Sometimes it is an uninformed, but deliberate act when an invasive plant takes hold in an area and other times it is by pure accident. The result is the same. When conditions are right, a plant that normally behaves in its home ecosystem often reproduces with a vigor that crowds out natives.
Phragmites australis is an Eurasian native known as the common reed and is an introduced species in many areas around the world. It is an aggressive marsh or wetlands plant that reproduces so quickly that many other plants, including our native reeds have no chance of survival. These alien reeds weren't consciously transported and planted in their new homes. Chances are the seeds, or chunks of the fleshy roots were part of a ship's ballast hundreds of years ago and when this was dumped in rivers and harbors in favor of trade goods, they germinated and flourished. These invasives now stretch over vast distances, choking harbors, lakes and rivers. It has been a number of years since I was in northern New Jersey, just outside of Elizabeth and Newark where the marshlands along the Meadowlands were choked with reeds. I know that many of these areas are protected and important for the wildlife, but the reeds have turned the area into a sea of grass. It would be impossible at this point to remove them without permanently harming the local ecosystem, so they are allowed to remain. Hardy throughout most of North America, it seems at home just about anywhere there is enough moisture except for USDA zone 3 and colder.
While these alien invaders do harm to the environment in the wrong setting, reeds were very important to the survival of mankind. We thatched our roofs, built boats, wove baskets and made paper, pipes, writing implements and flutes from a number of reed species. The plant even makes a nice, light green natural dye. The stems are strong, pliable, rot-resistant and in the right environment essential for some people's existence, so it isn't a bad plant but it sometimes grows in the wrong places.
Most reed species are edible as well. The starchy roots were often prepared as a type of bread or dumpling. The young leaves were cooked or steamed, much like we would use spinach today and the stems contain a milky sap that is slightly sweet. The seeds are high in fiber and contain a number of nutrients, but it takes quite a bit of effort to thresh and clean them. The young shoots, harvested before leaves appear and were used in a similar fashion as bamboo shoots and are reported to be quite tasty.
Modern science is working with reeds as a non-food plant for bio-mass in ethanol production and reports are quite encouraging. They reproduce quickly with an amazing amount of usable material that makes reeds attractive as a substitute for corn in this fuel production. Experiments are also underway testing reed fiber as a natural stuffing for upholstery instead of the petrochemical based fibers that are so popular now.
If you have a reed population nearby, you can easily tell the difference between the invasive and a native reed. Phragmites australis has bluish-green leaves and grows into an almost impenetrable curtain. Native reeds tend to be more yellow-green with reddish stems and very seldom compete to the point that other plants in their area are choked from existence.
There are still commercial sources offering the common reed, even though it has been designated an invasive or toxic weed, so please remember that even if something is for sale, that doesn't mean it is lawful to plant. Please research your plant and seed purchases wisely so as not to contribute to the environmental problems we already have.