Purple loosestrife (Lythrum salicaria) is an aggressive plant that spreads through extensive seed production - up to one million seeds per plant! It is native to Asia and Europe and was introduced to North America in the 1800s in ship's ballast and via imported wool. This weed is particularly devastating to riparian areas and wetlands.
Stems are woody, square and grow 1-1.5 m tall.
Leaves are stalkless, borne opposite (whorled at the base) and lance-shaped.
Flowers and Fruit
Purple loosestrife has purple-red (more rarely white or pink) flowers borne on a long stalk that blooms from the base upwards. Seeds are tiny (less than 1mm long) and can remain viable for 2-3 years under water.
Purple loosestrife prefers moist, highly organic soils often associated with wetlands, riparian areas, waterways and swamps. It tolerates shallow flooding and partial shade, which also helps it invade wet areas.
Once plants are established, their deep, strong taproots and extensive seed production make them very difficult to eradicate entirely. Purple loosestrife is highly invasive and will quickly choke out native vegetation, destroying nesting habitat for waterfowl and disrupting water flow through wetlands.
Purple loosestrife can sometimes be found for sale at garden centres, even listed under a different Latin name. Once an infestation is established, it is almost impossible to eradicate, so prevention and control of isolated plants is extremely important.
Mowing is not effective; however, hand pulling can eradicate small infestations. Any cut or pulled stalks should be bagged immediately and removed from site to prevent seed spread.
Glyphosate provides effective control if applied to individual plants or cut stems.
This plant is designated as Prohibited Noxious in Alberta, which means that under the Weed Control Act, it must be completely eradicated wherever it is found.