Introduction

Purple Loosestrife Lythrum salicaria is a well-known noxious invasive species in North America. It has invaded all of southern Canada and almost the entire continental USA. It invades wetland habitats where the soil remains moist most of the year and grows abundantly on the edges of open water wetlands, lakes, rivers, etc. Purple Loosestrife is an aggressive invade known to create monocultures. Its sturdy rhizomes enable its vegetative spread and it produces a huge number of viable seeds each year. It is particularly invasive in the northeastern USA and in eastern Canada. Efforts to control or remove Purple Loosestrife cost $45 million per year in the USA.

When I was first learning about invasive species back when I was still a teenager on the coast of British Columbia, Canada, Purple Loosestrife was the most well-known invasive species at that time. We were lucky in British Columbia at that time in the Purple Loosestrife did not arrive in BC until 1915 and it is only found in a handful of locations in the southern half of the province. It is still a concern, certainly. But, knowledge and awareness go a long way. Now that many people are aware of the Loosestrife problem early detection becomes easier. Early detection is always the best method to control or remove Purple Loosestrife. It is so much easier to remove Purple Loosestrife from an area when there are only 10 or 20 plants to deal with as opposed to hundreds or thousands.

Description of Lythrum salicaria

Leaves & Stems

Purple Loosestrife is a herbaceous perennial producing 1 – 15 (-50) annual stems 60 – 120 (-200) cm tall from a perennial woody rhizome. The stems are angular and usually four-sided but occasionally grow 6-sided. They are stiff and woody and may be finely pubescent or hairless. Stems are green when young but turn reddish, brownish or purplish with age. The stems are slender and sometimes branched.

Leaves are sessile (no leaf stalk), entire (not toothed) and either lance-shaped or somewhat triangular. They may be smooth or finely pubescent. The leaves are usually arranged oppositely on the stem, one on each side of the node. But they may also be whorled with three or more per node. Upper leaves are usually smaller and arranged alternately on the stem. Leaf size varies with location on the stem as well as light levels, but may be anywhere from 3 – 12 cm long. Leaves turn bright red in autumn when the plant dies back for the winter.

Flowers & Fruits

The showy flowers bloom from June to September, depending on the location. They are usually deep pink but can vary to purple, light pink, or rarely white. Flowers are arranged in dense terminal spikes 10 – 100 cm long. Each flower is 1 – 2 cm in diameter with 6 (5-7) petals that are each 7 – 10 mm long and appear somewhat wrinkled.

In the yellow center of each flower is 12 crowded stamens and a single pistil with a long exserted style and capitate (rounded) stigma. The flowers are trystylous, having three different morphologies of styles and stamens coming in short, medium and long varieties. They are pollinated by a variety of insects that aid in the cross-pollination between the different flower morphs.

The fruits are an oblong–ovoid capsule 3 – 4 mm long, contained within a persistent calyx. The capsules open by two valves to release numerous tiny brownish seeds. Each capsule can contain more than a hundred seeds and a single mature plant can produce up to 2.7 million tiny seeds annually.

Similar Species Frequently Confused With Lythrum salicaria

There are a few native species that grow in the same habitat that Lythrum salicaria is sometimes confused with. They can be differentiated as follows:

• Chamerion angustifolium Fireweed – found in much of the same ranges in North America but usually in drier habitats, though they are sometimes found in moist habitats as well. However, the stems are rounded and smooth, not angular or pubescent and the leaves are always alternate and hairless. The flowers are in looser terminal racemes and each flower only has 4 petals.
• Verbena hastata Blue Vervain – also found in much of the same areas in North America, and in wetland habitats, and it also has stems that are angular and rough-haired. However, they can be distinguished by the leaves that have stalks (as opposed to sessile) and the flowers that smaller, blue-purple, and weakly two-lipped to funnel-shaped.
• Lythrum alatum Winged Loosestrife – found in wetland habitats but is endemic to eastern North America only. Their square stems are always hairless and are winged. Also, the leaves are always stalked and the pink-purple flowers are born solitary in the leaf axils instead of in dense terminal spikes.
• Decodon verticillatus Swamp Loosestrife – also found in wetland habitats but is endemic to eastern North America only. It also has 4-6 sided hairy stems but they are taller, up to 2.5 m. Also, its leaves are short-stalked and shorter, only up to 4 cm long. Its flowers are produced in axillary cymes with 5 – 7 triangular sepals.

Native Distribution of Lythrum salicaria

Lythrum salicaria is native in wetland areas throughout Europe, temperate Asia, and northwest Africa. There is some debate on its status in Australia where it was thought to be a recent introduction. However, pre-colonial pollen samples collected in New South Wales suggest that it may either be native or have been introduced at a much earlier time in human history.

Habitat Types Where Purple Loosestrife is Found

Purple Loosestrife is a semi-aquatic moisture-loving species that can be found in wetlands, marshes, wet meadows and along the side of lakes and other riparian areas. It can also be found in wet ditches, wet gravel pits, and other disturbed areas providing there is an adequate amount of moisture. It is generally not found in areas subjected to extended droughts where the soil remains dry for most/all of the year.

Human Uses of Purple Loosestrife

Purple Loosestrife sadly is still occasionally sold online and in garden stores, despite the widespread knowledge of its invasiveness. It was and still is frequently used as a garden plant for wet areas and ponds for its pretty pink flowers. Many supposedly sterile cultivars have been developed in an attempt to deal with the plant’s invasiveness. However, many of those sterile cultivars that were said not to produce seeds have since been observed producing seeds and spreading. They are particularly at risk of spreading when cross-pollinating with wild varieties. Even though the hybrids themselves may be sterile they are still replacing native vegetation because they spread vegetatively through their rhizomes. The true sterility of these cultivars must be thoroughly investigated before they are allowed to be sold in stores. However, so far, this is not the case.

In herbal medicine Lythrum salicaria is used for its tannins and salicarin that act as astringents (drying agents). This makes it useful in teas to help treat diarrhea and dysentery. It is even considered safe for use by children and even babies. It is also occasionally used in teas for menstrual problems and bacterial infections. Sometimes it is applied externally for bleeding gums, hemorrhoids, varicose veins, and eczema. If you are harvesting Purple Loosestrife in North America remember that the rules of Ethical Wildcrafting do not apply when it comes to invasive species. Remove more of the plant than you even need for your own use. Just be sure to harvest before it goes to see so that you do not further spread the plant.

Distribution of Lythrum salicaria in North America

The species was first brought to North America in the early 1800s. The earliest official report was in 1814 on the eastern seaboard of the US. It was at first probably brought there through accidental introduction by way of seeds traveling in ship ballasts or possibly with livestock in their bedding and feed. It was then spread purposely by settlers and is still sometimes planted in aquatic gardens to this day.

In Canada, Lythrum salicaria has been recorded in all the southern provinces including British Columbia, Alberta, Saskatchewan, Manitoba, Ontario, Quebec, New Brunswick, Nova Scotia, Prince Edward Island and the island of Newfoundland but so far has not been reported in Labrador. It is also not currently found in any of the Northern Territories.

In the USA, Purple Loosestrife has spread to almost every state. The only continental states where it is not currently found are Arizona, Louisiana, Florida, Georgia, and South Carolina. It is surprising that is not found in Louisiana, Florida, Georgia, or South Carolina since there is a lot of habitat there suitable for invasion. It may in part be due to its preference for temperate habitats. However, given its presence in other southern states this would not rule out invasion in these areas. These areas would be good candidates for monitoring as early removal is the best means of dealing with an infestation of Purple Loosestrife. It is also not currently reported to be found in either Alaska or Hawaii.

In Mexico Lythrum salicaria has so far not been confirmed in any state.

Purple Loosestrife has been introduced to much of the temperate world including southern Africa, North America, South America, and New Zealand.

How Purple Loosestrife Spreads

Lythrum salicaria is primarily spread through long distances by deliberate human introductions in garden plantings where their prolific production of small seeds and their vegetative production by way of their rhizomes then helps them spread to nearby wild areas.

Purple Loosestrife is easily spread short distances by its prolific seeds. A single plant can produce 2.7 million tiny seeds annually that can be carried by wind, water, and animals. While its seeds can germinate in water they are much more prolific when the substrate is moist but not flooded. Seed viability is over 90% and the seeds can remain viable in the soil seed bank for many years.

Purple Loosestrife is also able to regenerate from pieces of roots that are broken off and float away in the nearby water to re-establish elsewhere.

Habitats at Risk of Invasion in North America

Purple Loosestrife is a water-loving species so any ecosystem with seasonal or permanently wet soils is at risk of invasion. These include marshes, bogs, fens, lake shores, and river or creek banks where the water is slow-moving. It is also tolerant of brackish water and will even invade tidal marshes. Purple Loosestrife will also invade wet ditches, canals, wet gravel pits, and other disturbed areas providing there is adequate moisture for it. It will tolerate drier conditions as well, but these habitats are more marginal for Purple Loosestrife.

Purple loosestrife prefers full sun to part shade, however, it can even tolerate full shade. It prefers cool temperate habitats but has been found in warm temperate habitats as well.

The only habitats that are not at risk are ecosystems subject to severe droughts including deserts and rocky mountainous terrains, and any other area where there is not adequate moisture.

Currently in North America it is not found in Louisiana, Florida, Georgia, South Carolina, Alaska or Hawaii but it is expected to invade these areas in the future. The Northern Territories of Canada will likely be at risk in the future as Purple Loosestrife has already been seen to adapt to the colder temperatures and shorter growing seasons as it moved northwards out of southern Ontario into central and northern Ontario. In northern Ontario the plants are smaller and go to seed sooner. In conjunction with climate change, it is not believed that the Northern Territories are exempt from potential invasion. It will likely invade Mexico in the future as well, particularly in the interior and mountainous regions with cooler conditions.

Impacts of Invasion by Lythrum salicaria

In rivers, creeks, canals and other waterways Purple Loosestrife infestations can seriously impact the flow of water. This can lead to costly Purple Loosestrife removal programs and loss of water to farmers in their irrigation canals. Its stiff stems trap debris and sediment and they also die back annually leaving plant litter to decompose. The combination of those two factors gradually change shallow water wetlands into more terrestrial wet meadow habitats. It can significantly reduce the recreational value of wetlands and waterways.

Because Lythrum salicaria is very competitive and grows very quickly it rapidly replaces native vegetation leading to the creation of monocultures. The creation of Purple Loosestrife monocultures affects habitat and food for numerous native birds, insects, and wildlife. It reduces nesting sites and shelter for birds and reduces shelter and habitat areas for fish. Significant declines in species richness (biodiversity) are always observed in habitats where Purple Loosestrife invaded. Increased biodiversity and ecosystem complexity are directly associated with ecosystem resilience. Monocultures create ecosystems that are further susceptible to serious declines.

It even commonly replaces cattails, which themselves are sometimes considered invasive even though they are native. Read this article here for more on common cattails and their supposed invasiveness in North America. The loss of cattails in marshes affects the native muskrats and other animals that feed on the rhizomes, young stalks and flowers. It also significantly reduces bird nesting sites as Purple Loosestrife is not suitable for nesting for most birds that would use the cattails.

Purple Loosestrife loses its leaves and decomposes faster than the native species it replaces. This causes a change in the timing of the release of nutrients into the ecosystem leading to eutrophication of the waterways downstream of the wetland habitat. This impacts native tadpole development as well as reduces the species richness and diversity in those ecosystems.

Potential Benefits of Invasion by Lythrum salicaria

Purple loosestrife is widely used by native and introduced bees alike. It is also visited by a number of native butterflies. However, the native vegetation Lythrum salicaria replaces would have also been a good source of nectar for the bees and butterflies and would have been associated with a multitude of other biodiversity and ecosystem values as well.

Methods to Remove Purple Loosestrife

As always prevention is the preferred method of control. It, like most invasive species, is still widely sold online and in most local garden stores. Do not buy or transport any Purple Loosestrife. Do not plant it in your yard.

If you see them being sold online or in your local garden stores please inform them of their invasive status and ask them to do their part and cease selling them. Even if they inform you that they have only sterile cultivars, sometimes these are mislabeled. Even worse, sterile cultivars have many times now been proven not to be sterile and even if they themselves are sterile many have been shown capable of interbreeding with wild populations. Furthermore, their vegetative spread via their rhizomes still puts them at risk of escaping cultivation by vegetative means. Avoid all cultivars.

Instead, asks stores to sell more native species as ecologically friendly garden alternatives to invasive species. And encourage these stores to do so by only purchasing species that are native to your region. We have so many beautiful native aquatic species all over North America. Just do a little research for your region and you will find many lovely alternatives.

Physical Control of Purple Loosestrife

Once already established, however, physical control is almost always the most effective means. Physical control is labor-intensive and time-consuming but it usually causes the least amount of environmental damage and results in the greatest increase in biodiversity compared to chemical control.

Aquatic and semi-aquatic invasive species are notoriously difficult to control. Choose a time of the year when the water is low and the soil is fully exposed as this will make it easier to dig out the rhizomes. Since Lythrum salicaria is simply a water-loving plant and not a fully aquatic one this should be relatively simple. Monitor the area and find the best time. The soil should however still be wet as this loosens the attachment of the roots to the soil making them easier to pull out. Ideally, remove the plants before flowering but certainly before they have gone to seed. Purple Loosestrife removal while in seed will only spread the prolific seeds around. Since they are herbaceous perennials they will need to be removed during the growing season and cannot be done in the winter after the plant has died back.

When removing single isolated Purple Loosestrife plants hand pulling or the use of a narrow shovel or weed puller can be very effective means of control. It is best to dig them out before the patch spreads to prevent them from spreading to become the dominant ground cover. Younger plants are also much easier to dig out as older rhizomes are larger and more difficult to remove. Simply dig out the plants, gently, trying not to break the rhizomes and taking care to remove any broken pieces from the soil. Also, dig around the edge of the patch a little bit to ensure you get all the rhizomes. Any piece left behind can create a new plant so it is important to get as much of the root as possible. However, also be aware that any soil disturbance can encourage the germination of seeds in the soil seed bank. Therefore, try to disturb the soil as little as is physically possible. It is a delicate balance between getting all the roots and not disturbing the soil too much.

Cutting the plants before they go to seed is not an effective control method. While it is less labor-intensive in the short term it is much more labor-intensive in the long term. This is because it requires multiple visits to the site to cut back the regrowth, and visits each year for multiple years to ensure that it does not regrow and produce seeds. This can be an effective method if done diligently. This is because after some time the rhizome in the soil will eventually wither and die. Without above ground growth for photosynthesis the plant is not able to store energy in its rhizome for the following year’s growth.

Mechanical mowing or the use of machines is generally never recommended as they are nonselective and will destroy any native vegetation still in the patch. If it is a dense monoculture of significant size then this should be assessed by a professional who will then decide the best strategy. In almost all cases, however, this course of action is not recommended.

Flooding of less than 30 cm will not prevent germination. Short-term flooding tends to enhance the survival of Purple Loosestrife because it can adapt to changing environments. Short-term or long-term flooding may negatively impact the native vegetation on site so this method is not recommended. On-site burning is also not recommended for the same reason. Burning can negatively affect any native wetland vegetation on site as these plants are seldom adapted to fire.

Disposal of the Lythrum salicaria Plants Once Removed

In all cases of physical removal any tools that are used will need to be cleaned, solarized, sterilized or heat-treated to kill any viable seeds or rhizome parts that may be on them before moving them to another site. The same goes for the boots you wear on-site, they should be washed and sterilized to ensure that you are not spreading them to any new sites you visit.

Upon removal, the plants must either be burned or solarized. In either case, they are best removed from the site in black garbage bags to prevent the spread of any seeds or rhizome pieces. If you are not able to burn them in your area due to fire bans then you will need to place them in thick black garbage bags and leave them in the full sun for 6 – 8 weeks to be sure that all seeds are no longer viable. While some sources recommend a week of solarization I have seen disposal of solarized bags of invasive species still grow if not solarized long enough. This is in part due to differential temperatures within the bag as well as sunlight intensity and duration during solarization. This is why I always recommend a much longer period of solarization to ensure all seeds are no longer viable before they are disposed of. To dispose of them after treating bring them to your local garbage dump but be sure to inform them that they are invasive species so that they are dealt with accordingly.

See Integrated Management below, as all removal methods require additional management and monitoring.

Chemical Control of Purple Loosestrife

Chemical applications are almost never an ideal method of control for any invasive species. That is because chemical alteration of the environment often makes the environment more suitable for invasive species than native species. Furthermore, it is often difficult to keep the chemical control method contained so that it does not directly affect any native species that are there during the application process itself. As a result, plots where chemical control is used usually show a decrease in species richness. On the other hand, in plots where only physical control is used species riches significantly increases.

Furthermore there are no chemical control methods that effectively target only Purple Loosestrife. Because they grow in wetlands and near major water sources in most cases the use of herbicides is not permitted.

Chemical control is not recommended.

Biological Control of Purple Loosestrife

Biological control involves the use of a predator, herbivore, disease, or some other biological agent to control an invasive species once it is established in the environment. The idea is an attempt to restore the natural balance that likely existed in its native environment where it had natural enemies that are not present in its new environment, The problem with biological control is that the agent used must be entirely specific to only the target organism before releasing it into the environment. This is often difficult to determine since the agent of control is also not native to the environment and could behave differently when released there. Take the example of the mongoose and the rat. The mongoose was released in Hawaii in the late 1800s to help control the rat. To this day there are still rats in Hawaii but the mongoose has helped to decimate many native bird populations.

Biological control methods are extremely risky and should only be carried out by professionals after years of rigorous study. The use of biological control methods can never be used alone. They must be part of an integrated pest management approach. This is because the control agent would need to effectively destroy over 99% of the target organism to actually control it on its own. However, using biological control in conjunction with an Integrated Management approach can be very effective. Following is a list of biological control methods that have been used in an attempt to help reduce the population densities of Lythrum salicaria.

Two species of leaf beetle have been used for the biological control of Purple Loosestrife. Neogalerucella (Galerucella) calmariensis The Black-Margined Loosestrife Beetle is a brown beetle with a black line down its thorax. It eats the leaves leaving characteristic round holes. Its larvae feed on leaf buds and leaves. A very similar and closely related species Neogalerucella (Galerucella) pusilla the Golden Loosestrife Beetle also feeds on the plant in a similar manner. It looks nearly identical to N. calmariensis but lacks the black line on its thorax. Both beetles have shown to be extremely effective and have defoliated over 90% of the plants in some areas. Both species have been released in North America.

Two other biological control agents have been released in North America. The Loosestrife Root Weevil Hylobius transversovittatus is a large red nocturnal weevil that feeds on leaves and buds while its larvae burrow into the root and feed on it for over a year. The root damage stunts the plants’ growth and hinders seed production. The Flower Weevil Nanophyes marmoratus is a tiny weevil that lays a single egg in each flower. The larvae emerge and feed on the flower’s ovaries preventing seed production. It then hollows out the flower bud and pupates there.

Caterpillars of the Engrailed Moth Ectropis crepuscularia are also known to feed on purple loosestrife, but according to some sources they are also a pest themselves so they are not recommended as a biological control method.

Integrated Pest Management & Ongoing Monitoring

Integrated management is always the best approach. In its simplest and least impactful form this involves physical removal methods, possibly biological control methods, replanting, and ongoing monitoring. The key point of integrated management approaches is that you cannot simply perform one method, once, and hope it works. All methods usually require ongoing monitoring, repeated applications, and in some cases multiple methods. When it comes to invasive species replanting with native species is also often crucial to prevent the re-establishment of invasives after disturbing the environment in the process of removal.

Replanting With Native Species is Crucial

In most cases of Purple Loosestrife removal the newly disturbed site should be replanted immediately because the bare soil will allow the seed bank of Purple Loosestrife seeds in the soil to germinate. They will then quickly re-invade the patch they were removed from. Exceptions include small isolated populations that have plenty of native species in among or around them as these will invade the bare soil given the chance. In this case, simply implement a monitoring program to ensure that any undesirable seedlings are removed before they regain a foothold there.

In the case of larger patches, a replanting program should already be planned and ready to implement immediately upon completion of the removal of the Purple Loosestrife. To decide which native species to plant examine the local environment to see what is already growing nearby. Ideally, if possible, collect seeds the year before from the area and use those to replant upon removal of Purple Loosestrife. Otherwise, purchase native seeds or native plants from a local nursery specializing in producing plants for restoration. Do not simply buy plants from a regular garden nursery as you may be getting cultivars created by humans even if you think you are buying the native species. Finally, be sure to plant a variety of species to help recreate the biodiversity that was lost.

Ongoing Monitoring is the Last But Most Important Step

In all cases of invasive Purple Loosestrife removal, ongoing monitoring is absolutely essential. Yearly monitoring programs should be put in place to ensure that any surviving individuals are removed so that the population is not able to recover. This is required whether the area is replanted or not. Purple Loosestrife is an aggressive and prolific invader. If given the chance it will reestablish and outcompete planted vegetation if yearly monitoring is not put in place. Monitoring is simple. Each year return to the removal site and destroy any young plants before they have a chance to become established and set seed. Ideally, return to the site twice per year for at least five years, and then once a year after that to be sure they do not return. The added benefit, you will get to see the fruits of your labor when the ecosystem is returned to a healthier and more biodiverse state filled with native species.

References and Resources

Canadensys Plant Search https://data.canadensys.net/vascan/search

Dictionary of Botanical Terms – Lyrae’s Nature Blog Dictionary of Botanical Terms

iNaturalist Plant Search https://www.inaturalist.org/home

Michigan Department of Agriculture and Rural Development Weed Risk Assessment for Lythrum salicaria https://www.michigan.gov/documents/invasives/Lythrum_salicaria_WRA_576399_7.pdf

Ontario Invasive Plant Council Best Management Practices Purple Loosestrife http://www.ontarioinvasiveplants.ca/wp-content/uploads/2016/07/Purple-Loosestrife-BMP-April-2016-final.pdf

Purple Loosestrife ‘Naturalized’ in a Marsh photo from Wikipedia By Saffron Blaze. Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=15287781

Purple Loosestrife RxList.com https://www.rxlist.com/purple_loosestrife/supplements.htm

USDA Plants Database https://plants.sc.egov.usda.gov/home

Wikipedia Purple Loosestrife

Willis, Lyrae (2022).  Plant Families of North America. Not yet published.

Currently Seeking Funding To Continue This Non-Profit, Ad-Free Work

If you are able to donate so that I can continue this non-profit work of supplying people with scientific information on the plant families, native plants, and invasive species found throughout North America, please donate using the GoFundMe link below. Thank you!

---