The Invasion of Super-Weeds
Some plants arrive in areas where they have never been seen before, take a liking to their new lodgings, and smother out native species. It’s a bit like that distant cousin who arrives unannounced and settles in. Such super-weeds as kudzu, purple loosestrife, and water hyacinth head the list of super-villains.
Vigorous Growth Habits of Kudzu
The British Columbia Ministry of Agriculture and Lands doesn’t have much that’s good to say about kudzu, a vine that’s native to eastern Asia: “Kudzu is a very aggressive plant and can out-compete or eliminate native plant species thereby upsetting the natural diversity of plant and animal communities.”
The perennial plant can grow up to 30 metres (60 feet) in a year “smothering and shading plants and trees from light.”
According to the U.S. National Park Service, “Kudzu was introduced into the U.S. in 1876 at the Philadelphia Centennial Exposition, where it was promoted as a forage crop and an ornamental plant.”
According to sciencedaily.com “kudzu infests 20,000 to 30,000 square kilometres of land in the United States and costs around $500 million annually in lost cropland and control costs.” It consumes about 61,000 additional hectares each year, that amounts to an area covered by 50,000 baseball fields.
The government of Canada notes that “It grows rapidly and forms dense, ropey mats over other vegetation and structures. The plants produce massive tuberous roots, making them difficult to control or eradicate. Kudzu reduces biodiversity and causes significant productivity losses to the forestry industry.”
Kudzu on the Move
The botanical name is Pueraria lobata but it has gathered a number of unflattering titles as it has relentlessly progressed across the U.S. southeastern states – “foot-a-night-vine,” “mile-a-minute-vine,” “cancer of the vegetative world,” and “the plant that ate the south” are some of the milder, family-friendly ones.
The climbing and coiling plant loves a hot, humid climate. The vine has been seen in Pennsylvania and the Ohio Valley, and that is outside its normal range. In the summer of 2009, it showed up in the southwestern corner of Ontario, Canada. Kudzu doesn’t like frost much so there’s something to be said for living in a cold climate. However, global heating might change that.
It’s such an attractive plant that it’s hard to believe that water hyacinth (Eichhornia crassipes) is filled with evil intent, but it’s choking Lake Victoria in Africa.
It’s native to South America and was introduced to Africa sometime around 1879. By 1989, it had found its way into Lake Victoria and it took just seven years to clog 80 percent of Uganda’s shoreline. In its homelands, the plant is kept under control by insects that eat it, but those predators did not make the trip to Africa.
Water hyacinth plants double in area every six to 18 days, forming thick mats. France24 notes that “The plant blocks out light, dramatically reducing the amount of oxygen in the lake, and killing off fish. It also makes access to the open lake increasingly difficult for fishermen.”
But, there may be an upside. The foliage contains an unusually high level of nitrogen and carbon, making it a good candidate for being turned into a biofuel. And, its prolific growing habit means a generous supply of source material is always available.
Another attractive plant with purple flowers, a purple loosestrife plant (Lythrum salicaria) can grow up to 30 stems, producing as many as 2.7 million seeds a year.
It arrived in North America from Europe early in the 19th century, probably as seeds in soil used as ship’s ballast, or embedded in the hooves of imported cattle.
It’s fond of wetlands such as river flood planes, damp roadside ditches, and marshes, and quickly out-competes native species. Purple loosestrife is rugged; it can survive drought, changes in pH, and has adapted to the colder Canadian climate. The rascal spreads its seeds by wind, bird poop, stuck to the boots of humans or vehicle treads, and even carried around by turtles.
Purple loosestrife knows better than most plants how to colonize a region. When it sets up residence it does a lot of damage the environment that most of us may not be aware of:
- It has a negative effect on frog spawn, therefore, fewer frogs;
- It increases the nutrient salts in water that leads to blooms of algae causing fish die-offs;
- By smothering native plants, it removes food, shelter, and nesting places for local wildlife;
- The changed environment has knock-on effects throughout the natural food chain; and,
- The plant clogs waterways, affecting fishing, hunting, and boating.
Eradication is difficult. It involves pulling up plants, introducing predator insects, and spraying with herbicides. But, the New Hampshire Department of Environmental Services warns that “None of these methods will completely eliminate purple loosestrife, but they will control the populations within ecologically acceptable limits.”
Invasive Plants in Our Gardens
Some nurseries still sell purple loosestrife as an ornamental garden plant. They should know better. However, there are many other plants we stick in our herbaceous borders that are questionable:
- Japanese honeysuckle has such a sweet fragrance but if it escapes the gardener’s control it can smother other plants and even break tree limbs;
- Tansy is a herb with claimed medicinal properties but it can cause skin irritations, and, as with so many invasive species, it chokes local plants;
- Privet hedges are a common feature of urban landscapes, but, if it escapes from captivity, it out-competes native species and reduces bio-diversity;
- Gardeners plant English ivy because it throttles weeds; it does so because of its energetic growth habits. Too energetic if the gardener is not vigilant in controlling its spread; and,
- Vinca: See English ivy above.
The list goes on and on without a mention of the bane of most gardeners―the accused dandelion.
- During the Great Depression, people in Franklin D. Roosevelt’s Civilian Conservation Corps were engaged in planting kudzu as a way of controlling soil erosion. This stopped in the mid-1950s when it was realized just how destructively invasive the plant is.
- Pablo Escobar was a Colombian drug lord who was killed in 1993. He had a huge ranch that he populated with imported exotic animals, among them four hippopotamuses. When the government of Colombia seized the ranch it placed the animals in zoos, except for the hippos, which had made a successful break for freedom. There are now about 40 of the animals and they are destroying the natural habitat of the Magdalena River.
- “This Invasive Plant Is Swallowing the U.S. at the Rate of 50,000 Baseball Fields per Year.” Michael Graham Richard, treehugger.com., July 18, 2014.
- “Kudzu - Pueraria Montana.” Government of Canada, May 14, 2019.
- “Water Hyacinth Re-Invades Lake Victoria.” NASA Earth Observatory, undated.
- “Could Lake Victoria’s Water Hyacinth Invasion Have a Silver Lining?” Wassim Cornet, France24, November 28, 2019.
- “Purple Loosestrife.” Nature Conservancy of Canada, undated.
- “Purple Loosestrife.” New Hampshire Department of Environmental Services, 2019.
- “16 Invasive Plants to Avoid.” David Beaulieu, thespruce.com, July 5, 2019.
© 2019 Rupert Taylor