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Purple Loosestrife

Purple Loosestrife

Lythrum salicaria

Class A Noxious Weed

This plant is new to the state and has been designated a Class A Noxious Weed by the New Mexico Department of Agriculture.

Riparian Threat

This plant is known to invade riparian areas or otherwise damage the health and abundance of our water resources.
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Wildlife & Ecosystems

This plant is detrimental to native ecosystems, negatively impacting the quality of forage for wildlife and reducing biodiversity in plant communities.
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QUICK FACTS

Purple loosestrife, native to Eurasia, was likely introduced to North America in the 1800s via discarded ship ballast or by colonial gardeners. It quickly spread along waterways and is now found across much of North America, thriving in disturbed wetlands and riparian areas.
The plant is highly invasive, outcompeting native vegetation by forming dense stands in wetland ecosystems. This disrupts local biodiversity and creates challenges for agriculture, as its root systems can clog irrigation channels and affect water flow.
Purple loosestrife produces an astonishing number of seeds—up to 2.7 million per plant annually—which are spread by water and animals. Its ability to rapidly reproduce and spread makes it a persistent problem for natural habitats and agricultural areas alike

1. Overview

family	Lythraceae – loosestrifes	origin	Eurasia
life cycle	Perennial	other names	Bouquet violet, Purple Lythrum, Willow herb, Spiked loosestrife, Red sally, Long purples

The merciless invader of stream banks, purple loosestrife, may have been introduced to North America as an ornamental plant, but it has since become a highly invasive species.

Purple loosestrife, a native of Eurasia, has spread across much of the northern hemisphere, including North America, where it has become quite a nuisance. Likely introduced in New England through discarded ship ballast or by colonial gardeners, it quickly spread inland via cargo vessels and waterways. Purple loosestrife is a robust, perennial plant with tall, upright stems, vibrant purple flowers, and prolific seed production. It thrives in moist or flooded environments, where it crowds out native plants, disrupts local ecosystems, and presents challenges for agriculture and infrastructure. The spread of purple loosestrife continues to be a significant ecological and economic concern.

History of Purple Loosestrife

Purple loosestrife is native to Eurasia and has spread across the northern hemisphere, excluding extremely cold environments. It was likely introduced in New England from discarded ship ballast or intentionally by colonial gardeners. The plant spread inland via cargo vessels, which traveled along various canals, lakes, and rivers. It has spread prolifically in the northeast, and by the 1830s, it was so well established that it was mistaken for a native plant. It now reaches many parts of the West, where it is spreading from wetland to wetland and is found in all but two US states. [1,4,8]

The most complete infestations of purple loosestrife occur along the Atlantic seaboard and throughout the Midwest and Great Lakes areas. However, the most rapidly expanding infestations are in the western US. This was likely due to the commercialization of the plant as an ornamental. This is especially true of private properties along riparian zones, which are frequently landscaped with purple loosestrife, resulting in seeds traveling downstream and starting new colonies. [1,4]

The weed was used in early medicines, increasing its likelihood of propagation in herbal gardens. Early literature promoted the use of purple loosestrife as an ornamental to be planted along riverbanks. [1]

2. ID Guide

What does it look like?

Purple loosestrife, a stout perennial, can grow up to 10 feet tall and has strong, upright stems that become woody as they age. They can last through the winter and remain for up to 2 years. Older plants can reach widths of 5 feet. As they age, the plants can start to look more like a bush because they produce more stems each year from the same root. Stems are 4 to 8-sided and either smooth (glabrous) or hairy. Wand loosestrife (Lythrum virgatum) is commonly mistaken for purple loosestrife due to its similar flowers and tendency to hybridize. It can be distinguished by narrower leaves that are pointed instead of cordate at the base. Because of the similar nature of L. virgatum, Minnesota has banned the sale of any plant in the Lythrum genus. [1,8]

Having trouble identifying a weed? Contact us.

Key Features

Roots: Thick and woody roots form in mature plants. The root crowns produce vegetative buds each year, producing 30-50 new shoots. [1,2]
Leaves: Opposite or in a whorled pattern, the leaves are hairy, non-toothed, and 1-5 inches long. They are lance-shaped, widest below the middle, gradually tapering to a pointed tip, and occasionally have a clasping base. [1,3]
Flowers: Dark pink/purple flowers display radial symmetry and are arranged in rings on a 2-36 inch spike. Each inflorescence has a yellow center with five to seven petals with ~9 stamens, each of varying length. The sepals and petals are largely fused, forming a slender, hairy tube (called a hypanthium) with eight to twelve nerves. [1,3,4]
Seeds: Each flower spike can produce upwards of 120,000 seeds, with plants producing up to 2.7 million seeds every year. Light tan seeds are very small (about 0.012 inches) and are contained within 0.13 to 0.19-inch brown capsules. [1,3]

deistung, inaturalist.org

Dark pink/purple flowers display radial symmetry and are arranged in rings on a 2-36 inch spike.

Bruce Ackley, The Ohio State University, Bugwood.org

Seedlings appear in the spring from either seeds or vegetative buds.

charlesgodbout, inaturalist.org

Thick and woody roots form in mature plants. The root crowns produce vegetative buds each year, producing 30-50 new shoots.

J, inaturalist.org

Opposite or in a whorled pattern, the leaves are hairy, non-toothed, and 1-5 inches long.

Jennifer Andreas, Washington State University, Bugwood.org

Purple loosestrife can grow up to 10 feet tall and has strong, upright stems that become woody as they age.

Gary L. Piper, Washington State University, Bugwood.org

Light tan seeds are very small (about 0.012 inches) and are contained within 0.13 to 0.19-inch brown capsules.

3. Infestation Basics

“The formerly unique and endemic flora of the estuary is being rapidly obliterated by the crowding and handsome but overwhelming purple loosestrife… gorgeous to look upon but unscrupulous and without mercy.”

-M.L. Fernald, 1938 [5]

Impacts

Where does it grow, and how does it spread?

Purple loosestrife is commonly found in disturbed wetlands and riparian areas, though it can grow in untouched stream banks as well. The seeds often escape private water gardens and travel down streams to invade other areas. Much of the weed’s expansion in the west is likely due to the presence of irrigation systems such as acequias, as they can facilitate movement for the weed between waterways. [1,6]

Purple loosestrife is a perennial that reproduces mainly through enormous quantities of seed and regeneration via buds on the root crowns. Seeds are primarily spread by waterways or by wetland creatures that can carry the seeds in their fur or feathers. Seeds will sink to the bottom of water bodies, where they can remain viable for up to 3 years. Seeds germinate while submerged, then become buoyant and travel along streams to take root in other locations. The weed begins to flower typically around 8 to 10 weeks following germination, and it blooms, usually in July, and continues until September, but can sometimes continue until October. [1,3]

Roots become dense and woody with age. Surface growth dies off each fall but leaves dead stalks that can remain for one or two years, leading to incredibly dense stands of vegetation. New shoots emerge from the root crowns each spring, creating 30-50 new stems. [1]

This plant is associated with temperate climates, and its germination rates are best between 60 and 72 degrees Fahrenheit. Unshaded, newly exposed, moist soil is best for seedling establishment. Purple loosestrife prefers full sun but can tolerate up to 50% shade. It continues to grow well in moist or saturated soils. Soil types can range from clay, sand, muck, and silt. Because it is flood resistant, it can stand growing with water two inches below the ground or with four inches of standing water above the ground. It usually grows on floodplains or the disturbed banks of water bodies. It prefers organic soils but has a relatively low nutrient requirement and can grow just about anywhere there is water. [1,6]

Key takeaway: Purple loosestrife thrives in disturbed wetlands and riparian areas, often spreading via irrigation systems, and can grow in many soil types and conditions, including flooded environments.

Why is it so invasive?

This weed spreads quickly anywhere water is present. Purple loosestrife damages natural wildlife habitats by suppressing food and shelter sources. I can be an agricultural threat in areas with irrigation ditches, as the dense matting root systems can clog up acequias and streams, leading to the need for costly and difficult removal of plant matter. [1]

Purple loosestrife is a prolific seed producer, and a mature plant may produce 2.7 million seeds per plant annually. These seeds quickly germinate and take hold in stream banks, particularly those with fresh sediment. Infestations aggressively crowd out native riparian plants like cattails, severely reducing biodiversity and wildlife carrying capacity in infected areas. Because the seeds are so tiny, they do not provide a viable food source for songbirds. Waterfowl have difficulty finding nesting spots in dense stands of purple loosestrife. [1,3]

Key takeaway: Purple loosestrife spreads rapidly in water, harming wildlife habitats, clogging irrigation systems, and reducing biodiversity by crowding out native plants and offering little food or shelter for animals.

Common risk factors for invasion

Human activity: Purple loosestrife has become invasive in areas where water plants have been disturbed, such as construction sites, stream projects, and water gardens. These disturbances create openings for purple loosestrife to invade. Additionally, the movement of soil, agricultural products, and equipment can inadvertently introduce purple loosestrife to new areas. This weed originally spread from an intentional introduction to private water gardens. It may still pop up in unexpected areas where it has been introduced as an ornamental. Purple loosestrife seeds may be found in some “wildflower” seed mixes, so be careful when purchasing generalized seed packets.
Moisture availability: While purple loosestrife has adapted to a wide range of soil and water conditions, it generally requires adequate moisture to establish and spread, making irrigated lands, streambanks, and areas with consistent rainfall particularly vulnerable. Recently flooded streams are particularly vulnerable as the plant can take hold in new sediment deposits with almost no competition.
Riparian Areas: Seeds are easily carried by streams and may be subsequently deposited on river terraces. Purple loosestrife germinates underwater before becoming buoyant and floating downstream to latch into the stream bank.

Ecosystem Health

Purple loosestrife can displace native plants like cattails, leading to a loss of biodiversity for wetland creatures and plants. Because purple loosestrife produces very little food, this displacement may reduce forage availability for waterfowl, especially during the winter months. However, recent studies suggest that the impact on birds and other native species may not be as severe as previously believed. According to the USDA, further research is necessary to fully understand this issue. Infestations of purple loosestrife do pose a threat to the survival of federally endangered species, such as the bog turtle and certain orchids. [1,3,4,6]

Infrastructure Issues

Purple loosestrife can block waterways and wetlands, hindering boating and other recreational activities. Its impact on agriculture includes changes to the hydrology and soil conditions of wetland pastures and meadows, as well as the obstruction of irrigation systems. [1,6]

Agriculture and Food Security

Purple loosestrife can invade rice fields and has the potential to affect a significant area of land. In North America, approximately 190,000 hectares of wetlands, marshlands, pastures, and riparian meadows are impacted annually, leading to economic losses of millions of dollars. The economic burden on farmers and the potential for decreased food supply contribute to the overall negative impact on food security in regions affected by purple loosestrife. [1,4,6]

4. Management Strategies

Land managers should prioritize small, newly established populations before addressing large, well-established ones. The most significant long-term challenge is the buildup of seeds in the soil, which creates a persistent seed bank. Efforts to deplete seed banks in well-established areas are difficult, as purple loosestrife seeds can remain dormant for extended periods, and disturbing the soil may inadvertently spread the weed. [1,4,7]

Flooding may reduce purple loosestrife by inhibiting its growth and reproductive capacity, although results vary, and research is needed to optimize this approach. [4,7]

Physical and mechanical methods, like cutting stems and pulling plants, can also prevent seed production and spread, but they are more effective when done early, before the plants become deeply rooted. Herbicide use may also be necessary when other methods are insufficient. [4,7]

DO’s

Plant native species in affected areas to outcompete purple loosestrife and restore biodiversity.
Pull or cut purple loosestrife plants before they flower and seed to prevent further spread.
Check areas that have been treated to ensure purple loosestrife doesn’t regrow from remaining seeds or root fragments.

DON’Ts

Avoid planting this species as an ornamental or in water gardens to prevent unintentional spreading.
Underestimate the seed bank: Purple loosestrife seeds can remain viable in the soil for up to three years, requiring ongoing management efforts.
Disturb the streambank unnecessarily, as this can inadvertently spread seeds or root fragments to new areas.

** The following information is provided courtesy of the UC Weed Research and Information Center. The Taos Soil and Water Conservation District does not endorse the use of any particular product, brand, or application thereof. **

Integrated management, combining multiple techniques, is most effective for long-term control. Methods should consider factors like land-use objectives, habitat, and environmental conditions. Cultural methods, such as seeding competitive vegetation and flooding areas, can be effective.

Hand-pulling	Manually digging or hand-pulling is effective for early infestations to help prevent the establishment of dense colonies. Early detection and removal is essential because established plants are too large and deep- rooted to remove easily. All plant material, especially the root crown, should be removed to prevent resprouting. Pulled plants should be dried or burned. Repeat visits are important to ensure there is no regrowth.
Mowing	Mowing purple loosestrife can be impractical due to the sites it occupies. Mowing or cutting stems can help reduce seed bank accumulation. Late-season cutting was found to reduce shoot production to a greater degree than mid-summer cutting. Mowing or cutting may be more effective when used as part of an integrated approach with herbicides.
Tilling	Tillage is probably not an effective control measure for purple loosestrife. Wetland sites where it grows are not conducive to tillage operations. In addition, any disturbance that fragments live stem or root tissue is likely to spread purple loosestrife, and its extensive soil seedbank is likely to reinvade open areas created with tillage. Three to four consecutive years of tillage might be effective.

Purple loosestrife grows in wet areas that are not usually grazed and it has poor palatability.

There is little information on the effectiveness of fire, but some sources indicate that purple loosestrife does not burn well and it would be difficult to get a fire to carry through an infested area. Additionally, it is doubtful that burn temperatures in wetland areas would get high enough to kill the massive crown.

However, burning removes biomass and may improve the effectiveness of herbicides.

Continuous flooding has been somewhat effective for large infestations where the water level can be controlled. The duration of the flooding appears to be more important than the depth of flooding. The precise parameters for maximum effectiveness need further study.

Black plastic mulch was found to be marginally effective, but while it can reduce growth and seed production, it does not kill the roots of mature plants.

Several non-native insects have been released in the United States to control purple loosestrife. These include the black-margined and golden loosestrife beetles (Galerucella calmariensis and G. pusilla), whose larvae and adults feed on the foliage and flowers, reducing seed production. In addition, the larvae and adults of the root weevil (Hylobius transversovittatus) feed on the root. Larvae of the loosestrife flower weevil (Nanophyes marmoratus and N. brevis) feed on flowers, and the adults feed on foliage and flowers.

The two Galerucella spp. have been the most successful of these biocontrol agents. These beetles were released between 1992 and 1994 and have become established in some states. They are not present in California yet but have shown promising results in Oregon. The root weevil and the flower weevil have been released at test sites in California with mediocre results.

There is ongoing research on the use of pathogenic fungi as biocontrol agents for purple loosestrife.

The following specific use information is based on published papers and reports by researchers and land managers. Other trade names may be available, and other compounds also are labeled for this weed. Directions for use may vary between brands; see label before use. Herbicides are listed by mode of action and then alphabetically. The order of herbicide listing is not reflective of the order of efficacy or preference.

Triclopyr

Garlon 3A

Rate: Broadcast foliar treatment: 6 to 8 qt Garlon 3A/acre (4.5 to 6 lb a.e./acre). Spot treatment: 1.5 to 2% v/v solution

Timing: Postemergence at bud to mid-flowering stage.

Remarks: Thorough coverage and a minimum of 50 gal/acre spray solution is recommended. Follow-up applications should be made to regrowth the following year. Triclopyr is broadleaf-selective and safe on most grasses. It is most effective on smaller plants and has little or no residual activity. Garlon 3A and other amine formulations are registered for aquatic use. Triclopyr (1 to 2 qt Garlon 3A/acre) can be mixed with aminopyralid (Milestone) at 7 oz product/acre or with 1 to 2 pt product/acre 2,4-D.

Glyphosate

Roundup, Rodeo, Aquamaster, and others

Rate: Broadcast foliar treatment: 1 to 2 pt product (Roundup ProMax)/acre (0.56 to 1.1 lb a.e./acre). Spot treatment: 1% v/v solution

Timing: Postemergence to rapidly growing plants in the full to late flowering stage. Seedlings may be treated in spring following a fall treatment.

Remarks: Nonselective, no soil activity. Effectiveness is increased by addition of ammonium sulfate. Aquatic registered formulations, e.g., Rodeo and Aquamaster, are available for use close to water.

Imazapyr

Arsenal, Habitat, Stalker, Chopper, Polaris

Rate: 1 to 2 pt product/acre (4 to 8 oz a.e./acre)

Timing: Postemergence to rapidly growing loosestrife after mid-bloom until killing frost.

Remarks: Nonselective, long soil residual activity. Leaves more bare ground than other treatments, even a year after application. Habitat is an aquatic registered formulation available for use close to water.

Metsulfuron

Escort

Rate: 1 to 2 oz product/acre (0.6 to 1.2 oz a.i./acre)

Timing: Postemergence from seedling to flowering stage. Most effective at flower-bud and flowering stage.

Remarks: Primarily active on broadleaf species. Always use a surfactant. It can be tank-mixed with 2,4-D for quicker burndown. Other premix formulations of metsulfuron can be used at similar application timing. These include Cimarron Max (metsulfuron + dicamba + 2,4-D) and Cimarron X-tra (metsulfuron + chlorsulfuron). Metsulfuron is not registered for use in California.

5. Citations

[1] Sheley, Roger L., and Janet K. Petroff. Biology and Management of Noxious Rangeland Weeds. Oregon State University Press, 1999.

[2] Minnesota Department of Natural Resources. “Purple Loosestrife (Lythrum Salicaria).” Dnr.state.mn.us, 2019, www.dnr.state.mn.us/invasives/aquaticplants/purpleloosestrife/index.html.

[3] USGS. “Purple Loosestrife (Lythrum Salicaria) – Species Profile.” Usgs.gov, 2009, nas.er.usgs.gov/queries/FactSheet.aspx?SpeciesID=239.

[4] Munger, Gregory T. “Lythrum Salicaria.” Www.fs.usda.gov, U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory, 2002, www.fs.usda.gov/database/feis/plants/forb/lytsal/all.html.

[5] Fernald, Merritt Lyndon. “Must all rare plants suffer the fate of Franklinia?.” Journal of the Franklin Institute 226.3 (1938): 383-397.

[6] Benefield, Carri. “Lythrum Salicaria.” Cal-Ipc.org, California Invasive Plant Council, www.cal-ipc.org/resources/library/publications/ipcw/report61/. Accessed 18 Feb. 2025.

[7] Anderson, Lars W., et al. Weed Control in Natural Areas in the Western United States. Davis, California, University Of California Weed Research And Information Center, 2013.

[8] Humphrey, Jude. “History, Impacts, and Control of Purple Loosestrife (Lythrum Salicaria) in Minnesota.” Scholarly Horizons: University of Minnesota, Morris Undergraduate Journal, vol. 11, no. 2, 28 June 2024, https://doi.org/10.61366/2576-2176.1154.

Steve Dewey, Utah State University, Bugwood.org

Fun Facts

Purple loosestrife is still commonly used as a nectar source for beekeepers. Apiaries value its long growing season and huge flower spikes to produce a unique honey that reportedly tastes like butterscotch. [8]

Leslie J. Mehrhoff, University of Connecticut, Bugwood.org

Fun Facts

This weed has been used for medicine for millennia, first being recorded by the Greeks and Romans. Studies are still being conducted on the medical properties of purple loosestrife. Traditional herbalists still use it today to treat bleeding, stomach illnesses, eczema, blood flow issues, and sinus problems. [8]