Purple Loosestrife (Lythrum salicaria)
Introduction and spread in North America
Adapted from USGS website.
Few people in the continental United States can say that they've never seen Purple Loosestrife. The bright purple flowers of this invasive species bloom profusely during the summer, and the plants form great stands across wetland areas. Purple Loosestrife was first introduced into the United States in the early 19th century, and has been enjoying great prosperity ever since. Apparently this plant was introduced in several different ways. Some were intentional such as the introduction for herbal (medicinal) uses, as a food source for bees (pollen) and as an ornamental garden plant.
Powders were made from dried leaves and concoctions from the roots. These were used to treat chronic diarrhea and dysentery and various other diseases including “blood spitting”. Green or dry leaves were also used to heal wounds, ulcers, and sores.
Beekeepers planted purple loosestrife as a source of nectar and pollen in the 1940’s and 50’s.
The profuse and attractive floral displays of purple loosestrife have made the plant well known to gardeners throughout the world. Its use in Europe probably extends back into the Middle Ages. In the western United States, at least two national plant companies still offer Purple Loosestrife as potted plants.
Other introductions were accidental, including being transported in ship's ballast and in imported wool.
In the19th century sailing vessels carried lumber, cotton, and tobacco from North America to Europe. On the trip back, these vessels required cargo or ballast (more often both) to have the necessary stability to be sailed efficiently. The material used for ballast was gravel, rock, rubble, or even moist sand from exposed tidal flats. The moist sand would have offered an opportunity for alien plant seeds to find their way to North American shores. On arrival at their New World destination, passengers, livestock, and commercial cargo were discharged first. Then the ballast along with any “stowaway” seeds was hoisted and dumped.
American demand for raw wool began with the rapid growth of woolen mills in New England following the War of 1812. The large importation of flocks of sheep and raw wool brought seeds, which readily adhered to the wool. In the processing and cleaning of the wool all the impurities including seeds were simply discarded.
There are several reasons that Purple Loosestrife has been such a successful invasive species. One is the ability for massive seed production, with a single plant often producing over one million seeds! The resulting seedlings will grow in a dense mat in the area surrounding the parent plant, crowding out neighboring species in the process. Also, Purple Loosestrife resprouts easily when cut, and can reproduce vegetatively (a small broken off piece of root can start a whole new plant). These factors, as well as the lack of natural enemies that might browse on the foliage or destroy roots, make Loosestrife a formidable (strong or fearsome) opponent in wetland ecosystems.
What does all this mean for the ecosystems that Purple Loosestrife invades? Diverse native plant communities have been replaced by large populations of a single species, resulting in a loss of biodiversity and threatening rare plants. Wildlife that depend on these native plant communities are now also at risk. Purple Loosestrife also fills in areas of open water, threatening the habitats of waterfowl.
Purple Loosestrife is just another example of a non-native plant species whose invasive potential has only recently been realized, even though the species was introduced over a century ago. There are experiments currently underway to test the ability of a beetle from Purple Loosestrife native habitat to control the plant here in the United States. For now, we can all do our part by refusing to buy and plant Purple Loosestrife in our gardens. Recent evidence has indicated that garden varieties of Loosestrife can hybridize with the wild Purple Loosestrife and therefore contribute further to the problems caused by this species. We can also make sure that if we are hiking through wetland habitats, that our bodies and our clothing do not help spread the seed of this species to new locations.
Think about the methods of introduction (both intentional and accidental) mentioned above. Choose the method that you think may have been the most responsible for introducing Purple Loosestrife to North America. Give several reasons to support your hypothesis.