Introduction and Spread of Purple Loosestrife in North America
Via the Erie Canal to the Interstates
Purple loosestrife (Lythrum salicaria) is an erect, non-woody perennial that came to North America from Eurasia in the early 1800s. Seeds of the plant came here in several ways -- in the ballast of ships, in the wool of sheep, and as an herb. Many times soil (which contained plant seeds) dug near shipyards in Europe was used as ballast. Once the vessel reached its destination in North America, the ballast was dumped overboard near shore, resulting in an opportunity for weed colonization. Seeds that adhered to the wool of large flocks of sheep brought in from Europe for the woolen mills were another source of purple loosestrife. Finally, the plants were brought over by well-intentioned immigrants for their herb gardens and by beekeepers because the flowers are an exellent source of nectar.
By the 1830s, purple loosestrife was so common along the east coast that, in the first edition ofA Flora of North America, the authors mistakenly thought it was "probably native". By the late 1800's, loosestrife had spread throughout the northeastern United States and southeastern Canada, and its exotic origin was recognized. The weed needed two factors to expand its range -- disturbance in native wetland communities and a method of dispersal. In 1817 the construction of the middle section of the Erie Canal began, which provided both the disturbance and a dispersal method. By 1840, the spread of loosestrife was closely related to canal traffic moving inland from the northeast shipping areas.
Two public works programs during the 1940s broke down barriers that had prevented the spread of looseestrife into western states. Under the Federal Reclamation Act of 1902, acreage under irrigation significantly increased. Off-water swales and seepage slopes along ditches were the perfect habitat for loosestrife to gain a foothold. Also, construction of the network of interstates led to later increased spread. Specification for interstates called for well-drained crowns, creating major disturbances. Superhighways cut through previous blocking barriers (such as mountain ranges) and offered opportunities for weeds to colonize new areas.
"Stunning in Leaf and Flower, and Grows Just About Everywhere"
Characteristics that make the plant a formidable invader-- hardiness, tolerance of many moisture and nutrient regimes, and virtual freedom from insect pests and disease -- also have made this plant an attractive garden perennial. Southern Living magazine stated, "Loosestrife is a perennial every garden needs. It's stunning in leaf and flower, and grows just about everywhere."
Purple loosestrife is a broad-leafed perennial that can range in height up to 8 feet. The leaves are long and narrow and are usually opposite, but may also be in whorls of three and four. Its angular stems become woody with age and persist through the winter. The plant has a spike of six-petaled purple flowers. The plants bloom from July to September and seed set begins by mid-July. Seeds are shed throughout the winter. Each seed capsule averages 90 seeds, each approximately the size of ground pepper. With a 1,000 capsules per stem and 30 stems per plant, a single plant can produce over 2 million seeds! The lightweight seeds can be transported by the wind, but they are usually dispersed by flowing water or by adhering to the fur, feathers, or feet of animals and birds.
Although the spread of loosestrife is usually by seed dispersal, vegetative reproduction also occurs in areas of disturbance. Once the crown has reached maximum size, lower stem and root pieces can produce adventitious roots and begin to move about. Mature plants can tolerate a broad range of water levels, soil types, and climate, making this a plant that does indeed "grow just about anywhere".
Information on this page maintained by David Voegtlin.