At restored Broad Branch stream, we must control invasive species so natives can thrive

by Katrina Weinig

Consider this: Approximately 42% of threatened or endangered birds, animals or plants in the U.S. today are at risk because non-native species have invaded their habitat. Thus, any environmental restoration project that is to be successful in the long term must begin with the removal and control of invasive species.

One important feature of the restoration work at the Broad Branch Stream site will be the removal of invasive species. There are many invasive plants growing in the woods surrounding the stream, including some you might have in your own garden: Norway maples, burning bush euonymus, bush honeysuckle, English ivy, bamboo, barberry, double file viburnum, Japanese maple, Japanese privet, loosestrife, Japanese (vine) honeysuckle and porcelainberry, to name just a few.

What are invasive species? “Invasive species” are defined by the USDA as species that are non-native (i.e., alien or exotic) to a particular ecosystem, and whose introduction causes harm to the environment. Invasive species can be plants, animals, or other organisms, although in this case we’re concerned with invasive plants.

Where do invasive species come from? Invasive species are usually introduced into a habitat by humans, either intentionally or by accident. Most of the invasive plant species in Rock Creek Park and around Broad Branch Stream have spread from residential gardens in the surrounding neighborhoods! The plants listed above have been popular landscaping plants for decades. Even today many landscapers are unaware of the invasive qualities of these plants, and continue to plant them in local gardens – even when there are many equally beautiful and more beneficial native alternatives available.

Why do these garden plants become invasive? Exotic species can become serious invasive pests. One reason for their success as pests is that their new habitat lacks the natural controls that would normally occur in their native range – for example, herbivores (mammals or birds) or insects that would normally feed on them and thereby control their growth and spread.

So what? What’s so bad about invasive plants? Invasive alien plants threaten native species and habitats by competing for critical resources like sunlight, water, nutrients, soil and space. They succeed through vigorous growth and prolific reproductive capabilities. Some spread through bird and other wildlife droppings; some seeds are wind dispersed; others spread through vigorous and deep root systems. Invasive plant species displace native plant communities, impede forest regeneration and natural succession, change soil chemistry, and even cause genetic changes in native plant relatives through hybridization. In short, invasive plant species can be devastating to our native forests, meadows, and wetlands – and to the birds, butterflies, frogs, and other animals that live there and depend upon them for food.

What can I do? Remove invasive species from your garden, and replant with species native to the mid-Atlantic. There are many good resources available to get you started! Some of these are listed below. And contact Rock Creek Conservancy to volunteer to remove invasives from Broad Branch, Rock Creek Park and other sensitive local areas.

Broad Branch Stream site case study: Norway maple. Norway maple, Acer platanoides, is a large tree, usually growing 40 to 60 feet tall, and one that many people would consider attractive. However, it is an invasive, exotic tree that was introduced into the U.S. from Europe in 1756. Since then, it has spread aggressively throughout forests in the northeast, mid-Atlantic, and into parts of the Pacific northwest.

The impact of Norway maple’s aggressive spread into our forests is problematic for two distinct, but related, reasons. First, as it spreads aggressively into forests it shades out native understory vegetation, such as spring ephemerals (including some of our most beloved wildflowers such as Virginia bluebells, trillium, lady slippers, and may apples), and it out-competes native tree species in the forest canopy, taking advantage of available sunlight, nutrients and water.

Marty Frye, an arborist with Casey Trees who’s working at the Broad Branch site, says it can reduce native species diversity and literally change the structure of forest habitats.

“If you walk into sections of forest with mature Norway maples, you’ll notice that little else is growing there – few understory shrubs, and few native trees such as the oaks, hickories, tulip poplars, beeches or native maples that you’ll find elsewhere in Rock Creek,” Frye says.

Understory shrubs and wildflowers provide important food and habitat for beneficial insects, songbirds, small mammals and amphibians. Once they’ve crowded out the natives, these invasive trees provide little of these “ecosystem services” themselves.

As Frye explains, “The increasing presence of invasive trees in our forests, at the expense of our native trees, has a cascading impact on insect and animal species as well.”

Controlling the Norway maple involves “girdling” them to kill them, after which they can be removed and the area replanted with similarly-sized native trees that provide valuable food and habitat for wildlife – trees such as tulip poplar, oaks, hickories and native maples. At Broad Branch, Casey will be planting over 100 native trees this coming November, both to replace the Norway maples and to add biodiversity to the overall site.

Resources: Invasive Species

National Invasive Species Information Center, USDA: invasivespeciesinfo.gov
Center for Invasive Species and Ecosystem Health: invasive.org
Mid-Atlantic Invasive Plant Council: maipc.org
Maryland Invasive Species Council: mdinvasivesp.org

Native Species:

Native Plant Database, Ladybird Johnson Wildflower Center & University of Texas: wildflower.org/plants
Maryland Native Plant Society: mdflora.org
Virginia Native Plant Society: vnps.org

Rock Creek Conservancy Invasive Species Removal Sign-up:
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