Today I flipped through a reference book called A Handbook of Global Freshwater Invasive Species (always a cheerful read). My eye was caught by purple loosestrife (Lythrum salicaria L.), since I have often encountered it on wetlands hikes around the country.
Purple loosestrife is native to Eurasia, but has become an aggressive invader in North America, where it chokes out some local species. According to my Handbook, the plant with pretty purple flowers has also arrived in Australia and New Zealand.
The plant is perennial (meaning individuals last more than one season) and typically grows to a height of 5 feet (1.5 meters), although it has been known to reach 10 feet (3 meters). Thirty to 50 stems often emerge from a single root, so to say the thing is weedy is an understatement.
According to the Handbook, the first scientific mention of purple loosestrife in North America was in 1814, when it was described in the harbors of New York, Philadelphia, Boston, and New Bedford. Based on that, scientists estimate that the plant first arrived at the end of the 18th century or beginning of the 19th century. Experts theorize that the plant arrived in the form of seeds stuck to sheep wool or carried by ship ballast.
Purple loosestrife then spread along the Erie Canal and as roads were built. By the 1850s, people began to appreciate the plant for landscaping, and it may have been purposely planted in the Great Lakes and farther west, according to the Handbook. Purple loosestrife has long been used medicinally as an astringent and as a bug repellent too.
In the 1930s, the plant began aggressively spreading across North America. Some scientists think it was spurred by the extensive public works programs of the New Deal. Highway construction may also have played a role by helping disperse seeds. Bee keepers are also known to have purposely spread the plant, since the flowers are favored by bees.
The plant usually needs a disturbance to take hold in a wetland, but once it gets established it often takes over, crowding out natives. To combat this invader, managers often employ a mix of hand pulling and chemical and biological control (beetles and weevils). While there often can be success on local stands, overall the plant remains quite ubiquitous and aggressive.
As a result, purple loosestrife is listed as a “noxious weed” in at least 32 states, where it is illegal to sell or plant it. Many ecologists have suggested that the plant hurts native plants and wildlife, although a review of the scientific literature in the Handbook found that actual evidence of ecological harm is inconclusive. More studies are needed.
Is purple loosestrife in your community?
Brian Clark Howard covers the environment for National Geographic. He previously served as an editor for TheDailyGreen.com and E/The Environmental Magazine, and has written for Popular Science, TheAtlantic.com, FastCompany.com, PopularMechanics.com, Yahoo!, MSN, and elsewhere. He is the co-author of six books, including Geothermal HVAC, Green Lighting, Build Your Own Small Wind Power System, and Rock Your Ugly Christmas Sweater.