Purple Loosestrife

Purple Loosestrife (Lythrum salicaria)

Report

Photo credit: John D. Byrd, Mississippi State University, Bugwood.org

Management Category

ID Characteristics

General: Purple Loosestrife is a woody, shrub-like perennial that lives in wetlands. It is up to 3 m tall and 1.5 m wide, and mostly recognized by its showy flowers.

Flowers: Small, purple flowers that grow in dense, showy spikes. Each flower is about 7 – 10 mm wide.

Stems: The woody stems have 4 – 6 sides with stalkless leaves. They grow opposite one another or in spirals (whorls) along the stem. The plant grows up to 3 m tall and 1.5 m wide. New growth is green, but older stems are reddish-brown or purplish.

Leaves: Narrow and lance-shaped or triangular (broad near the base and tapering towards the tip), with smooth edges and fine hairs. Leaves are typically 3 – 12 cm long. They are green in summer and red in the fall.

Fruit and seeds: Seeds are enclosed in tiny, rounded capsules, 3 – 6 mm long and 2 mm wide. Each capsule contains over 100 flat, light brown seeds.

Roots: Strong, persistent taproot is deeply rooted and becomes woody with age.

Similar Species

Invasive

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Photo credit: J. Hallworth

Dame’s Rocket (Hesperis matronalis), which has 4 petals, alternate leaves, and seed pods that are longer than Purple Loosestrife’s.

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Butterfly Bush (Buddleja davidii),which is a deciduous shrub that lives in dryer conditions than Purple Loosestrife. Its leaves are also bigger (3-25 cm long), and its flowers have an orange eye.

Native

Hardhack (Spirea douglasii): Can be distinguished from Purple Loosestrife by its oval leaves and reddish brown, round stems. It is native in the Sea to Sky, but it grows in much drier soils.

Fireweed (Epilobium angustifolium or Chamaenerion angustifolium) is another native often mistaken for Purple Loosestrife. Its long (up to 20 cm) leaves and unbranched stems are a great ID characteristic. In addition, fireweed’s purple-pink flowers have 4 petals, and the leaves have wavy edges.

Blue Vervain (Verbena hastata) is sometimes mistaken for Purple Loosestrife, but its leaves have a rougher texture and doubly-serrated margins.

Exotic

Dotted Blazing Star (Liatris punctata) flowers are on a long curving stem and stiffer, more narrow leaves.

Lastly, Winged Loosestrife (Lythrum alatum) is in the same family as Purple Loosestrife but it is not invasive in the Sea to Sky. It is recognized by its winged bark, with edges that run parallel to the edge of the stem.

Habitat and Origin

In the early 1800s, Purple Loosestrife was accidentally introduced to eastern North America in the water ballast of a ship from Europe. The plant was also intentionally introduced as a medicinal plant and for beekeeping.

As many other attractive invaders, Purple Loosestrife also arrived via garden centres that stock the plant as an ornamental.

Purple Loosestrife thrives in wet areas, such as brackish or fresh standing water, including marshes, stream banks, pond edges, ditches, irrigation canals and lake or river shorelines. It prefers full sun, but can also tolerate partly shaded conditions.

How it Spreads

Purple Loosestrife reproduces by seed, with a single plant producing over 2.5 million seeds in a season. The seeds have an extremely high germination rate and remain viable in the soil or underwater for many years.

Purple Loosestrife seeds are small and abundant; they can be dispersed by the wind, or in water. Seeds are also spread by birds and wildlife. Purple Loosestrife is also spread through the horticultural trade, and pollinated by bees.

Impacts

Ecological:

Outcompetes native species for light, space and pollinators.
Reduces habitat and food availability for wildlife.
Dense stands trap sediment, which can alter the landscape and lead to a rise in the water table.
In the US, it is estimated that 200,000 hectares of wetlands are lost each year due to Purple Loosestrife.

Economic:

Clogs irrigation systems and obstructs waterways used by recreational boaters.
Honey produced from loosestrife-feeding bees is of low quality.

Prevent the Spread

Purple Loosestrife is found in the Sea to Sky region, but with a limited distribution. The goal is to eradicate this species from the region, and to prevent new introductions.

What to do if you spot it: You can report any Purple Loosestrife sighting by visiting our reporting page.

The best approach to controlling the spread of Purple Loosestrife is PREVENTION.

Maintain riparian and aquatic environments in a healthy condition to ensure a productive natural plant community
Regularly patrol your property for Purple Loosestrife plants and immediately control new infestations
Re-vegetate disturbed soils with native plants that provide dense, early colonization to prevent weed invasion
Clean boats of plant material and soil before leaving a loosestrife-infested area

Control

Mechanical control:

Small, isolated infestations of Purple Loosestrife can be hand-pulled or dug up.
The entire root of the plant should be removed to avoid re-growth from root fragments.
Pulling is easiest when plants are young. Older plants have larger root systems that are better removed with a garden fork or a spade. Pulling should also occur before seed production.
Purple Loosestrife reproduces through fragmentation, so special attention must be paid to ensure all plant parts are removed. This method should be applied before seed set to prevent scattering.
Cutting plants at the base may prevent seed spread and inhibit growth, but it should only be used as a stop-gap measure.
Mechanical control for large infestations has not proven successful.

Chemical control:

Since Purple Loosestrife is often found growing in wet soils or near waterbodies, herbicide application is often not possible (under Canadian and BC regulations).
In other countries, aquatic formulations of glyphosate have been effective. These formulations are not currently registered for use in Canada.
If site conditions permit the use of herbicides, best results are achieved by applying herbicide before flowering, to minimize seed production.

We recommend that any herbicide application be carried out by a person holding a valid BC Pesticide Applicator Certificate. Before selecting and applying herbicides, you must review and follow herbicide labels and application rates; municipal, regional, provincial and federal laws and regulations; species-specific treatment recommendations, and site-specific goals and objectives.

Biological control:

Two species of leaf-eating beetles, Neogalerucella clamariensis and N. pusilla have been released, and have proven effective against Purple Loosestrife in other areas of BC, including the Okanagan and the Lower Mainland. These beetles feed on the foliage and flower heads, stunting growth and reducing seed production of the plant.
Please contact SSISC to find out more about biocontrol.