holistic, ecological-based pest management practices, successful IPM examples in 3 different landscape habitats on Capitol Hill—turfgrass lawns, urban forestry, and ornamental horticulture display beds—are described. Ultimately, a list of current programming and future opportunities for expanding ecological IPM practices best suited for solving the CGA’s unique landscape pest management challenges are summarized.

Introduction

Urban landscapes constitute multifaceted layers of complexity that challenge any pest management professional with developing an effective, successful plant health care (PHC) program. The adoption of integrated pest management (IPM) strategies, while founded on conventional agricultural systems, must consider the diversity of ornamental and turfgrass plant material, management intensity, environmental conditions, perceived expectations, as well as other influential factors that can greatly impact their efficacy in controlling problematic landscape pests (Braman et al. 2000). The Architect of the Capitol (AOC), Capitol Grounds and Arboretum (CGA), is a large expanse of managed greenspace within the highly urbanized center of Washington, D.C. The purpose of this article is to present the unique horticultural pest management challenges that occur within the public grounds of the Nation’s capital and highlight the historic, current, and prospective IPM programming aimed at maintaining a healthy, aesthetically appealing landscape at Capitol Hill.

CGA Background

The AOC, U.S. Capitol Grounds encompasses 295 acres of ornamental landscaping and built infrastructure—Congressional offices and their facilities, utility systems above and below ground, paved surfaces—across Capitol Hill (Fig. 1, Supplementary Fig 1). The property includes the U.S. Capitol, Senate and House office buildings, the Capitol Power Plant, and Union Square, which is a 13-acre site located between the U.S. Capitol’s West Front and the National Mall. The AOC property also encompasses a diversity of managed greenspace that includes forested parklands (which house historical, memorial, and commemorative native and exotic trees), turfgrass lawns, green roof systems, intricately designed courtyards with many horticultural plant species, interior and exterior decorative plant features, historical fountains and water features, and seasonal rotational display beds comprised of different mixtures of woody and herbaceous ornamentals.

To better understand the composition, design, and architecture of the landscape on Capitol Hill, it is important to briefly describe the history of this space and the figures who were instrumental in the evolution of the U.S. Capitol Grounds from its inception to the present day. Today, Washington D.C. is a highly developed urban city, but back when the U.S. Capitol Building site was selected in 1791, the land was primarily unmanaged wetland habitat dominated by scrub oak (Quercus ilicifolia Wangenh.) (Dietzen et al. 2020). It was not until 1874 that the overall design of the Capitol landscape fell to the direction of Frederick Law Olmsted along with the English architect, Thomas Wisedell (Heritage Landscapes, LLC 2012, Cultural Landscape Foundation 2022). Unlike other landscape projects, Olmsted designed the surrounding landscape to channel one’s attention to the architecture of the U.S. Capitol Building as the focal point. Thus, the paths, periphery, and woody ornamental planting arrangements on Capitol Square are situated to give the visitor expansive vistas of the building from different vantage points. Even today, new plan installations must maintain the “original intent” of the historical landscape designed by Olmsted back in the late 19th century.

The core mission of the CGA is “to maintain a landscape that serves the United States Congress and that provides an aesthetically pleasing” experience (Architect of the Capitol 2022). Thus, by working together, the CGA shall strengthen and showcase the foundation, facilities, and functions supporting American democracy. It is important to highlight here that the AOC CGA workforce includes duties beyond overall PHC, of which IPM programming is a major component. Furthermore, the CGA ultimately reports to members of Congress and the Supreme Court; all of whom are relevant stakeholders of CGA’s activities in and around Capitol Hill.

The CGA staff is comprised of managerial, supervisory, and administrative members along with trained individuals in horticulture, arboriculture, plumbing, masonry, and automotive mechanics, averaging around 90 managerial and operational staff at a given time over the past 10 years (Capitol Grounds and Arboretum Organizational Handbook 2022). The PHC program within the CGA falls under the purview of an IPM specialist in conjunction with technical expertise in the 3 divisions: Gardening, Turfgrass, and Urban Forestry. Working collectively as 1 team, the CGA supports the AOC in preserving its heritage assets, which include cultural landscapes, living botanical assets, decorative water features, and architectural structures on the grounds (Architect of the Capitol 2008).

IPM Programming and Current Challenges

Little historical information is available about the early years of U.S. Capitol Grounds landscape care and maintenance programs. Most likely, conventional chemical pesticide spray programs were used to control major landscape pests by workers and work animals (Figs. 2 and 3). The only documented statement to reflect such controls was in 1913, the Architect of the Capitol, Elliott Woods, remarked in his annual report for a vigorous spray program to control for “pit scale afflicting English oaks on the grounds” (Heritage Landscapes, LLC 2012). To further complicate pest management programs on the property during that time, plant specimens brought back from various botanical collection trips to other North American and international lands were installed on the AOC grounds without any proper quarantine or inspection process. A June 1825 article from the Daily National Intelligencer made note of their “exotic” presence and quite possibly included plant species that originated from Asia, Europe, and western North America (Heritage Landscapes, LLC 2012). These plantings unintentionally resulted in the introduction of several nonnative invasive, and even federally defined noxious weed species, thus providing ample opportunities for the onset of an invasive insect pest or disease infestation that may have been overlooked over the years (Reichard and White 2001, Beck et al. 2008, Lehan et al. 2013, Beaury et al. 2021).

Before reorganization, CGA pest management could only be described as a reactionary means of control of episodic, repeated, or suspect pest outbreaks with poor documentation or management strategy. Interventions often depended on the discretion of Congressional constituency demands or CGA Superintendent perceptions. After reorganization, the AOC CGA employment infrastructure expanded to include technical staff specifically trained in IPM to establish and implement a successful PHC program based on current IPM principles and horticultural Best Management Practices (BMP) (Capitol Grounds and Arboretum Organizational Handbook 2022). That increased technical expertise led to an overall improvement in the assessment of the health status of the plant specimens across the AOC. As a result, the CGA identified major cultural and pest issues that initiated the development of a more formalized pest management plan for future landscape maintenance efforts. While the CGA Pest Management Plan is still in development, below we highlight CGA’s PHC approach with select examples of improved IPM-based programs that have been incorporated by the CGA within the last 5 years. These examples aim to illustrate the most recent achievements in IPM programming to the U.S. Capitol landscape upon the onset of the newly restructured organization of CGA staff and management.

Plant Health Care

Plant Health Care started to include preventative approaches to pest management in the form of concerted efforts to proactively avoid new pest introductions or their spread on Capitol Grounds. At least 5,000 perennial and more than 12,000 annual ornamental plants are delivered from different nursery growers and installed on the Capitol Grounds each year. Government-contracted vendors of live plant material must provide proper phytosanitary documentation along with purchase orders; plant materials are inspected upon delivery; and damaged or infested plants are either rejected, discarded, or treated immediately upon receipt. When possible, selections of woody plants are done at the contract nursery or grower site to assess the BMPs used in rearing the plants while also selecting for the healthiest, potentially disease-free, plants. These improved sanitation methodologies aim to prevent the arrival of new invasive problems such as the spotted lanternfly [Lycorma delicatula (White) (Hemiptera: Fulgoridae)], elm zigzag sawfly [Aproceros leucopoda Takeuchi (Hymenoptera: Argidae)], beech leaf disease which is caused by the nematode Litylenchus crenatae mccannii, vascular streak dieback disease, among others (Carta et al. 2020, Murman et al. 2020, Bily and Bush 2023, Oten et al. 2023, Urban and Leach 2023).

When a pest or disease of concern is detected on incoming plant material, infested materials are isolated, and proper lab diagnostics (if time permits) confirm the specific pathogen or pest before suppliers are alerted of the infestation. Once confirmed, treatment of infested plant material includes double-bagging and discarding it at the nearest waste facility. If infested plants or sod has already been installed, the newly infested areas are treated with the most appropriate control tactics for the target location. In high-traffic areas, for example, only horticultural oils and soaps can be applied to control sap-feeding insects, and cultural pruning can remove isolated patches of diseased or damaged foliage.

For any pest or disease problem on the AOC property, historical precedence was reactionary, as mentioned previously, without confirmed diagnostics nor established action thresholds in place. Basic principles in IPM and BMP are currently being reinforced with particular emphasis on correct diagnosis of the issue, routine monitoring, and adoption of advanced biological control methods. Recent pest data records are currently being used to develop action thresholds for major disease, weed, and insect pests. Nonchemical pesticides are used when appropriate and require additional training in pest biology and timing of susceptible life stages. Dormant oil sprays in winter months have replaced all conventional broad-spectrum insecticide sprays that were used in years past to control major wood-boring and scale pests. Most importantly, outdated inventoried pesticide products were properly disposed of and replaced with less hazardous, target-specific biopesticides.

Augmentative biological control was implemented for the first time in 2022 to evaluate the efficacy of releasing green lacewing larvae [Chrysoperla spp. (Neuroptera: Chysopidae)] to reduce azalea lace bug [Stephanitis pyrioides (Scott) (Hemiptera: Tingidae)] aphids (Hemiptera: Aphididae), and armored gloomy scale [Melanaspis tenebricosa (Comstock) (Hemiptera: Coccomorpha: Diaspididae)] at the susceptible crawler stage (Parsons and Frank, 2019). This was the first attempt at using live biological agents to reduce problematic pests in a public park within the AOC and was well-received as an environmental-friendly form of pest control among CGA staff, AOC constituents, and local residents.

In addition to improved phytosanitary measures, pest diagnostics, and targeted biological controls, current investigations of unhealthy, symptomatic, and dying plants also involve identifying the underlying abiotic causes and conditions leading to premature plant failure. Girdled tree roots from poor nursery stock or installations, soil nutrient composition, soil compaction levels, and soil moisture content are all abiotic factors that contribute to plant stress and increase the risk to disease or pest attack. Soil sampling has become routine when comparing plant health to these fore-mentioned conditions. These analyses of nutrient composition in the soil from more regular soil testing have revealed current gaps in soil fertility programs, thereby creating awareness for a more targeted approach to soil amendment plans. The goal is to remediate soils for optimal nutrient uptake by plant roots in discrete locations of the grounds.

Another major improvement to CGA PHC programming is the shared responsibility of landscape pest control efforts among CGA staff. Before 2019, landscape pest control sprays were primarily carried out by a select few technical expert individuals that were trained and licensed to apply pesticides. With experience and training, advanced gardening and arborist CGA personnel now also qualify to obtain their DC public pesticide applicator certification and annual licensure through examinations given by the DC, Department of Energy and Environment, Pesticides Division. Once certified and licensed, CGA pesticide applicators can independently implement pest control products based on IPM plans developed by CGA’s technical experts and supervisors of the targeted area(s) facing landscape pest issues. Currently, the AOC gardening staff includes more than 70 trained and licensed pesticide applicators; all well-informed in the basic IPM principles and BMPs to effectively manage problematic pests of turfgrasses and ornamental plants. For reference, Chapter 11 of the AOC’s Environmental Manual provides a summary of the AOC’s integrated approach to pest management on the property and it clearly states the requirements and responsibilities for its employees with respect to the management of pesticides (IPM Environment Chapter AOC policy 2019).

To meet the AOC’s commitment to environmental sustainability, the CGA must move away from the use of broad-spectrum chemical pesticides and include more preventative and augmentative biological controls in pest management. The application of such selective biological products requires an in-depth understanding of pest life cycles and proper timing of applications. Ironically, an increase in staff as trained pesticide applicators would appear counterproductive to the AOC’s mission, yet this advanced knowledge and training in proper pesticide handling builds upon basic IPM principles. As certified pesticide applicators, CGA staff learn how to properly diagnose pest issues, keep detailed records, and implement appropriate pesticide products for target pests, including biopesticides such as Bt toxin and mycoinsecticides.

Given its landscape complexity, high profile location, and influential constituency, CGA’s PHC program, of which IPM is a main component, must overcome certain challenges to its sustainable landscape pest management. Next, we highlight these unique factors that greatly impact pest management programming on Capitol Hill.

Unique PHC Challenges

Continual Habitat Disturbance

Construction and renovation projects to maintain the aesthetic of the Nation’s Capitol constantly occur across the AOC, which heavily impacts the landscape surrounding the government office buildings (Fig. 4). These multiyear projects can severely damage existing trees and plants within the target area and often include completely new landscape designs that are installed near project completion. Residing soils are negatively affected by compaction from the heavy traffic flow and equipment, mixed with poor-quality backfill soils. Additionally, biotic agents that degrade sites such as plant pathogens and weeds are introduced from the use of contaminated equipment and imported materials.

Fragmented Irrigation Infrastructure

Decades of building renovations and construction projects have resulted in a mixed patchwork of new and old irrigation systems across the AOC property. While certain areas are watered automatically on irrigation timers, other areas must still be manually turned on/off. This creates the problem of having some areas being overwatered due to overlapping irrigation pipes while other areas do not receive sufficient or any supplemental watering within a given season. Irrigation pipes are constantly being interrupted, damaged, redesigned, or ultimately ignored upon the completion of any given construction project. Prolonged water stress contributes to increased insect pest pressure, root rot diseases, and premature plant death (Williamson-Benavides and Dhingra 2021). Advanced wireless-based and Bluetooth irrigation technologies have been installed in select areas of the grounds, yet their use is restrictive and must be managed through multiple plumbing branches across multiple AOC jurisdictions.

Microclimates and Plant Hardiness Zones

According to the updated 2023 U.S. Department of Agriculture Plant Hardiness Zone Map, the entire property of the AOC is characterized as Zone 8a, with minimum average temperature within 10–15° Fahrenheit and maximum temperatures ranging from 45 to 90° Fahrenheit (USDA Plant Hardiness Zone Map 2023). However, as is well supported by research, many local and landscape-scale factors such as climate variability, percent hardscape cover, greenspace composition, and proximity to office buildings can create distinct microclimates that differ from hardiness zone maps called urban heat-islands (Bede-Fazekas and Somodi 2023). As a result, actual minimum average temperatures that occur across the AOC fall within USDA Hardiness Zones 7, 8, and 9 in Washington D.C. These subtle environmental differences can complicate plant selection, irrigation, maintenance processes, and IPM programs for CGA staff.

Species Uniformity with Susceptible Plant Types

The AOC’s detailed Cultural Landscape Reports (CLR) honor Frederick Law Olmsted’s original landscape design and aim to preserve, as well as restore, the AOC greenspace to a period designated as historically significant. The CLRs serve as a primary reference for the types and locations of plant specimens on the U.S. Capitol Grounds. Select areas of the U.S. Capitol Grounds showcase formal horticultural displays, other locations have mass plantings of specific plant types, and yet still other, more public areas appear to have a less formal appearance across the AOC property. While restoring the cultural heritage of the national landscape, ornamental plant bed designs inconsequentially have become restrictive in species and diversity. To date, common boxwood [Buxus spp. (Buxales: Buxaceae)], azalea [Rhododendron spp. (Ericales: Ericaceae)], rose [Rosa spp. (Rosales: Rosaceae)], yew [Taxus spp. (Taxales: Taxaceae)], magnolia [Magnolia spp. (Magnoliales: Magnoliaceae)], dogwood [Cornus spp. (Cornales: Cornaceae)], oak [Quercus spp. (Fagales: Fagaceae)], and ornamental cherry [Prunus spp. (Rosales: Rosaceae)] dominate the landscape (Urban Canopy Works 2021). Most of these plant taxa are well known to have a suite of key pests that frequently reach damaging densities and require management in urban settings. The repetitive installation of these susceptible plant genera not only greatly enhances the likelihood of continual pest pressure but also magnifies the need for vigilant, ongoing pest management measures by the CGA.

Public Perception

The U.S. Capitol Grounds provides a symbolic representation of the national landscape of the Founding Fathers and current legislators of the United States. Visitors from all over the world come on any given day to tour the U.S. Capitol Building and its historical surroundings. Visitors’, AOC constituents’, and other relevant stakeholders’ opinions of how the Capitol Grounds should be managed greatly influence IPM-related programs, which can delay timely pest control efforts, or prevent the use of certain control tactics for managing major urban landscape pests. For example, contracted work for the application of dormant oil sprays to scale-infested trees in March 2023 unintentionally created an influx of complaints (and later resulted in additional restrictions on times permitted for oil sprays on the AOC grounds), whereby the contractors were accused of spraying “dangerous chemicals” because they were wearing personal protective clothing, such as Tyvek coveralls, rubber gloves, and particulate respirators. The public misperception led to the delay of further appropriately timed dormant oil sprays and heightened oversight of pest control applications. CGA staff have communicated their hesitancy in spraying for fear of negative public reaction.

Successful IPM Program Developments

The U.S. Capitol Grounds is home to a high quantity and a diverse array of woody plant species at varying stages of development. Between 45 and 175 young tree saplings and new woody shrubs are installed throughout the grounds each year. Our inventory also includes 42 of the oldest trees in Washington D.C., which are referred to as the “1894 Olmsted Originals.” These trees were installed at the inception of Frederick Law Olmsted’s Landscape Design for the U.S. Capitol in 1894. This diversity in age, species, habitat type, and public visibility of woody plants on the grounds, compounded by missing pest and abiotic survey data, imposes yet more complexity in the CGA’s PHC program development.

To address these current and emerging challenges, pest management on the U.S. Capitol Grounds can no longer rely mainly on conventional practices of routine chemical pesticide applications to solve all pest-related issues. The following section highlights 3 successful examples where the CGA has adopted IPM programs that incorporate holistic, ecological-based practices to effectively manage pest populations. Each example is paired with a particular landscape type to illustrate how these unique PHC challenges were overcome to positively impact AOC greenspaces as a whole.

Transition from Input-intensive to Ecological-based Turfgrass Management

Heavy foot traffic, compaction, constant disturbance from building construction projects and special event equipment use, extreme weather events like flash floods and summer droughts, overwatering, lack of operational standards, and the repetitive use of input-intensive and disease-susceptible cool-season fescue cultivars are major challenges that curtail optimal turfgrass growth and development on the AOC grounds (Figs. 4 and 5). In 2018, the CGA created a technical turfgrass position to formally develop and implement a turfgrass management program for the U.S. Capitol Grounds. Before then, the more than 75 acres of turfgrass lawns had been managed separately by individual crews and relied on conventional, pesticide-intensive turfgrass management practices.

Historically, tall fescue, [Schedonorus arundinaceus (syn. Festuca arundinacea) Schreb. (Poales: Poaceae)], a common perennial cool-season grass, has been the predominant turfgrass species for lawn establishment and long-term maintenance on the U.S. Capitol Grounds property. Fescue sod is easy to purchase in bulk commercially and provides an aesthetic quality that constituents and the public find appealing. To maintain a healthy, well-established fescue lawn, CGA turfgrass managers have traditionally relied on intensive irrigation, fertilization, and pesticide programs. Conventional calendar-based pesticide spray applications were essential to prevent severely damaging turfgrass diseases and insect pests from attacking the neatly manicured lawns. Major cool-season turfgrass diseases and insect pests included brown patch (Rhizoctonia solani Kuhn), pythium blight [Pythium aphanidermatum (Edson) Fitzp.]; gray leaf spot (Pyricularia oryzae Cavara), rusts (Puccinia spp.), fall armyworm [Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae)], black cutworm [Agrotis ipsilon (Hufnagel)], among others (Potter et al. 1996, Settle et al. 2001, Uddin et al. 2003, Cutulle et al. 2014, Karakkat et al. 2017, Althoff and Rice 2022, Khan and Joseph 2022).

At present, the CGA turfgrass system has taken on a more ecological approach to pest management. One main component of the turfgrass IPM management program is the use of improved turfgrass species for our area. For example, warm-season grasses have been introduced and adopted within the AOC as the maintenance of fescue lawns has become more challenging with the increase in insect pest and disease pressure, more erratic weather patterns, and a growing public aversion to pesticide use. The AOC landscape is situated within a turfgrass “transition zone,” whereby both cool-season and warm-season grass types can thrive and grow under specific microclimates. Until recently, other turfgrass species had not been evaluated for their performance, resiliency, or suitability on the U.S. Capitol Grounds. Since 2009, as the CGA initiated change in conventional turfgrass management practices, the CGA has been diversifying turfgrass species incorporating more improved, climate-resilient, warm-season zoysia grass, Zoysia spp. Willd. and advanced bermudagrass, Cynodon spp., cultivars like “Tifway” and “Tahoma 31” (Gopinath et al. 2021). These improved turfgrass types have successfully established in shadier portions of the lawns, rarely succumb to fungal disease or insect pest attacks, require few to no pesticide inputs within the growing season, quickly rebound after high-disturbance events, and have sustained healthy plant vigor under poor, compacted urban soils.

Establishment of Routine Pest Data Collection and Monitoring Methodologies

In the Urban Forestry Division, a main concern at the AOC is the potential safety hazards that dying or damaged trees pose to humans. Routine tree risk assessments evaluate the physical conditions of mature specimens and determine appropriate mitigation actions required to ensure human safety on the grounds. Included in these routine inspections are forestry insect pest, disease, and abiotic concerns affecting the health status of these living assets. With a property that houses more than 4,600 trees and 9,600 shrubs, the most common ongoing landscape pest issues are rodent damage, root rot, infestations of sap-feeding insects like mites and scales, and foliar disease outbreaks (2021 CGA Management Plan). The majority of CGA’s tree specimens are vulnerable to known forestry pests and diseases, yet the AOC lacks a standard IPM-based pest monitoring program to account for their presence or absence (Lovett et al. 2016). Only a few, select historically important specimens are proactively injected with pesticides to protect against the onset of Dutch elm disease, caused by the fungus Ophiostoma ulmi or from attack from the invasive emerald ash borer [Agrilus planipennis Fairmaire (Coleoptera: Buprestidae)] (de Andrade et al. 2021, Copeland et al. 2023). Less obvious, yet sometimes more pervasive, are abiotic issues, such as compaction, soil nutrient imbalances, and root girdling. Despite their persistence in the urban landscape, the impact of these nonliving factors on individual tree health has not been tracked, measured, or recorded previously at the AOC. Efforts are currently underway to accurately monitor the effects of these abiotic stressors on the AOC landscape.

Traditionally, pest-related information, if collected, was retained on paper forms that were filed away, easily lost, or disposed of during staffing transitional periods. Digital platforms have since been engaged to help improve recordkeeping within the jurisdiction. Since 2017, the AOC CGA Urban Forestry program records observations through ArcGIS urban forestry management system software called TreeKeeper (The Davey Tree Expert Co., Kent, OH). Every tree on the property is uniquely identified and georeferenced; any insect pest and disease information can then be easily captured and archived throughout the life history of the individual specimen (Fig. 6). Furthermore, to better account for PHC issues on the grounds, the CGA initiated an ArcGIS Survey123 (Environmental Systems Research Institute, Redlands, CA) survey tool in 2022 for logging biotic, abiotic, and other PHC concerns that CGA staff come across during their daily work activities. Through smartphones and other hand-held electronic devices, any AOC staff member can easily submit relevant plant pest information and images, which is made available to all end-users in real-time.

Congruent with the retirement of paper records, the CGA adopted IPM-based pest scouting methodologies that have since been incorporated into routine work maintenance schedules (Fig. 7). The successful management of the recent arrival of crape myrtle bark scale (CMBS) [Acanthococcus lagerstroemiae (Kuwana) (Hemiptera: Eriococcidae)] at the AOC best illustrates the benefits from these recent changes in PHC programming. In 2021, an AOC staff member reported white, fuzzy colonization with dark sooty mold growth along the undersides of branches of select crape myrtles [Lagerstroemia spp. (Myrtales: Lythraceae)] near the James Madison Memorial Building of the Library of Congress. The white colonies were later confirmed to be the exotic A. lagerstroemiae (Wang et al. 2016). Since its reported citing on the property, visual inspections, passive sticky card trapping, and double-sided tape monitoring methods have been carried out to track the seasonal population dynamics and dispersal of CMBS as well as natural enemy presence on a subset of the 408 crape myrtle trees on the U.S. Capitol Grounds (Vafaie and Knight 2017). The improved pest monitoring and recordkeeping guided the proper timing of controlled releases of biological control agents during the target immature crawler stage and timely dormant oil sprays only for crape myrtle trees with heavy CMBS infestations. The arrival of the CMBS initiated the first year-round monitoring of an insect pest on the property and provided initial baseline information as to where the insect was most problematic, which cultivars were prone to scale damage, and targeted times of the year when control interventions would be most appropriate (i.e., at peak crawler stage). The AOC even explored the use of high-pressure water sprays as a potential tactic for reducing CMBS populations in highly sensitive areas, like entryways to congressional office buildings and pesticide-prohibited areas such as daycare centers. Using TreeKeeper and Survey123 to survey sites for the presence or absence of scales, CMBS populations, and the effects of the pest control measures can now easily be evaluated in successive years.

With the adoption of digital platforms and trapping technologies for pest monitoring, the CGA is transitioning away from a traditional, reactionary pest control program and progressing toward an ecological-based holistic IPM systems approach to PHC programming. As in the case with CMBS control in the CGA’s Urban Forestry Division, improved recordkeeping and data collection allowed for the use biological controls in timely fashion that targeted the specific pest of concern before spreading further to other susceptible plant hosts across the AOC or harming other nontarget species. Common ornamental plants such as Spirea (Spiraea L.), American beautyberry (Callicarpa americana L.), and other rosaceous species have been reported as susceptible to CMBS feeding and have been included in pest survey efforts to determine whether any control is warranted (Xie et al. 2021).

Belowground Assessment and Remediation of Ornamental Horticulture Beds

The CGA’s Gardening Division manages all ornamental landscape beds, which, like the Urban Forestry program, are broadly diverse in composition, location, size, age, formal design, and habitat type. Equally challenging is their proper care and maintenance. Currently, the AOC CGA implements an informal PHC program for its woody shrubs and herbaceous planting material based on the American National Standards Institute’s arboricultural and horticultural BMPs.

Beginning with Olmsted’s landscape design, the main focal point on the Hill has always been (and continues to be) the U.S. Capitol. Eyes focus on the building’s unique architecture while little attention is given to belowground features. Belowground factors—like irrigation and soil quality—play a critical role in any successful PHC program. The CGA Plumbing Branch works with landscape developers, engineers, and contractors to address irrigation issues as they arise while also supporting gardening and arboreal staff in routine irrigation scheduling, maintenance, and access.

Interestingly, in 2022 alone, 95% of the symptomatic plants submitted for analysis at Land Grant University pest and disease diagnostic plant clinics were suffering from over-irrigation issues, which led to a reassessment of irrigation scheduling in 2023 with all relevant CGA staff. Occurrences of soil-borne plant pathogens that persist in wet soils, such as Phytophthora spp., Pythium spp., and Armillaria spp., were often detected in symptomatic or dying plants in areas where irrigation was excessive. Other secondary issues from over-irrigation are also pervasive across the property. Such issues include the absence of water-soluble nutrients in soil profiles, aggressive growth of water-loving weeds like yellow nutsedge [Cyperus esculentus (Poales: Cyperaceae)] and common chickweed [Stellaria media (Caryophyllales: Caryophyllaceae)] and premature plant death or leaf yellowing of water-sensitive species like yews and rhododendrons [Rhododendron spp. (Ericales: Ericaceae)].

Previous research emphasizes the importance of soil quality in reducing pest pressure and for maintaining healthy plants, yet routine soil testing has not been a common practice on the U.S. Capitol Grounds (Magdoff and Van Es 2021). Stressed plants in urban environments are often impacted by abiotic factors, such as soil nutrient imbalances, and can be misdiagnosed as pest or disease problems. Furthermore, the increased frequency of nutrient-related plant symptoms across the U.S. Capitol Grounds suggested a more detailed inventory of current and conventional plant fertility practices among contractors and internal staff operating across the property. The lack of a centralized framework of soil health information has resulted in the misapplication of fertilizers, improper dose applications of particular plant macronutrients—nitrogen (N), phosphorus (P), potassium (K)—and micronutrients—boron (B), manganese (Mn), iron (Fe), sulfur (S), magnesium (Mg), copper (Cu), and zinc (Zn), and differing fertility programs among contracting agencies and CGA staff.

Proper plant nutrition requires knowledge of the plant species present in the landscape. Equally important are the status of the soils in which the desired plants are currently growing. Beginning in 2018, soil samples submitted for nutrient, chemical, and textural analyses have been georeferenced and test results entered in a main database for CGA staff use and information (Fig. 8). CGA staff have been trained in proper soil sampling, have their own soil sample probes, and a standard protocol for soil sampling has been developed to further promote soil testing among CGA crew members. While this practice has only been incorporated into routine CGA maintenance programs in the past few years, sampling efforts have steadily increased each year (35 samples were taken from 2018 to 2020, followed by 26 in 2021, 69 in 2022, and 114 in 2023). More importantly, customized soil amendments based on soil test results are being implemented by staff to improve soil conditions and address plant nutrient imbalances. This added practice also extends to contracted agencies that import soils from outside sources; soils brought onto the property must meet standard soil specifications to maximize plant growth and development. Collectively, these incremental achievements in the PHC program on the U.S. Capitol Grounds successfully demonstrate how improved IPM strategies can meet constituency demand and AOC sustainability objectives while also effectively managing destructive landscape pests common in Washington D.C.

Future Directions and Opportunities

The recent developments in CGA leadership, administration, and staffing have created new, unexplored avenues for enhancing PHC programming on the U.S. Capitol Grounds. The previous paragraphs detailed examples where IPM principles have enhanced the care and maintenance of different landscape types at the AOC. These discrete modifications have contributed to a much larger strategic approach to PHC program development within the CGA. Furthermore, they revealed critical program areas in urban landscape management that are weak or poorly developed. Moving forward, the CGA envisions opportunities to strengthen 3 critical areas in landscape planning, partnership building, and educational outreach.

Engaging with Project Planners and Designers

For roughly 150 years, historical preservation has been a primary objective in the maintenance of the U.S. Capitol Grounds with project landscape designs and future plant selections based on Frederick Law Olmsted’s General Plan from the late 18th century. During the planning process, different teams of landscape architects and planning engineers refer to CLRs when making historical plant selections for upcoming landscape projects (Heritage Landscapes 2012, 2015a, 2015b, 2018a, 2018b, 2020). Often, AOC landscape plans are replanted or redesigned shortly after project completion because of the poor establishment of original plant designs. Several factors can play a role in plant failure; some characteristics of standard AOC landscape designs that contribute to their poor establishment are: (i) selection of outdated or susceptible plant species, (ii) disregard for microclimates in project areas, (iii) mismatched species for targeted project locations, and (iv) very dense plant spacings.

A recently completed AOC project contract illustrates these challenges. The AOC landscape design was compiled years in advance of the project contract, which led to outdated plant installations that were not suitable for their projected sites. Roughly 300 patented groundcover roses [Rosa × hybrida (Rosales: Rosaceae)] became heavily diseased (Fig. 9) or dead within a 2-year period due to the heightened presence of Rose rosette disease (RRD) on cultivated Rosa spp. within the Washington metropolitan area (Tipping 2000, Byrne 2013, Ong 2022). In less than 1 year post-installation, between 25 and 30% of planting material at the project site failed to establish.

To address the unexpected losses and prevent their repeated occurrence in the future, the CGA has been given more opportunities to be present at the project planning phases of AOC landscape restoration projects. This proactive PHC approach is a recent development; technical CGA staff members are gaining traction with the AOC’s project designing processes. More frequently, CGA representatives are actively involved in project planning phases, encouraged to provide plant selection recommendations, and permitted to provide inputs on contracted AOC landscape restoration projects. In 2023 alone, at least 40 advanced plant cultivars with disease-resistant traits replaced susceptible types in new perennial garden bed designs installed across the Capitol Grounds (Fig. 10, Supplementary Fig. 2) (K. Serock, personal communication).

The remodeling of the O’Neill House Office Building successfully reflects these positive changes. The O’Neill Building was remodeled in 2014; in 2017, the General Services Administration handed over building ownership to the AOC. In the remodel, the building’s exterior became home to a green roof, water-wise drip irrigation system, and native-forward plant palette contributing to its Leadership in Energy and Environmental Design (LEED) building certification. This presented an opportunity for the CGA to enhance the infrastructure and plant design to continue to support the LEED certification and promote environmental sustainability in urban areas.

Factors that impacted the garden design and decision-making in the O’Neill Building were large glass windows, which face both the east and west, and refract light that warmed the planting beds below. By measuring the soil and building temperature throughout a 1-year period, CGA staff were able to track the temperature changes that created a microclimate in the east and west ornamental plant beds. The west garden beds stayed consistently moist, while the east beds remained dry. Ultimately, the final design reflected these environmental differences and matched appropriate plants best suited for those climate conditions.

Building Collaborative Partnerships

Intra-agency improvements provide benefits to all program types, however, for an IPM-based PHC program on the U.S. Capitol Grounds to be successful, collaboration is key. Engaging partners at the local, district, and national level has not been a common practice within the AOC in the last 5 years. Highlighting a select few of the latest cooperative partnership developments below, the CGA aims to continue to encourage, maintain, and strengthen these formative relationships with partner organizations.

Without prior forest pest knowledge or survey records within the CGA, the current status of major forestry pests is poorly understood and complicates any urban tree PHC program development. Building relationships with local university plant disease and insect pest diagnostic clinics have been critical for properly identifying causal agents of symptomatic and dying plants. To better track the wood-boring insect populations on the Capitol campus, the CGA partnered with DC’s Department of Transportation Urban Forestry Division and the United States Forest Service, to set up Lindgren traps for monitoring wood borers on highly stressed young or mature trees on the property (Lindgren 1983). To help address the newly invasive bark scale on crape myrtles, the CGA has also partnered with university researchers to determine how best to monitor CMBS and reduce their populations using biological control methods. Through these collaborations, local and national foresters along with AOC staff gain more insight into forest pest abundance, diversity, and best management options within Washington D.C.

Numerous studies have shown the benefits of increased plant and habitat diversity on urban pest suppression and the abundance of beneficial insects (Bennett and Gratton 2012). Where possible, plant selections for garden beds on the Hill now factor in species or cultivars with tolerance to urban conditions, host plant resistance, native origin, and potential for conserving beneficial insects. To realize this plant diversity objective, the CGA has been expanding its working relationships with horticultural industries that can provide commercial-size quantities of these advanced, desired plant types.

Establishing Public Education and Outreach Programs

At least 1 million visitors travel through the CGA each year (Destination 2023) and, due to strict signage policies, few ever realize the CGA’s positive impact on the grounds through the implementation of ecological-based pest management practices. Fortunately, recent requests for CGA-scheduled tours have initiated windows of opportunity to share these practices with special interest groups in Urban Forestry, Public Gardens, and Horticulture. Growing trends in social media, like the AOC’s blog post, inspire readers to learn more about beneficial insects and biological control and even participate in on-site events (Fig. 11) (Capitol Grounds and Arboretum 2023).

The CGA’s latest success came with Congressional approval to erect signage to protect the nesting habitat of a commonly misidentified beneficial insect (Fig. 12). The informational sign creates public awareness of cicada killers as natural predators of urban forestry pests and provides correct insect identification to passersby to curtail further requests for the CGA to immediately remove the “swarming hornets” or “harmful wasps.” The CGA aims to continue to capitalize on these educational moments to heighten awareness of its PHC achievements and better inform relevant stakeholders of the AOC’s ecological-based principles in action.

Evidence-based decision-making will take time as the CGA initiates monitoring protocols to better track pest outbreaks and maintain detailed pest records over time. Experimental trials are ongoing as new biological-based products come on the market and are objectively compared to traditional pest management practices. Plant palettes with combinations of native species and advanced cultivars are currently being assessed to determine which plant types are the best adapted to urban-stressed conditions. Still in its infancy, the PHC program at the CGA continues to grow, expand, and build upon the framework of a holistic ecological-based approach to pest management while providing a welcoming greenspace for all to enjoy at Capitol Hill.

Acknowledgments

The authors would like to thank the internal reviewers and legal counsel at the AOC for their time, commitment, and assistance with developing the manuscript. Special thanks to Michael Naas for his contributions on turfgrass management, to Melissa Westbrook for her contributions on urban forestry, Katie Serock for her contributions on ornamental horticulture, and James (Jim) Kaufmann for his informative contributions on the mission, history, and direction of the CGA.

Author contributions

Emily Mueller (Conceptualization [Lead], Project administration [Lead], Resources [Equal], Supervision [Lead], Visualization [Equal], Writing – original draft [Lead], Writing – review & editing [Lead]), Michael Naas (Investigation [Equal], Methodology [Equal], Resources [Equal], Validation [Equal], Writing – review & editing [Equal]), Katie Serock (Investigation [Equal], Methodology [Equal], Resources [Equal], Validation [Equal], Writing – review & editing [Equal]), Melissa Westbrook (Methodology [Equal], Resources [Equal], Validation [Equal]), and James Kaufmann (Conceptualization [Equal], Project administration [Equal], Supervision [Equal], Validation [Equal], Writing – original draft [Equal])

References