Fifty-nine species of plant have been introduced into the Great Lakes region since the 1800s, yet only two have had substantial impacts: purple loosestrife and water millfoil.
Purple Loosestrife (Lythrum salicaria)
Purple loosestrife is a wetland plant from Eurasia. It was introduced to the East coast of North America in the 1800s. It first spread along roads and canals and was later distributed as an ornamental plant. Loosestrife readily impacts native plants by out-competing them, forming dense, impenetrable stands, that are unsuitable for cover or nesting sites. It invades marshes and lakes, replacing cattails and other wetland plants. In North America about 190,000 hectares of wetlands are lost every year with an estimated cost of 45 million dollars.
Part of the success of purple loosestrife is due to the enormous number of seeds it produces. One plant can distribute 2 million seeds annually! It is also able to resprout from roots and broken stems that fall to the ground.
There are few predators of purple loosestrife in North America. Several European insects that only attack this species are now being tested for their potential as biological control agents.
Eurasian Water Milfoil (Myriophyllum spicatum)
Water milfoil is a plant with long, flexible stems with feather-like leaves attached in whorls of four. Water milfoil grows fast, quickly choking out native plants and harming fish habitat. It grows in thick mats, interfering with boating, fishing, and swimming.
It was accidentally introduced from Europe, probably through aquarium release or ballast water release. It is spread mainly by boats and some waterbirds. The success of milfoil is due to its ability to reproduce through stem fragmentation. A single segment of stem and leaves can take root and form a new colony. The fragments cling to boats and can start new colonies when they move from one lake to another. Mechanical clearing of weed beds also creates thousands of new colonies.
Poison Hemlock (Conium maculatum)
This poisonous plant was used medicinally as a powerful narcotic. It was introduced to North America from Europe in the early 1800's. Poison hemlock is now common in on stream banks, and other damp areas throughout the Great Lakes drainage area.