Purple Loosestrife – Lythrum Salicaria

What is it?

If there is any invasive species Canadians may want to keep, it is the purple loosestrife. One of the more beautiful invasive species, the purple loosestrife is appropriately named for the colourful plumes of pinkish-purple flowers blooming throughout summer. While eye-catching, the purple loosestrife is not just a pretty bloom; it was once widely regarded as one of the most aggressive and pesky invasive species threatening wetlands across North America!  

Purple loosestrife at Rockwood Conservation Area. Photo taken by Mandy Huynh.

Native to Eurasia, the purple loosestrife is an herbaceous flowering wetland plant first introduced to eastern North America in the early 19th century. While some were planted in gardens for their showy blooms, others were brought over to help in beekeeping, due to the many blossoms, providing an abundance of nectar. Even more grew spontaneously around harbors and rivers due to sailors dumping ballast soil dug up in Europe and carrying the small seeds of the purple loosestrife. Like other hardy invasive species, it spread rapidly throughout North America and by 1992, became such a threat that it is one of the few invasive species which the Canadian government authorized the use of the controversial technique of biological control (releasing non-native species) to manage these flowers – releasing European leaf-eating beetles – the natural predator of the purple loosestrife into the environment to try to slow its spread. 

Galerucella calmariensis (a Leaf beetle) 1
Galerucella calmariensis, an European leaf-eating beetle species released to manage population of purple loosestrife. Photo taken by Jonathan, available on Flickr.

Although the release of these beetles reduced purple loosestrife populations by 90% in Ontario, it can still be found almost everywhere in North America, save for Florida, Alaska, and Hawaii. Purple loosestrife is especially common throughout the Great Lakes-St. Lawrence region where it can usually be found growing in wetlands, floodplains, and damp roadsides. 

Why you should care

Prior to the release of these beetles, rampant overgrowth by this deceptively pretty plant clogged our wetlands and waterways. Due to its aggressive and rapid growth, growing up to 30 flowering stems per plant, per year. Each of which produces millions of seeds every year that spreads easily by water, wind, and wildlife and stay dormant for decades before sprouting. It also regenerates from the smallest root fragments, allowing the purple loosestrife to overtake almost any shoreline and riverine habitat in Ontario. 

Once established, its dense thickets choke out other plants, while its thick roots make accessing waterways difficult and increase costs to keep riverways, and irrigation and drainage ditches clear of purple loosestrife. Most insidiously, this deceptively magenta bloom greatly degrades soil quality. Its dense growth strips vital nutrients, interrupt river flows, and alter downstream lakes, increasing the occurrence of scummy algal blooms; a real trickle-down effect. Native plants that are most at risk because of purple loosestrife in Canada include: 

  • The dense blazing star (Liatris spicata)
  • Eastern prairie fringed orchid (Platanthera leucophaea)
  • False hop sedge (Carex lupuliformis)
  • Swamp Rose-mallow (Hibiscus moscheutos)

The loss of these native plants reduces the amount of habitat, good foraging material, and shelter available to all wildlife, further degrading an infested wetland. Especially at risk are the spotted turtle (Clemmys guttata), and spiny softshell turtle (Apalone spinifera). Even beavers, critical to wetland ecosystem health, find purple loosestrife detestable. Perhaps the only good purple loosestrife brings is that at least the bees still find their nectar good! Were it not for leaf eating beetles like Galerucella calmariensis and G. pusilla, purple loosestrife populations would be even more rampant!

However, all of this is standard for most invasive species. Why then, does the purple loosestrife receive special attention?

As everyone knows, British Columbia, California, and most recently, Europe are all burning. With the looming threat of climate change, carbon sequestration has never been more important, and one of the most important carbon sinks in Ontario are our wetlands. 

Purple loosestrife in a wetland. Photo by Liz West, available on Flickr.

Although purple loosestrife is a fast-growing and expansive plant, it also damages and endangers the health of our wetland ecosystems, altering their hydrology and reducing their overall carbon sequestration efficiency. It may no longer be as common as it once was, and our wetlands are bouncing back with conservation efforts, but if we are to commit to fighting climate change, management of purple loosestrife and curtailing its impacts are just one of many ways we can all help. 

What can we do?

The case of the purple loosestrife in Canada is one of the few stories of successful invasive species management. The tightly regulated release of leaf eating beetles for the purpose of controlling the growth of purple loosestrife led to the purple loosestrife now viewed almost as just another Canadian wildflower. 

The best way to help is to not plant any; after all, purple loosestrife was originally and sometimes still planted for its attractive flowers. Its pinkish purple blooms can be confused for other garden plants and native wildflowers such as:

  • fireweed (Epilobium agustifolium), 
  • blue vervain (Verbena hastata
  • blazing stars (Liatris spp.)
  • winged loosestrife (Lythrum alatum
  • swamp loosestrife (Decodon verticillatus)

Follow the description below to see if you’ve got any purple loosestrife stealthing about in your backyard!

How to identify Purple Loosestrife

  • Height
    • Can grow between 1.5 – 3m tall 
  • Stem
    • Woody square stiff stems with hair
    • Each plant can produce between 30 – 50 erect stems
    • Each stem or root, when fragmented can grow into new plants
  • Leaves
    • in opposite pairs or whorls of three
    • lance-shaped leaves 3 -12 cm long 
    • normally green but can turn scarlet red in autumn
  • Flowers
    • Found at the top of the stems in dense footlong spikes of showy pinkish-purple blooms
    • Each flower has 5 – 7 petals, 10 millimetres long and small yellow centres 
    • Bloom from May to June
  • Fruit
    • 3–6-millimetre long and 2mm wide capsule resembling a dried brown umbrella or cup containing dozens to a hundred tiny 1-millimetre seeds 
    • Each seed resembles a tiny, reddish-brown wrinkled almond 

To prevent accidentally spreading fragments, always make sure watercraft are cleaned before and after use, and dried for at least 21 days before changing water bodies. Also, never dump water or soils that may hold purple loosestrife seeds from one location to another, and dispose of any fragments in the trash.   

Can you remove Purple Loosestrife?

Purple loosestrife is very well known to many conservation managers. Over the past 60 years, the best methods for controlling purple loosestrife have been determined and optimized to the point that purple loosestrife in the wild is no longer considered to need regular removals, and is carefully balanced with the beetles used to control it. 

As always, feel free to remove any purple loosestrife on your own property from which it could spread, with the younger the infestation, the easier it will be to manage it. However, you’re in for the long haul; this zombie plant can take years to clean up.  

You can remove whole stands, root and all, by hand for individual plants, but try shovels and weed pullers for larger plants. Never mow down purple loosestrife; the fragments will be like zombies growing back, and always remove any purple loosestrife that has been dug up to be dried for at least a week before disposal via burning. However, removing purple loosestrife always runs the risk of releasing fragments and seeds, so be sure to plan your removals during and before the summer, when the flowers are still blooming. Also check with your local municipality to see if they have any special instructions for disposal or composting programs for purple loosestrife! 

Because purple loosestrife tends to grow near the water, it is inadvisable to use fire or herbicides to control any stands. Fire will be ineffective due to the wet soils, while herbicides could poison downstream habitats. Also do not try flooding purple loosestrife; you will only spread it more.

If you see any plants that match the purple loosestrife, please contact the Invading Species Hotline at 1-800-563-7711 or the Natural Resources Information Support Centre at 1-800-667-1940 or 1-800-387-7011 to submit your observations! Also consider visiting the EDDMapS Ontario which has partnered with both organizations above to report sightings!

Interested in learning more? See also:

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