(Editor's Note: This article was originally published on March 3, 2011. Your comments are welcome, but please be aware that authors of previously published articles may not be able to promptly respond to new questions or comments.)
There are several unrelated plants that have borne the common name purple loosestrife. This article will focus on the plant that bears the Latin name Lythrum salicaria, and to an extent its cousin, Lythrum virgatum. At various times, it has also been called "spiked loosestrife." The name is derived from "lytron," the Greek word for blood. The leaves bear a certain similarity to the willow; hence the species name "salicaria." It is the common name, loosestrife, that intrigues me, however. The earliest explanation I could find was a reference to an ancient text by Pliny, in which he attributed the plant with the ability to dissolve strife, and bring harmony. Ancient farmers would even hang garlands of the herb around the neck of stubborn oxen, to encourage them to plough in harmony.
A more recent name, coined after its introduction into the North American wetlands, is the "Purple Plague." The reference to the deadly and speedy spread of the plague, and its devastating impact, needs little explanation. In areas where purple loosestrife becomes established in North American wetlands, isolated from its natural predators and controlling factors, it can quickly form vast monospecific stands, excluding all other vegetation. In its natural habitat, in Eurasia, it would comprise only 1 to 4% of the vegetative cover.1 It is all a matter of context.
It is believed that purple loosestrife was introduced to North America in the early 1800s, as colonists began arriving from Europe in greater numbers. Various theories abound, primarily blaming the rocks and sand used as ballast to steady the sailing ships as the source for the seed. The ballast was usually dumped near the coast when the ship arrived, before it returned to its own country. It is also likely that some colonists intentionally brought seeds with them, to establish medicinal herb gardens in the new world. In times past, the astringent and antiseptic properties of the plant were used both internally, to treat diarrhea, dysentary, and ulcers, and externally for bleeding and sores. It is mentioned in Dr Lindley's Flora Medica (1838) as a treatment for cholera.2 There is no question that purple loosestrife is a beautiful plant. There are gardeners in the United States and elsewhere, and even here at Dave's Garden, that would argue that the threat has been exaggerated, and insist that their own use of purple loosestrife poses no danger to the native wildlife. It is still available from a few isolated nurseries as an ornamental plant, despite the fact that it is listed as a noxious or invasive weed in 33 of the 48 contiguous United States. Canada faces similar levels of infestation, and is likewise taking steps to limit its spread. Purple loosestrife in its various cultivars can range from 18 inches to nearly 10 feet tall, and as much as 5 feet wide. It sports lovely spikes of magenta-purple blooms, each sporting 5 to 6 petals, and those blooms persist from mid-summer to late autumn. The leaves, which are smooth, occasionally sporting fine hairs, grow in opposing pairs along a square stem. Certain variations have 6-sided stems. It is a perennial, which only compounds the difficulty of eradication. Though some cultivars have been marketed as sterile or self-fertile only, this is only true in areas in which there is no chance of cross-pollination with wild varieties. Cross-pollinated plants can set viable seed, though it will not come "true" to the hybrid that was planted.3 The seeds can then spread by wind, water, or mechanical means to nearby wetlands. Even though you see no evidence of purple loosestrife acting as a bully in your own garden, you can't be sure that it isn't having a detrimental effect elsewhere!
At first, the extent of the problem wasn't recognized. However, purple loosestrife thrives in a wide variety of conditions and soil types, ranging from zones 3a to 9b, and has spread relentlessly westward, throughout much of the United States and Canada. The United States Department of Agriculture has mapped the spread of the plant, and makes this map available to the public:
It is found in every contiguous state except Florida, Louisiana, and Arizona, though it is not considered noxious or invasive in all of the marked states. "An estimated 190,000 hectares of wetlands, marshes, pastures and riparian meadows are affected in North America each year, with an economic impact of millions of dollars."4 It is most dangerous to those areas with extensive wetlands. The problem is not limited to North America, however. It has also been listed as invasive in Australia, Ethiopia, Georgia (the nation), New Zealand, Saint Pierre and Miquelon, and South Africa.
Once purple loosestrife becomes established in an area, it quickly forms a dense root mass that chokes out any competing vegetation. It does not spread by root runners, as some invasive plants do. It has a long tap root, which makes it difficult to manually pull. Instead, it spreads by means of plentiful and unusually viable seeds. A single mature plant can produce up to 2.7 million seeds annually, with an almost 100% germination rate. The seeds maintain 80% germination rates even after three years. Burning the plants does little to reduce germination or prevent return of the plant from the deep roots. The seeds travel along the water currents, and can be transported from one location to another in the mud or dirt stuck to animals, human foot traffic, or vehicles that pass through an infested area.
The plants displace native vegetation, such as bulrushes and cattails, which support wildlife, both as a food source and as shelter. "Waterfowl, fur-bearing animals, and birds vacate wetland habitat when they lose their food source, nesting material, and ground cover due to native vegetation loss and replacement. . . .Recreational hunting or trapping grounds are lost, decreasing the land value to those that own or manage operational wetlands."5 As the stands become deeper and thicker, they decrease waterflow and quality, and disrupt irrigation systems necessary for nearby agricultural uses. Economic impacts are high in agricultural communities when irrigation systems are clogged or when wetland pastures are lost to grazing. Purple loosestrife is responsible for the destruction of wetland pastureland, as most grazing animals will not eat the plants, perhaps due to their high tannin content; even songbirds will not eat the seeds. In the Pacific Northwest, several wetland-dependent cultivated crops are also affected, including the cranberry bogs where much of the nation's cranberry crops are grown. It also impacts wild rice beds and riparian meadows.
There are no natural predators, either herbivores or pathogens, for purple loosestrife in North America, or in many of the other nations in which the spread of purple loosestrife has become problematic. It is with great caution that scientists began exploring the possibility of introducing a few select insect predators, starting in 1986. Very controlled experiments were conducted first, to insure that the insects would consume only purple loosestrife, and not cause even greater devastation to the already endangered wetland plants. Beginning in 2000, these insect controls have been incorporated into closely monitored biocontrol programs in the Pacific Northwest. Two beetles, Galerucella pusilla and Galerucella calmariensis, feed on buds, leaves, and stems while in the larval stage, and on newly formed buds and leaves as adults. Two weevils have also been introduced, Hylobius transversovittatus and Nanaphyes marmoratus. The former is a root mining weevil, which can completely destroy a mature plant, and the latter consumes the flowers, preventing the formation of seedheads. The advantage of using the insect bio-control is that the beetle or weevil population will survive as long as there is an available food source. If the insects eliminate one stand of purple loosestrife, they will either move to another nearby colony of the plant, or will die out. Research has shown that they will not move on to other secondary food sources when purple loosestrife is no longer available.
Images of Biocontrol in Action at the
Neponset River Watershed Project,
Release of Galerucella beetles
Close up of Galerucella beetle
on Purple Loosestrife Galerucella beetles swarm over
the leaves and buds of a
purple loosestrife plant Larval and adult Galerucella beetles
on purple loosestrife
Of course, introduction of biocontrols isn't practical in every situation. If you have only a mild infestation, perhaps 100 plants or less, you can still eradicate the plant by diligently hand-pulling the plants. Because the plant has an extensive tap root, you must be sure to remove the entire root. If broken off at the crown, the plant will just send up new shoots, perhaps even more numerous than the original stems. Complicating the matter is the fact that it can be propagated by cuttings. All stems and plant matter must be bagged and removed, as broken stems or branches left on the damp ground may form new roots. For this reason, bulldozing or mowing a stand of purple loosestrife is generally not effective. Ideally, you should pull the plants early in the flowering season, before they have set seed, to avoid spreading the seeds more widely. If you must remove plants after they have set seed, place bags over the seed heads, bend them over, and cut them off, before attempting to remove the plant and root.
In limited situations, chemical controls have been found to be effective against purple loosestrife, though this must be attempted with great caution to minimize the impact to the very native plants and wetlands you are trying to preserve. In certain situations, Glyphosate can provide good control of purple loosestrife when applied from July to early September. Glypro Rodeo, a trade name of glyphosate, can be used in water. Triclopyr will provide good to excellent purple loosestrife control when applied during the pre-to early-flower to lateflower growth stage. Garlon 3A, triclopyr, is labeled for use in wetland sites. Contact your local county extension agent for recommended use rates, locations, and timing. In some states, you must request permission from the Department of Ecology or a similar governing body before using any herbicides in or near water, so do your research before any herbicide application. I would urge great caution in pursuing this method of control! Whatever method of control you use, you must be prepared to continue over the course of several years, as none of these methods of control will eliminate the seeds that are already in the soil, and are likely to germinate.
If you determine that you have an area that is becoming overrun with purple loosestrife, your first course of action should be to research what methods of control are already being used successfully in your area. There are many governmental and independent, community-sponsored organizations committed to stopping the spread of purple loosestrife. Working in concert with such an organization may yield better results than attempting to tackle the problem alone.
For more information and to locate an organization dedicated to the elimination of PL in your area, consult:
I would like to extend my special thanks to Carly Rocklen, Outreach Director and Restoration Manager of the Neponset River Watershed Association (NepRWA) for very graciously granting her permission to use some of her photographs to illustrate this article. I sincerely enjoyed my correspondence with her, and exploring her website to learn about her work to restore the Neponset River Watershed to its native vegetation and wildlife. If you would like to learn more about how a diverse community in Massachusetts is coming together to battle the invasion of Purple Loosestrife in their Neponset River Watershed wetlands, visit:
You can also view a photostream that Ms. Rocklen posted on Flickr, illustrating the ongoing project at the Neponset Watershed, here:
There are many concise, authoritative sources of information on the internet regarding purple loosestrife. For more information, consult:
Thumbnail at beginning of artice: photo made free for use in Wikimedia Commons, by H. Zell
Field of purple loosestrife at the Neponset Watershed, provided courtesy of Carly Rocklen, Outreach Director and Restoration Manager of the Neponset River Watershed Association (NepRWA)
Map, made available for use in the public domain, from the United States Department of Agriculture
Purple loosestrife at the Exeter Canal in the United Kingdom, made free for use at Wikimedia Commons by Derek Harper
Images of Biocontrol efforts at the Neponset Watershed, courtesy of Carly Rocklen and NepRWA
Again, my sincere thanks to those who have made their pictures available, to further the effort to educate the gardening community about the issues surrounding the spread of purple loosestrife!