During the settlement of North America, many exotic plants were introduced into the environment either by design or by accident. Some of these exotic species have become invasive in Minnesota, due in part to a lack of natural predators that control the growth of these plants in their native lands. Due to the large number of lakes and the countless miles of lake shore land, the most invasive exotic plants found in Minnesota are primarily aquatic.
Purple loosestrife (Lythrum salicaria) is a native of Europe and was introduced into North America in the early 19th century. It has since become invasive over much of the United States, and is a threat to the many wetland habitats throughout the state of Minnesota. It is a deep-rooted perennial that can produce up to 50 sprouts per mature plant, which then form a dense mat, crowding out other vegetation. Purple loosestrife grows predominantly in wetlands but is increasingly seen in uplands and meadows, causing growing concern for agricultural fields.
Eurasion watermilfoil (Myriophyllum spicatum L.) was introduced into North America sometime between the late 19th and mid 20th centuries. Native to Europe and Asia, it is an aquatic plant that rapidly grows to form a dense canopy on the surface of the water. Eurasian watermilfoil can reproduce from small fragments of roots and spreads rapidly. The mats that it forms on the water's surface can impede boat navigation and cause dense shade, which can kill off native vegetation.
Curly-leaf pondweed (Potamogeton crispus) is native to Europe, Africa, Asia and Australia and was introduced into the United States in the 19th century. Curly-leaf pondweed is tolerant of low light and can grow in turbid water or even beneath an ice-covered surface. Its rhizomes dig into the sediment on the lake bottom and allow it to grow in running streams or in lakes with strong waves. Curly-leaf pondweed goes dormant during summer and begins to sprout from dormant stem buds lying on the lake bottom during winter. Rapid spring growth shades out native plants and produces flowers in May. The parent plants die off in late June, leaving stems with their dormant buds on the lake bottom until winter, when they re-sprout.