By Ellen Oordt, land steward, Longwood Gardens
As a land steward with an understanding of invasives, how can you manage these species on a large scale? For those new to invasive management, check out Where to Start? Understanding Your Invasives from October’s edition of Into the Weeds!
Invasive plant management, or the removal of non-native plants that cause harm to an ecosystem, can seem like a never-ending task that expands in scope as property size increases. Professional land stewards also face capacity constraints, such as time, money, and labor, which can impact success. To be effective in landscape-level efforts, it is important to have a strong invasive species management plan that is feasible, adaptable, and aligned with broader stewardship goals for natural areas.
At Longwood Gardens, I work as a land steward on the Land Stewardship and Ecology (LSE) team. Longwood Gardens is one of the world’s great gardens, welcoming 1.7 million guests annually and encompassing 1,100 acres of gardens, woodlands, meadows, and fountains. The LSE team manages 750 acres of natural areas that surround the formal gardens for the purpose of driving science-led stewardship to conserve biodiversity, support resilient ecosystems, protect watershed health, and connect people with the beauty of native landscapes (Figure 1). Using the experience of Land Stewardship at Longwood as a case study, we’ve learned how to develop effective and efficient invasive species management plans at any scale through: 1) assessing the landscape; 2) prioritizing efforts using the invasion curve; 3) tracking management efforts for adaptive planning; and 4) becoming proactive in management.
Assessing the landscape
Effective land stewardship begins with an assessment of what ecosystems and plant communities make up the landscape. Understanding forest age, canopy conditions, degree of fragmentation and connectivity, and land-use history can help determine the quality of ecosystems. This information provides a foundational understanding of the landscape that will allow prioritization and management goals to be applied consistently.
Longwood Gardens is set in a suburban setting, approximately an hour west of Philadelphia. This area of the country has one of the highest recorded numbers of established non-native species in the United States. While not all non-native species are invasive, or will become invasive, ongoing introductions and naturalization increase the risk of a species becoming invasive and establishing in the area, especially as the climate shifts. The land where Longwood sits was deforested and used as farmland throughout the 18th and 19th centuries before being converted to formal gardens with natural areas surrounding it. This land history indicates that these natural areas are situated within a highly disturbed and fragmented landscape with a long history of invasion. As a result of these factors, there is a higher risk of reinvasion after management is conducted, highlighting the need for consistent and ongoing population management. It can take several years to control an already established plant population, so understanding the history helps land stewards set realistic expectations and develop effective long-term management strategies.
Figure 1: The plant communities map used by Longwood’s LSE team as a framework for planning. Each polygon represents a unique plant community with data attributes containing information on community structure and species abundance.
To form an effective management plan, a system to organize and document information provides a strong foundation for all current and future action. Many land stewards divide their properties into management units based on habitat type, land use history, geographic features, or stewardship values. These units provide a framework for organizing information, planning work, and communicating priorities. Knowing what plant communities are present and how they function helps land stewards use their time and resources more effectively. It allows them to set realistic goals, choose the right actions, and track progress over time instead of reacting to problems as they arise.
At Longwood, we implemented a vegetation study in 2021 to identify plant communities and assess their structure and species composition across the landscape (Figure 1). We use this Plant Community Study as the structure for our management planning. This project ties quantitative data on community structure and species abundance to a map of our landscape which equips us to apply science-driven frameworks that guide realistic decision making.
Prioritizing efforts using the invasion curve
A widely used framework for helping land stewards understand which management outcomes are realistic is the invasion curve (Figure 2). The invasion curve illustrates that the longer an invasive species has been established in the landscape, the more widespread it becomes and the more resources are required to control it. A land steward following the framework of the invasion curve will prioritize fully eradicating newly introduced species with small populations before they spread. They do not expect to eradicate a common and widespread invasive plants from the landscape, but rather aim to suppress its population, especially if the species is likely to be reintroduced from nearby areas. A good management plan will identify which species fall into the various categories of the invasion curve, particularly the eradication category, so that continuous scouting and removal of those populations can occur.
Figure 2. The Invasion Curve, Adapted from Invasive Plants and Animals Policy Framework/State of Victoria, Department of Primary Industries, 2010, Public Domain, https://www.fws.gov/apps/media/invasion-curve
Because of the high invasive pressure in Southeastern Pennsylvania, there are species across the full spectrum of the invasion curve within the natural areas of Longwood Gardens. For example, incised fumewort (Cordyalis incisa) falls into the eradication category. While small populations may be found throughout the broader landscape, it is not considered widespread and there is only one known population at Longwood. This species is actively scouted for and fully removed with the prospect that it may be eradicated from the property.
At the other end of the spectrum, garlic mustard (Alliaria petiolata) is a common and widespread species across the region. It is found in almost all woodland and edge habitats in our natural areas. While putting resources toward controlling a species like garlic mustard can seem futile, the LSE team is strategic as to where and how much management should take place. This is done through prioritizing management of garlic mustard populations in very high-quality forests with the aim of reducing the population and giving native plants some advantage over these invasive plants. Additionally, since it is easily recognizable and easy to remove, we often engage in volunteer corporate groups to help control the spread.
Eradication of invasive species is closely linked to prevention by enabling early detection, maintaining up-to-date plant species lists for the property and engaging with local networks to stay informed about new invasive species that may appear. We have an active scouting program led by trained volunteers that monitors species already present on the property, as well as species not currently present but likely to be introduced due to their occurrence in the region.
Tracking Management Efforts for Adaptive Planning
Adaptability is a major component of the planning process. An adaptive management approach addresses uncertainty in decision making by integrating data collection, analysis, and review into management actions. Building dynamic datasets on invasive species management can shape plans, guide follow-up actions, and improve practices.
Figure 3: An output of the management tracking tool for a project where we utilized large groups of volunteers to hand pull garlic mustard (Alliaria petiolata). Each of the shapes represents a session of management in a year.
We built a management tracking tool to document which invasive species were managed where, using what methods, and —as a proxy for effort— how many hours of management were required. We use GIS tools in the field to record this data at the time of management, ensuring accuracy and timely reporting. By reviewing this regularly, we can monitor outcomes of our management strategies and adapt plans as needed. Tracking effort also allows the team to assess the labor and time required for various projects and tasks. As these resources are a limiting factor for completing these types of projects, incorporating that data into planning can help land stewards ensure that they are utilizing such resources strategically.
While there are often guiding principles for how to control many invasive plant species based on their biology and mode of reproduction, optimal management often requires different methods for different species. These methods can also be very site specific depending on growing conditions, competing species, and property location. Widespread invasive species, those further along the invasion curve, typically have well-documented best management practices. In contrast, invasive species that are less common or newly identified as invasive often have limited published guidance. As stewards experiment with various management methods, having a robust way to document methods, outcomes processes, and lessons learned become even more important. For example, research trials evaluating the effects of management methods for Japanese stiltgrass (Microstegium vimineum) on native plant communities have informed stewardship practices. Results from these trials supported the use of a preemergent herbicide treatment to control Japanese stiltgrass in select areas within the natural lands, which is tracked using our invasive species management tool. This integration allows management strategies to be evaluated and refined over time, ensuring that research findings directly support stewardship practices.
Becoming Proactive in Management
Understanding property-wide stewardship goals and incorporating them into an invasive species management plan creates a holistic approach that is both more effective and more efficient. An effective invasive species management plan is dynamic, but not reactionary. When invasive control is a primary component of land stewardship goals, it can become easy for land stewards to be distracted by noticing new patches emerging as the seasons progress. Most land stewards have a plethora of stewardship goals outside of controlling invasive species—many of which are interconnected—which requires coordinated long-term restoration planning. These goals, often focused on habitat improvement, can include reforestation, stream restoration, meadow management, and more.
A proactive approach to invasive management, up to two years ahead of any restoration, reforestation, or reintroduction project, improves native species establishment and limits the amount of management needed to be conducted after planting. Focusing efforts on asset-based protection (higher-quality ecosystems) allows management to provide the greatest ecological benefit while conserving resources.
Effective invasive plant management at scale requires moving beyond reactive control toward intentional, long-term stewardship. While it can feel overwhelming at the beginning of any landscape-scale plan, taking step-by-step, coordinated action can improve outcomes, as well as team morale. The experience at Longwood Gardens demonstrates that invasive plant management is most successful when it is proactive and data informed. While no single approach will eliminate invasive species entirely, combining assessment, prioritization, documentation, and prevention allows land stewards to reduce invasive pressure, support native plant communities, and build more resilient landscapes over time.