Invasive species – "Phragmites"

(Published December 2003)

Exotic species are a global problem. Before humankind could easily transport people and goods around the world, ecosystems evolved in physical isolation from one another. If new and different species are suddenly introduced, either purposely or inadvertently, the results are seldom predictable. The “exotic” organism can be out-competed by the native species and die out, it can co-exist with the native ecosystem, or it can out-compete the natives and become invasive. Chesapeake Bay is (unfortunately) now home to many exotic species, which include microbes such as MSX (a protozoan fatal to oysters), plants (Tree-of-heaven, Japanese honeysuckle, Kudzu vine, Chinese privet, Alligator weed, Canada thistle, and many more), and animals (the Rapa whelk, Nutria, Mute Swan, and many more). Some organisms were introduced with the best of intentions (Grass Carp), and their desirability is argued. Other species were introduced without forethought (most ornamental plants), and still others like MSX were “hitchhikers,” which apparently arrived along with the introduced (and failed) Pacific oyster.

Seeds from Phragmites australis, the common reed, are known from archeological sites in North America that are thousands of years old, so some people claim it is not an exotic species at all. Recent research, however, suggests that there are at least two types of Phragmites, one native and the other exotic, and invasive. The exotic form was probably introduced from Europe. Phragmites is found in both freshwater and brackish wetlands. It is the largest of the wetland plants, and can grow up to about 12 feet tall. It is easily identified by its bamboo-like stalk and long-lasting, feathery, plume-like purple-brown seed-head. Unlike Big Cordgrass, with which small specimens of Phragmites can be confused, the edges of the wide leaves are smooth and not “saw-toothed.” Phragmites grows and spreads rapidly by both seeds and rhizomes (runners), or root-like stems that can extend beneath or at the sediment surface up to 30 feet from the main stem. Because it colonizes rapidly, Phragmites is especially characteristic of disturbed sites, such as ditches and dredge spoil.

The leaves of Phragmites were used by Native Americans to make mats and its stems were used to make cigarettes. Its leaves are still used today in Europe to thatch roofs. It is also one of the most useful plants in constructed wetlands designed for tertiary sewage treatment. Not only do the rapidly growing plants remove some of the nutrients nitrate and phosphate from the effluent water, but the anoxic (oxygen-free) environment associated with the roots and rhizomes promotes microbial de-nitrification, or the conversion of nitrate into harmless nitrogen gas.

Should Phragmites be controlled? This question must be answered for each specific situation. Most scientists agree that control is desirable when the plant becomes so abundant that it displaces other species and becomes a “mono-culture.” The plant has little value as food for wildlife, and the rapidly growing leaves and stems form a dense thatch which covers the marsh surface and prevents more desirable plant species from becoming established. The strong stems and tall plants form a nearly permanent “under-story” which impedes the feeding and nesting of waterfowl and wading birds, and the nesting of some declining bird species such as Henslow's Sparrow, the Sedge Wren, and the Salt Marsh Sharp-tailed Sparrow. On the other hand, Red-winged blackbirds, common Yellow-throats, and Marsh wrens often nest in Phragmites stands, and Black-crowned night-herons are known to nest in it when there is nothing else available. If the Phragmites stand is adjacent to the water, the dense rhizome mat can help resist erosion.

If individuals decided to try to control Phragmites, they must realize there is no simple one-step procedure. It is likely that treatment will need to be repeated, and that trial-and-error and patience will be required. The most effective method