NOVEMBER 2008

Washington State University
WHITMAN COUNTY EXTENSION

STEVE'S       

Purple Loosestrife

Also Known As: spiked loosestrife, salicaire, rainbow weed, purple lythrum

Purple Loosestrife is a Class B Weed. In regions where a Class B species is already abundant, control is decided at the local level, with containment as the primary goal.

Purple Loosestrife, (Lythrum salicaria L.), native to Eurasia, is a perennial that thrives in aquatic habitats. The plant has a woody taproot and fibrous rhizomes that form a dense mat. Mature plants can have from 30 to 50 stems arising from a single rootstock. The erect stems are square and woody, and average 5 feet in height, but can grow up to 10 feet tall. Leaves are simple, entire, lance-shaped, and opposite or whorled. Magenta- or purple-colored flowers are arranged in showy spikes, generally blooming from June to October. Each flower has five to seven petals surrounding a small yellow center. Seeds are highly viable and abundant. A mature plant can produce well over 2 million minute seeds in one season. Besides reproducing by seed, purple loosestrife can spread vegetatively by resprouting from cut stems and regenerating from root fragments and stem pieces. A highly invasive plant, purple loosestrife is able to form dense, homogeneous stands that impede water flow and displace native vegetation, reducing wildlife food sources and habitat. Purple loosestrife is on the Washington State Department of Agriculture quarantine list, prohibiting the buying, selling, and transport of purple loosestrife within the state of Washington. 
                                                                       Photo: Steve Dewey Utah State University                                                                                                                                             

Photo: Agri-Food Canada

Control Methods

Size, age and location of purple loosestrife infestations should be considered when selecting control methods.

Physical/Mechanical/Cultural: Small infestations can be removed byhand, preferably before plants flower. To avoid new growth, it is important to remove and properly dispose of all plant materials, including root fragments. While younger plants can be pulled, older plants can be dug or treated with appropriate herbicides. A cut-stem method can be used, where one person cuts the plant and coats the stump surface with herbicide while another person bags the severed plant top. The site should be monitored for several years so that any regrowth can also be treated. Mowing and burning are not recommended.

Chemical: Several herbicides are registered for purple loosestrife control. Because the plant tends to populate riparian areas, care should be taken when selecting and applying chemicals. Among those commonly used, glyphosate-type herbicides (such as Rodeo® for wetlands, Roundup® for uplands) should be applied at first bloom. To avoid injuring nontarget plants, apply as spot treatments. Also effective, triclopyr (Garlon® or Garlon 3A® for wetlands) and imazapyr (Arsenal®). For large infestations, 2,4-D is sometimes used.

More information can be found in the
PNW Weed Management Handbook.

Refer to the label for all herbicides
Always follow label directions prior to use

Biological: Biological control agents should be considered for larger infestations. Some infested areas of purple loosestrife in Washington have been completely managed by using biological control agents. Biocontrol insects worth considering are two leaf-feeding beetles, Galerucella calmariensis and G. pusilla. Grazing is not effective in controlling purple loosestrife.

Photo: UGA 1291008

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Questions: contact Steve Van Vleet or phone (509) 397 - 6290