Invasive Plants in Urban Stream Restorations

Globally, invasive plants are a threat to biodiversity and cost over $190 billion per year to manage once they become established (Fantle-Lepczyk et al. 2022). Understanding why invasive plants are disrupting ecosystems and how to identify common invaders are important first steps to minimizing their negative impacts. Invasive plants have negative effects on many native species, especially birds and insects (Fletcher et al. 2019; Schirmel et al. 2016). In stream systems, invasive plants can decrease the density and survival of macroinvertebrates (Custer et al. 2017; McNeish et al. 2015) and alter water quality or quantity (Fargen et al. 2015; Galster and Vanderklein 2023; Robertson and Coll 2019).

The main goals of this publication are to (1) explain the susceptibility of urban streams, especially restored streams, to invasive plants; (2) describe how to identify common invasive plants of the riparian zones of Mid- Atlantic streams; and (3) raise public awareness of why invasive plants can be problematic for stream ecosystems.

Boldface terms are defined in the Glossary.

Why Are Streams Susceptible to Invasive Plants?

Streams connect wide-ranging areas of a landscape as small streams flow into larger streams that then flow into rivers. Plants surrounding streams can drop seeds and other plant material into the water, which can carry the propagules to other locations downstream. Streams also experience frequent disturbances, such as flooding, which can draw propagules into the water and deposit them downstream. Seeds of invasive plants are often readily available in human-disturbed areas and can easily be dispersed. Invasive plants can have competitive advantages over native plants, including a lack of pests, broader environmental tolerances, a longer growing season, and abundant seed production.

Streams are already more susceptible to invasion than other parts of a landscape (Schirmel et al. 2016), but stream restoration projects can potentially create even more favorable conditions for invasion. When restoring a stream, existing vegetation is often removed or disturbed, and large trees may be removed (fig. 1). Disturbances to the soil and increases in light and space availability due to restoration construction can provide the ideal conditions for invasive plants to establish. Additionally, excess nutrients from urban and agricultural runoff or from fertilizer added to the soil during restoration can favor invasive plant establishment over native plants. Given their competitive advantages over native plants, invasive plants may also outcompete the native species planted on stream restoration projects, leaving them without resources, such as light, space, or soil nutrients, to establish.

What Are Common Invaders of Mid-Atlantic Streams?

Many species of invasive plants are present in the Mid- Atlantic; however, some are better at invading riparian areas than others (fig. 2). The more common invaders of Mid-Atlantic streams include Japanese stiltgrass, multiflora rose, Japanese honeysuckle, and porcelainberry. Oriental bittersweet, ground ivy, common mugwort, English ivy, reed canary grass, wineberry, Japanese barberry, and privets (e.g., border privet, Chinese privet) are also commonly observed along Mid-Atlantic streams (table 1). There are many other invasive plants that have become established in these watersheds; for more information on invaders and how to identify them, see “Plant Invaders of Mid-Atlantic Natural Areas” (Additional Resources).

How Do Invasive Plants Impact Streams?

Invasive plants can have various negative impacts on stream systems. Leaves, branches, and entire trees that fall into streams serve as important food sources for aquatic organisms. These organisms are adapted to consuming native plants; aquatic organisms may not be able to acquire sufficient nutrition from invasive plants due to differences in when leaves are dropped and how degradable the plant material is (Robertson and Coll 2019). The differences between native and invasive plants impact many different species — from aquatic invertebrates to fish — through the food web, i.e., the connections between species based on what species they eat and/or are eaten by. Invasive plants may have different root systems from native species, which can affect streambank erosion. For example, Japanese knotweed, a known invader of Mid-Atlantic streams, may cause streambank erosion due to a loss of native root structure holding soil in place because knotweed has shallower roots (Colleran et al. 2020). Japanese knotweed has also been shown to transpire more water through its leaves than native vegetation, thus reducing total water flow in the streams and rivers it invades (Galster and Vanderklein 2023).

Plants provide food or shelter for many native animals that live near streams, including insects, birds, reptiles, amphibians, and mammals. In many cases, insects have coevolved with specific native plant species and when invasive plants outcompete these native plant species, insects can experience population declines (Tallamy et al. 2021). The detrimental effects of invasive plants on insects can be felt by species higher up on the food chain, such as birds. For example, research has shown that Carolina chickadees have fewer offspring when there are more non-native plants (Narango et al. 2018). In this way, invasive plants can impact not only the stream itself, but they can also create cascading impacts for other species that rely on stream ecosystems. Thus, care should be taken to minimize the introduction and establishment of invasive plant propagules and to cultivate appropriate native riparian vegetation.

How To Identify Common Invaders

Correct identification is critical to effective invasive plant management, and several tools are available to aid identification of invasive plants, including several apps (e.g., iNaturalist, Seek). We also recommend the Virginia Tech Weed Identification tool (see Additional Resources).

Below we highlight several of the most common invasive plants in the Mid-Atlantic region.

Japanese Stiltgrass

Japanese stiltgrass (Microstegium vimineum) is an annual invasive grass known to dominate forest understories in much of the Eastern United States by displacing native plants. Japanese stiltgrass leaves are approximately 3 inches long and often have a silvery midrib. Leaves are arranged alternately along the stem. Japanese stiltgrass does not have an extensive root system and usually can be pulled out of the ground easily. Many individual Japanese stiltgrass plants are often found growing together, forming a dense carpet along the ground. Japanese stiltgrass seeds are known to disperse in forests through flooding (Tekiela and Barney 2013).

Multiflora Rose

Multiflora rose (Rosa multiflora) is a vining shrub that climbs on surrounding vegetation and forms dense thickets. Multiflora rose has an alternate leaf arrangement with groups of five to 11 leaflets. At the base of the leaf, multiflora rose has fringed stipules that differentiate it from native rose species, which have smooth stipules.

Sharp prickles are present on the light green or reddish stems. Flowers are usually white and have five petals that are somewhat heart-shaped with yellow stamens in the middle. Fruits of multiflora rose are red and are often present on stems from summer through winter.

Japanese Honeysuckle

Japanese honeysuckle (Lonicera japonica) is a climbing vine that can strangle native vegetation. Leaves are oppositely arranged on brownish stems and often oval-shaped, though some younger leaves may be lobed. Japanese honeysuckle flowers are tubular; they are initially white or cream and turn yellow, which differentiates them from native honeysuckle, which has red flowers. Fruits are present in the fall and turn from green to black when ripe.

Porcelain-Berry

Porcelain-berry (Ampelopsis brevipedunculata) is a member of the grape family with climbing vines. Leaves are alternately arranged, shiny green on the upper surface, and usually have three to five lobes. Young stems can be green or brown with lenticels and become woodier with age. Flowers are green and often hard to notice; they are typically present from June through August. Porcelain- berry fruits in September and October with bright and noticeable drupes, beginning as light blue before changing to green and ultimately dark blue. Porcelain- berry vines can grow in thick tangles, overtaking existing vegetation and ultimately killing it.

Additional Resources

Swearingen, Jil M., and Judith P. Fulton. 2022. Plant Invaders of Mid-Atlantic Natural Areas, Field Guide. Passiflora Press.

U.S. Department of Agriculture. Forest Service. “Invasive Plants.” https://www.fs.usda.gov/wildflowers/invasives/index.shtml.

For additional resources to aid identification, use the Virginia Tech Weed Identification tool available at https://weedid.cals.vt.edu.

Glossary

Alternate, alternately – A leaf arrangement where there is one leaf on one side of the stem at a node and one leaf on the opposite side of the stem at the next node, alternating sides of the stem.

Coevolved – When two or more organisms evolve together and are influenced by each other.

Disturbance – A change in an ecosystem process that can cause lasting changes to the ecosystem with varying severity, frequency, and seasonality (e.g., flood, fire, drought).

Drupes – Fruit with soft outer flesh and a hard seed in the middle (e.g., cherry).

Environmental tolerance – The range of environmental conditions that a species is fit to inhabit (e.g., temperature ranges, flood tolerance, shade tolerance).

Food web – Relationships among different organisms based on what they eat and what they are eaten by.

Invasive – A species that causes harm to an environment and often becomes overpopulated. Invasive species can be non-native or native to an ecosystem, although in this document the invasive species mentioned are all non- native.

Leaflets – the individual leaf-like parts that make up a complete leaf.

Lenticels – Appear as dots or raised bumps on woody stems.

Lobe, lobed – Projections from a leaf that are not split all the way to the midrib.

Midrib – The middle vein on a leaf; may not always be in the exact middle of the leaf.

Node – Point on a stem from which leaves grow.

Opposite, oppositely – A leaf arrangement where leaves on either side of the stem emerge from the same node.

Prickles – A sharp, stiff projection emerging from the surface of a plant stem.

Propagules – Plant material (e.g., bud, seed) that can grow into a new plant.

Resources – What a plant needs to establish and grow; includes sunlight, soil nutrients, and space.

Riparian – Adjacent to rivers or streams.

Runoff – Water and other substances it carries that drains over the land and into waterways.

Stamens – Male organ of a flower; often contains pollen and stalks stick up from the middle of the flower.

Stipules – A pair of leaflike projections where the leaf stalk meets the stem.

Watershed – The area of land that drains to a particular point along a stream or river.

Acknowledgements

Funding is provided by the Chesapeake Bay Trust grant #20594.

References