While the dunes on the Cape are characterized by a variety of plant species, including beach grass, bayberry, and seaside rose, there is a plant invader that is increasingly seen. Spotted knapweed (Centaurea biebersteinii), one of the most invasive plants in the western part of the United States, is now commonly seen here. Not only is this plant growing along the sides of roads and disturbed areas, but it can be found competing with the essential beach grass on dunes.
Spotted knapweed is a member of the composite family, the Asteraceae. Related to dandelions and daisies, it has many similar characteristics of these familiar plants. Introduced from Europe in the 1800ís, it now covers millions of acres in western states such as Montana and Idaho. It is included on the Massachusetts prohibited plant list which means it may not be sold or deliberately distributed in the state.
The plant can be easily identified. The flower head is pinkish-purple, and looks a little like a thistle. The bracts have a blackish tinge to them. The plant grows one to four feet high with alternate, compound leaves. It also has a two to four foot taproot which allows it to grow in dry, sandy habitats.
Like so many weedy species, spotted knapweed produces thousands of seeds that are easily distributed by the wind. This biennial plant can quickly colonize an area and spread its population. Very few herbivores like to eat this plant, although there are a couple of seed head fly species that have been used in biological controls.
What makes this plant so difficult to control as an invader is its ability to release a toxin, called catechin, into the soil. Catechin is a very effective substance that prevents other plants from growing by reducing their ability to absorb nutrients. Spotted knapweed does not re-absorb the chemical so it able to grow without any interference. Combined with its strong taproot, ability to grow in different conditions, and its enormous seed production, it is a very formidable plant invader.
As is true with most invasive species, spotted knapweed is an environmental problem. A reduction in plant diversity also reduces the diversity of animals which depend on plants for food and shelter. In a place like a sand dune, spotted knapweed is less effective than beach grass for controlling erosion. This could have an impact on many issues from endangered bird habitat to coastal property protection.
The use of herbicides create their own set of environmental problems, and have had limited success. Mechanically removing the entire plant prior to seed production is effective, but quite labor intensive. One must time it correctly as well. The seeds can remain viable in the soil for many years.
It is probably impossible to completely control this invader. However, selective and targeted areas could be managed, particularly where it may be competing with more desirable or even threatened plant species.
Copyright Gil Newton 2009 Thanks to Chris Dumas for logo image.