By Ellen Oordt
Do you want to start managing your landscape to support and increase native plant diversity? Whether it is the woods behind your house or a cultivated garden, there is a common obstacle to the success of these communities: competition of non-native, invasive plants.
Not all non-native plants qualify as invasive, though. An invasive plant is a non-native plant that spreads aggressively, causing harm to the environment, economy, and/or human health. Invasive plants threaten native plant diversity by establishing in gardens and natural areas, outcompeting native plants for resources that they need to grow. They crowd out native plants during crucial times in the growing cycle, making it difficult for native plant populations to bounce back.
To support native diversity, first learn to identify, manage, and prevent the spread of invasive plant species.
Learning what is invasive in your landscape
The first step in tackling invasive plants is learning how to identify them. Fortunately, you do not need to be an expert botanist to get started. Instead, look for clues that may indicate which plants are invaders. These clues will help you identify what plants in your landscape may be invasive, so that you can further investigate and identify them to determine if they are indeed invasive.
Clue 1: Is this plant spreading excessively or creating a monoculture?
A wall of Phragmites takes over this stormwater basin. Photo courtesy of Longwood Gardens, photographer: Joseph Thomas
Invasive species often dominate the areas where they grow. Take a closer look at the landscape you’re managing. Do you see the same plant growing in dense patches with little else in between? Or perhaps it’s a plant that seems to pop up in every corner of your landscape? These are signs that this plant is potentially an invasive species. For example, common reed (Phragmites australis) often grows in dense stands on roadside ditches, wetlands, and storm water basins. Japanese stiltgrass (Microstegium vimineum) can blanket edges of trails and roads, woodlands, and floodplains. Both species create monocultures. These dense monocultures prevent the natural diversity of the ecosystem by not leaving enough resources for native seeds to germinate, seedlings to grow, or mature plants to flourish.
Clue 2: Does it look like this species is harming the health of native plants?
This invasive bittersweet vine is girdling a native black cherry. Photo courtesy of Longwood Gardens, photographer: Colin McCallum-Cook.
Monocultures aren’t the only way that invasive plants may harm the health of your native plant populations. Sometimes, the damage is more visible. Do you see plants smothering native vegetation, growing over the top and blocking sunlight? Or vines wrapping around the trunks and branches of trees, encroaching into the canopy? These signs may point to an invasive plant. For example, bittersweet vine (Celastrus orbiculatus), will wrap itself tightly around a tree trunk. As the tree grows, the vine can girdle and eventually kill the tree by cutting off its ability to send resources to its roots. This is especially harmful for young trees and poses a serious threat to natural regeneration of native trees or the success of tree planting projects.
Clue 3: Is the plant out of the ordinary?
A little too eye catching
As you get to know the landscape you are stewarding, you may start to sense when something looks out of the ordinary. For example, you may look at a spontaneously growing plant and notice the plant might look “a little too eye-catching.” This could indicate that it is an escaped ornamental. Ornamental plants are cultivated to have features such as beautiful flowers, variegated foliage, or specific forms that might not be seen in a natural landscape. While not all cultivated plants are invasive, some do aggressively grow beyond the areas where they were planted. For example, Chinese silver grass (Miscanthus sinensis) has dramatic, feathery panicles that made it a popular ornamental grass in garden environments but now has become invasive in natural areas. Even variegated cultivars of Chinese silver grass have escaped cultivation and stand out even more conspicuously in native landscapes.
The dramatic flowers of Chinese silver grass make it stand out among the native vegetation of this meadow. Photo courtesy of Longwood Gardens, photographer: Candie Ward.
Visibility out of the regular growing season?
Certain times of year make invasive species, specifically woody invasives, easier to spot. In the late fall, early winter, and early spring, many native species have already gone dormant, while certain invasive plants stay green and stick out against the bare landscapes. Pictured are the brilliant green leaves of black jetbead (Rhodotypos scandens) in early winter when everything else has gone dormant for the season. Similarly, Norway maples (Acer platanoides) tend to turn colors later than native maples, and invasive shrubs like privets (Ligustrum spp.) and honeysuckles (Lonicera spp.) often stay green well into fall.
Use these clues, but don’t rely on them alone
Remember that these are only clues, not strict rules. They’re meant to help you know what to look for when learning about invasive plants. Keep in mind, some native plants also fit one or more of these criteria. For example, tall goldenrod (Solidago altissima), a common native throughout the United States, is known to dominate meadows and can even give the appearance of a monoculture. Roundleaf greenbrier (Smilex rotundifolia) creates dense thickets that may grow over native trees and shrubs, restricting growth and regeneration, yet these thickets provide valuable habitat for native birds and small rodents.
If you spot a plant that seems suspicious based on these clues, utilize reliable resources to confirm its identity, such as field guides, picture-based identification apps (such as iNaturalist), or your state’s extension website. The invasive species used as examples in this article are invasive to the Mid-Atlantic region of the United States, but invasive plants vary by region. Researching your state’s invasive species list is one of the best ways to become more familiar with what to look for.
Getting to know your invasives
Once you have an idea of which invasive species are in your landscape, it is time to get to know them. Knowing the characteristics of each invasive species can help you evaluate when and how to manage different species.
Get to know… the phenology
Phenology is the cyclic timing of a plant’s lifecycle—when leaves emerge, flowers bloom, and seeds set. Understanding a plant’s phenology can help you time your management efforts effectively.
Many invasive plants are easiest to find when flowering. Others are recognizable because they leaf out earlier, such as bittersweet vine (Celastrus orbiculatus), or stay green later, such as jetbead (Rhodotypos scandens), than surrounding vegetation. Finally, knowing when a plant sets seeds gives you a timeline for managing a species. Aim to act before seeds mature to mitigate further spread of these invasive plants into your landscape.
Get to know… the duration
A plant’s duration is its life span: annual, biennial, or perennial. Knowing the duration of a species can guide your management efforts.
Annual plants senesce soon after they go to seed. Once they senesce, the individual plant dies, leaving only seeds behind. For example, Japanese stiltgrass is an annual, so if it has gone to seed for the year, your efforts would be better spent managing another species until the next year.
Biennial plants, such as Poison hemlock, are typically recommended to be managed in the first year of its lifecycle when it is a basal rosette. If you miss that first-year window, you can still prevent seeds by removing the flowers and remembering to target the basal rosettes the next year.
Perennial plants return year after year from the same root system. They should be managed even after they go to seed because they don’t die after senescing, they just go dormant and will return the following year.
Get to know… Reproduction strategies
Understanding the reproduction strategies can help determine the best methods for managing an invasive plant to prevent further spread. Plants can spread through seed production and dispersal (sexual reproduction) and through vegetative spread (asexual reproduction). Many invasive plants do both, contributing to their aggressive growth. Knowing those strategies is important for determining the best management methods and preventing further spread.
Invasive golden bamboo (Phyllostachys aurea) spreads through a network of aggressive rhizomes. To control this species, you must target the entire rhizome network through extensive and frequent cutting, excavation, and herbicide treatment. Tree of heaven (Ailanthus altissima) is another species with an extensive root system, and mature female trees produce hundreds of thousands seeds per year. If cut, tree of heaven will send up dozens of suckers that you must be prepared to continually manage. In both cases, understanding how a species reproduces is essential to developing a sustainable control strategy.
Next steps…
Invasive species management is not a one and done task. It requires patience, persistence, and adaptability. Understanding invasive plant populations in your landscape is the first step to managing them. As you learn about the specific invasive species, learn about different methods to mitigate them and determine what works best for your landscape. Create a management calendar that aligns with the plant’s life cycle and your own seasonal schedule. This is an ongoing process, where progress may be slow and repetitive, but be persistent, and remember that your efforts are vital for restoring native plant diversity across the landscape and thus supporting all other forms of life.
To take this one step further, learn which native plants are currently in your landscape and assess whether key species can be added. From there, learn how to encourage native plants to grow and when you need to plant to add missing species to your landscape.
There are countless resources and communities to help you through this process, so stay committed and adapt your approach as you continue to learn more.
Ellen Oordt is a land stewardship and ecology technician at Longwood Gardens. The Land Stewardship and Ecology team is responsible for over 750 acres of natural lands that surround the formal gardens, which include meadows, forests, streams, and wetlands. Ellen’s primary duty involves leading the many invasive species management projects that are needed to promote native biodiversity in these ecosystems. She holds a B.S. in ecology and evolution from the University of Pittsburgh.