Purple loosestrife

Identification & Biology
Alias' : Spiked loosestrife, purple lythrum, blooming sally, rainbow weed
Latin Name : Lythrum salicaria
Category : Terrestrial Plants
Description :
  • Perennial plant that grows 0.5 to 2.0 m in height
  • Dense spikes of showy pinkish-purple flowers at the tops of stems. Each flower has 5-7 petals that are approximately 10 mm long
  • Stiff, four sided stems
  • Narrow, stalkless leaves 3-10 cm in length, sometimes covered in short hairs
  • Seeds are extremely tiny, roughly the size of a grain of sand


Purple loosestrife is a perennial plant that reproduces both by seed and vegetatively through root fragments. It can produce up to 2.5 million seeds per plant. Seeds disperse in the fall and stay dormant over winter before germinating in the spring. Seeds can remain viable for up to 20 years! Seeds are mainly dispersed by water, but can also be dispersed by wind, waterfowl, wildlife, and humans.

Purple loosestrife is commonly confused with fireweed (Chamaenerion angustifolium), a native plant. It can be distinguished from loosestrife by its 4 petaled flowers and unique leaves with veins that are circular and do not terminate at the leaf edges. Purple loosestrife may also be confused with the introduced Dame’s rocket (Hesperis matronalis) which can be distinguished by its 4 petaled flowers, long seeds pods, and alternate leaves. As well, the blooming period of Dame’s rocket is in the spring, much earlier than loosestrife.

Look Alikes

Native to Eurasia, purple loosestrife prefers moist soils which can be found near wetlands, lake and river shores, ditchbanks, marshlands, freshwater tidal flats, and riparian meadows. However, purple loosestrife can tolerate a wide range of growing conditions including partially shaded areas, calcareous and acidic soils, and standing water. Sometimes if the plant is already well established it can survive on drier soil types such as pastures or croplands. In British Columbia, purple loosestrife is primarily in the south, particularly on Vancouver Island, the Lower Mainland, Fraser Valley, and the Okanagan.

Impact & Risks
  • Invades wetlands, stream and river banks, and shallow ponds where it displaces valuable habitat.
  • Can form monospecific stands, dominate aquatic plant communities, and reduce biodiversity.
  • Dense infestations can clog canals and ditches, impeding water flow.
  • Reduces the forage value of land for livestock, and decreases the palatability of hay.
Prevention & Mitigation

The most effective way to ensure that your lands do not become infested with purple loosestrife is by prevention. Here are some recommendations to prevent invasion:

  • Regularly patrol your property for purple loosestrife plants and immediately control or remove infestations before seed set.
  • Cooperate with adjacent landowners and encourage them to take action to reduce purple loosestrife spread.
  • Immediately re-vegetate disturbed, bare soils with a suitable seed mixture that provides dense, early colonization to prevent weed invasion.
  • Do not move contaminated soils to a new area.
  • Take special care when controlling purple loosestrife near streams, or ditch lines, to prevent the movement of seeds and plant parts downstream.
  • Clean, drain, dry’ boats and remove plant material and soil prior to leaving a loosestrife infested area.
  • A few native and ornamental alternatives to plant instead of purple loosestrife include: Blazing Star, Tall Delphinium, Bloody Iris, Hardhack, and Spike Speedwell.
Treatment & Disposal
  • Small infestations can be removed by hand pulling or digging before seed production and when plants are young. Since purple loosestrife reproduces through fragmentation, special care must be taken to ensure all plant parts are removed and don’t move downstream.
  • Tarp or bag all removed plants, plant parts, and seeds and dispose of at a landfill.
  • Mechanical control for large infestations has so far been unsuccessful.
  • There are currently no chemical control options for purple loosestrife in BC due to its proximity to water.
  • As a biocontrol option, the root boring beetle (Hylobius transversovittatus) and two species of leaf eating beetles (Galerucella calmariensis and Galerucaella pusilla) have proven effective against purple loosestrife in B.C.
Okanagan Distribution

Priority Level Definitions

Watch For - Poses a significant threat (very high risk) and does NOT presently occur in the region OR is relatively new to the region and is very limited in extent.
High - High risk/impact; limited population with significant potential to spread in the region.
Medium - Medium risk/impact; limited distribution – broader population distribution with potential to spread further in a region.
Low - Low risk/impact; may be widespread or not, may be of concern in specific situations with certain high values – e.g. specific agriculture crops. Some species may be treated primarily with biological control agents.