Purple Loosestrife – A beautiful, but horrible invasive plant


Purple Loosestrife is a highly invasive plant introduced to North America. Scientifically known as Lythrum salicaria, it typically grows 2-6′ tall and blooms densely flowered purple spikes in full sun and moist soil along pond edges or wetlands. Spreading by seed and rhizome, it can colonize and outcompete native plants in a few years and become dominant.

In this article:

What is Purple Loosestrife

Purple Loosestrife is a herbaceous perennial flower native to Eurasia that dies back to ground each Winter. It can rapidly colonize wetland and choke out native plants such as grasses, flowers, and cattails. The result is less habitat for native insects to feed and lay their eggs. [1][2]

While bee keepers value this plant for it’s nectar and pollen, apparently the honey made from it is of low to marginal quality. [3] The sale of Purple Loosestrife is banned or controlled in at least 33 states.[1]

How did Purple Loosestrife come to North America?

Originally immigrants from Europe introduced Purple Loosestrife to North America to grow for it’s beauty, value to bee keepers as nectar & pollen source, and for it’s medicinal benefits. [1][2]

North American Range of Purple Loosestrife

Purple Loosestrife has essentially been reported in nearly all 50 states and most of Canada.

EDDMapS. 2022. Early Detection & Distribution Mapping System. The University of Georgia – Center for Invasive Species and Ecosystem Health. Available online at http://www.eddmaps.org/; last accessed December 15, 2022.

Purple Loosestrife Reference Table

Scientific NameLythrum salicaria
Common Name(s)Purple Loosestrife
Native Range, USDA ZoneEurasia, USDA hardiness zone 3-10
Bloom TimeJuly-September
Bloom Duration, Color2-3 months, pink to purple densely flowered stalks
Height2-6′ (60-180 cm)
Light RequirementsFull sun / partial shade
Soil TypesLoam to clay, rich with organic matter
MoistureMoist to wet soils
Fauna Associations / Larval HostsBees and butterflies
Source [1][2]

How Purple Loosestrife is harmful to the environment

Purple Loosestrife harms the environment three ways.

  • Displacing native plants reduces habitat for wetland animals and insects
  • Clogging waterways and wetland
  • Negatively effecting agriculture by modifying the hydrology of waterways and wetlands.

Purple Loosestrife is native to Eurasia, and is invasive in North America. Although we may admire the flowers, and even observe pollinators visiting them, we need to also think of the total pollinator lifecycle. Our native insects have evolved over millennia to lay eggs on various plants and have their caterpillars eat the leaves, feeding them until they make their final morph into butterflies, moths, or anything else.

Identification and Characteristics of Purple Loosestrife

Where you typically find Purple Loosestrife

Purple Loosestrife typically grows near water. It is often colonizing wetlands, pond or creek edges, or low spots. It likes full sun but can tolerate 50% shade, and generally outcompetes native competition.

Stalk

The stems are four-sided and typically grow between 2-6′ tall. There will often be more than one stalk or branching before the flowers. Stalks are light green and typically hairy near the flowers, and woody and smooth towards the base.

Leaf

Opposite leaves along the stalk are paired along the stalk, lanceolate and between 3-5″ long by 3/4″ wide. They have small white hairs, generally have to stem (petiole) and are light green in color.

Flower

The upper 6-24″ of stalk will be a spike of flowers. Individual flowers are 1/2-1″ diameter with 6 pink to purple petals.

The flowers slowly bloom for roughly two months. After blooming, an individual flower will be replaced by a seed capsule. Seed capsules can float and are also light enough to blow in the wind. Individual seeds also float, contributing to the spread of this invasive plant.

Root

The root system of Purple Loosestrife is fibrous roots that don’t penetrate too deep into the earth. This plant also produces underground rhizomes, which also can greatly contribute to the spread in somewhat open areas near water.

Growing Conditions for Purple Loosestrife

Purple Loosestrife will grow in full or partial sun. It prefers wet to moist soil, and is most often observed near ponds, streams, and wetlands. It is highly adaptable, and in low fertility environments has been found to have a competitive advantage over most natives. [1]

How to Purple Loosestrife spreads

Purple Loosestrife spreads via seed and underground rhizomes.

Purple Loosestrife spreading by seed

Purple Loosestrife seed needs light to germinate, as well as a period of cold stratification. A single Purple Loosestrife plant can produce thousands of seeds. These seeds are tiny enough to be dispersed by the wind, but they also float on the water.

Further complicating the spread is that seeds can stick to boats, hiking or hunting boots, or even clothes. So many people may unwittingly spread this invasive plant.

So, Purple Loosestrife is particularly damaging to waterways in how it aggressively spreads. It’s seeds easily germinate after a winter, resting on the surface of bare soil.

Underground Rhizomes

In addition to spreading by seed, Purple Loosestrife can also propagate itself via underground rhizomes. Rhizomes are underground stems that spread horizontally from a plant, and periodically sprout new shoots or plantlets.

Wildlife, Pests, and Diseases associated with Purple Loosestrife

Pollinators

A number of long-tongued bees are attracted to Purple Loosestrife as well as butterflies. Bumblebees can be observed frequently, and Linda Jeays of the Ottawa Field-Naturalists’ Club has documented a large number (17) of butterflies visiting such as the Black Swallowtail, Cloudy Sulphur, and Great Spangled Fritillary. [5]

Pests

No native fauna seem to damage Purple Loosestrife. Deer and rabbits do not eat it. However, there are some introduced beetles that feed on it, and are able to reduce it’s vigor.

Where you can buy Purple Loosestrife

Purple Loosestrife is banned or listed as a noxious weed in 33 states as of 2022. [1] Unfortunately it is sometimes still sold, or hybrids/cultivars are sold. But – do not purchase this plant. Just because you don’t think it is invasive in your yard doesn’t mean it won’t spread to other areas. For example I have personally pulled two Butterfly Bush plants in my flower beds, but I have never grown the plant (my neighbors have it though).

Native Alternative to Purple Loosestrife

If one has fallen in love with the aesthetics of Purple Loosestrife, then please let me implore you to consider some alternatives that are native to North America, look similar, and prefer similar growing conditions:

How to control Purple Loosestrife

The best method of controlling Purple Loosestrife will depend on the infestation size. Small infestations of 100 plants or less can be hand pulled, while larger infestations will likely require chemical or biological control. You should plant on control taking several years, as plants can resprout and new seeds can easily germinate.

Mechanical Control / Hand pulling Purple Loosestrife

If your infestation is a few plants, or less than one hundred, then you can pull up plants by hand or with the aid of a shovel or potato fork. You must get all the root, as any pieces of white fleshy rhizome left behind will likely re-sprout. You should only pull Purple Loosestrife in Spring before flowering, lest you risk spreading the seed. [2]

Do not mow Purple Loosestrife. Mowing is not effective means of control, the root system will simply create more shoots, and if done after flowering, you risk spreading seed.

Chemical Control of Purple Loosestrife

Several herbicides can be used on Purple Loosestrife. However, if the area you are treating qualifies as a wetland, you may need to hire out the work to someone certified.

But spot treating or hand painting foliage can work using various herbicides such as aquatic formulations of 2-4D, Glyphosate or Triclopyr. Don’t go spraying some regular herbicide you got at a big box store near water! [1][2]

Biological Control of Purple Loosestrife

Since 1992, the USDA has approved the release of several European beetle species to be used for controlling Purple Loosestrife biologically. There are two species, Galerucella calmariensis and G. pusilla that will eat every part of the plant (but not other flora), thus preventing it from dispersing seed without harming surrounding native plants. Additionally a weevil was introduced that eats the roots, and sometimes killing them outright. [6][7]

The success rate has been mixed, as surveys in British Columbia showed that near oceans and in tidal areas the beetles failed to establish. Researchers also found that predation on beetle eggs was a problem in some areas.[8] More recent surveys also found mixed results, with noticeably less flowers present, but the overall biomass of Purple Loosestrife was unchanged when compared with native, non-invaded areas. [9]

Growing at a local creek near my home.

Final Thoughts

Of all the invasive plants I’ve encountered, I must say that Purple Loosestrife would be one of the prettiest were I ignorant to it’s environmental impact. The seeds ability to float and spread give it a great vector to spread along waterways and creeks (a local creek is where most of my photos were taken). The fact that the tiny seeds can stick to clothes, boots, boats, and animal fur only enhance it’s spread.

This is one plant we will probably never get rid of, but fighting off infestations is a cause we should do if we are able. Every acre of native flora protected will help maintain our biodiversity, and thus our environment. It’s like the old saying – “How do you eat an elephant?” One bite at a time.

Find more invasive plants here

References:

[1] – Urbatsch, Lowell; Skinner, Mark; Purple Loosestrife Plant Guide, USDA. Accessed 16DEC2022.

[2] – Purple Loosestrife. Invasive plants of Ohio. Fact Sheet 4. Retrieved 16DEC2022

[3] – Feller-Demalsy, M. J., and J. Parent. “Analyse pollinique des miels de l’Ontario, Canada.” Apidologie 20.2 (1989): 127-138.

[4] – EDDMapS. 2022. Early Detection & Distribution Mapping System. The University of Georgia – Center for Invasive Species and Ecosystem Health. Available online at http://www.eddmaps.org/; last accessed December 15, 2022.

[5] – Jeays, Linda. Purple Loosestrife Good News For Butterflies, Trail & landscape by Ottawa Field-Naturalists’ Club. vol 42, no2, 2008 pp.54-57.

[bio1] – Malecki, Richard A., et al. “Biological control of purple loosestrife.” BioScience 43.10 (1993): 680-686.

[bio2] – Hight, Stephen D., et al. “Establishment of insect biological control agents from Europe against Lythrum salicaria in North America.” Environmental Entomology 24.4 (1995): 967-977.

[bio3] – Denoth, Madlen, and Judith H. Myers. “Variable success of biological control of Lythrum salicaria in British Columbia.” Biological Control 32.2 (2005): 269-279.

[bio4] – St Louis, E., M. Stastny, and R. D. Sargent. “Impacts of biological control on the performance of Lythrum salicaria 20 years post-release.” Biological control (2020).

Joe Foster

Hi - I grew up outdoors in nature - hiking, fishing, hunting. In high school I got my first job at a garden center where I learned to garden and landscape. I've been growing plants from seed and designing native plant gardens for over 10 years. I hope to share some of my knowledge with you! You may have seen some of my videos I create on our YouTube channel, GrowitBuildit (more than 10 million views!). You can find my channel here: https://youtube.com/@growitbuildit Additionally I am a wood worker / DIY enthusiast. I enjoy designing/building projects (with hand tools when I can!). I hope to give you some tips and useful information!

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