Common Name: purple loosestrife
Scientific Name: Lythrum salicaria
Symbol: LYSA2
Group: Dicot
Family: Lythraceae
Growth Habit: Subshrub; Forb/Herb
Duration: Perennial

What does it look like?

Purple loosestrife is a striking plant that grows less than five feet tall and displays brilliant, reddish-purple flowers. The flowers have a yellow center with five to seven pink-purple petals. Each year, purple loosestrife enlarges at the base to eventually become a round, bushy plant with thirty to fifty stems rising from a single rootwad. The leaves are long, narrow and opposite to whorled. The flowers on purple loosestrife are often less-widely spaced than those of other species (Bossard et al. 2000).

Purple loosestrife flowers from late June to September when seeds begin setting in mid-July and continues to flower into late summer. Each mature plant produces more than two million seeds. This invasive weed spreads primarily by seed, but it can also regenerate from broken stems and root segments. Most seedlings begin growing in late spring to early summer and can produce a floral shoot in the first year (Bossard et al. 2000).

Where does it grow?

Native to Eurasia, purple loosestrife lives in scattered freshwater wetlands in northern and central California (Bossard et al. 2000). It is a growing problem throughout Humboldt County and has been documented on 75 sites along an 18-mile stretch of the South Fork Eel River. Waterways often carry viable seed to other habitats, and there is great concern that loosestrife may spread to other major waterways in California (Jeff Dolf, pers. comm.).

The roots of purple loosestrife are suited for wet habitats, so it thrives in riparian areas. Adaptive root structures allow it exist in a variety of habitats and wide range of soil moistures, soil types, temperatures, and light availability (pers. comm. Jeff Dolf).

Is it in our watershed?

Purple loosestrife has not yet been identified in the Mattole River watershed. However, since it has been documented in this county, we should keep a vigilant watch for this highly invasive weed in the Mattole.

What problems does it cause?

Purple loosestrife has the ability to spread rapidly, often displacing native plant species and creating a monoculture, which replaces valuable food and habitat for native fauna. In riparian areas, loosestrife grows rapidly from large rootwads and often moves into the watercourse, trapping sediment behind them. This can alter hydrology, transform the streambank, and even move the course of rivers and creeks (pers. comm. Jeff Dolf).

How do you get rid of it?

Manual/Mechanical Removal: Manual removal can be successful in small, localized sites with fewer than one hundred plants that are one to two years old. To prevent scattering seeds, it is necessary to pull plants before they flower. In loose soils, it is preferable to dig and tease out the roots of older plants. After removal from the site, dry and burn the removed material (Bossard, 2000).

Mowing: If timed correctly, mowing can be a successful control by preventing flowering and seed set. However, poor timing can also lead to distribution of seeds and stem fragments, both of which can lead to large-scale reinfestation.

Biological Control: Studies in 1986 showed promise for the use of six species for biological control of purple loosestrife. A cecidomyiid fly (Dasineura salicariae) can reduce foliage by 75% and seed production by 80%; a stem and root boring weevil (Hylobius transversovittatus); two chrysomelids (Pyrrhalta calmariensis and P. pusilla) that can cause nearly 50% defoliation; and two weevils (Nanophyes marmoratus and N.brevis) can mine ovaries and seeds (Thompson, 1987).

Since 1997, three insect species from Europe have been approved for use as biological control agents by the U.S. Department of Agriculture, including the root-boring weevil (mentioned above) and two leaf-eating beetles (Galerucella calmariensis and Galerucella pusilla). In 1998, the State of California obtained a permit to conduct test releases of leaf-eating beetles (Galerucella spp.) where purple loosestrife suffered considerable damage in test release sites (Bossard, 2000).

Case Study: Removal efforts have been made on a small infestation of purple loosestrife off Highway 36 in the Van Duzen watershed, crews made repeated visits to the site. On less than 1/10 of an acre, they used spades to dig up the plants by the roots. The removal continued for four years, until the project appeared complete. Further monitoring revealed some resprouting and confirmed the need for continual monitoring and removal until the seedbank is depleted, (pers. comm. Lisa Hoover).

Author:
Unity Peterson, Invasive Plant Program Coordinator, Mattole Restoration Council, Petrolia, CA.

Photographs:
CDFA, 2001

References:
Bossard, CC., J.M. Randall and M. Hoshovsky. (eds.) 2000. Invasive Plants of California’s Wildlands. University of California Press, Berkeley, CA. Online version: http://groups.ucanr.org/ceppc/Invasive_Plants_of_California's_Wildlands/

Jeff Dolf, Biologist, Humboldt County Agricultural Commissioner’s Office, November 2004.

Lisa Hoover, Forest Botanist, Humboldt WMA, Six Rivers Forest, November 2005.

Thompson, Daniel Q., Stuckey, Ronald L. and Thompson, Edith B. 1987 Spread, Impact and Control of Purple Loosestrife(Lythrum salicaria) in North American Wetlands. U.S. Fish and Wildlife Service. 55 pages. Jamestown, ND: Northern Prairie Wildlife Research Center Online. http://www.npwrc.usgs.gov/resource/1999/loosstrf/loosstrf.htm