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An Assessment of Exotic Plant Species of Rocky Mountain National Park

Lythrum salicaria L.
Purple loosestrife, purple lythrum

Current level of impact
Known locations in RMNP: Presently not in Park, found in Jefferson, Larimer and Boulder Counties.

Origin: Introduced from Eurasia and Africa.
Geographic distribution: Locally abundant in Newfoundland, Quebec, New England, and Minnesota. South to Virginia and Missouri, also in western Washington.
Ecological distribution: Cultivated ornamental, now escaping and established in moist sites. Marshes, wet meadows, stream margins, shores of lakes. Often escapes to aquatic sites such as stream banks or shorelines of shallow ponds. Wetlands such as cattail marshes, sedge meadows, and bogs.
Soils: Commonly found in moist soils fertile soils. Can tolerate a wide range of conditions and can be found growing on calcareous and acidic soils. Also can withstand flooding of up to 30-45 cm.

Perennial, reproduces by seeds and rhizomes. Flowers June to September. New shoots arise from buds at the tops of rootstocks in spring. Can spread vegetatively from cut stems and pieces of rootstocks. However, vegetative spread is generally limited.
Seed production: A prolific seed producer. Each capsule contains approximately 120 seeds and an average plant may contain approximately 900 capsules.
Seed dispersal: Seeds are mainly dispersed by water (seeds are buoyant and can be carried by water currents). Seeds may also be dispersed by wind, livestock, and other animals. Humans can inadvertently carry seeds on clothing and shoes.
Seed longevity: Seeds may remain viable for several years.
Germination: Requires warm temperatures from 15-20 C and at least a 13 hour photoperiod to germinate (usually spring/summer). Spring germinated seeds generally have a higher survival rate than seeds which germinate in summer. Seeds are small and contain small food reserves so germination must occur in conditions where photosynthesis can occur rapidly.

An extremely successful invader of wetlands that have undergone disturbance. Expansion in wetlands can be rapid due to the prolific seed production and high seed viability. Purple loosestrife germinates at such high densities that it out competes native seedlings. In addition, loosestrife has low nutrient requirements, but has a high capacity for nutrient uptake. In poor conditions, for example, purple loosestrife produces additional roots which may give it a competitive advantage over other species.
Level of impacts: Often chokes out native vegetation. Reports of reduced wildlife habitat are common. loosestrife infestations can become dense and impede water flow in canals and ditches. Rapidly and aggressively spreading, noxious in some north-central states.
Shade: Can tolerate up to 50% shade.

Purple loosestrife usually is found in very wet soils, thus great care should be used when using herbicides because these may endanger other water plants. loosestrife populations which extend over three acres are difficult to eradicate and may be a better target for containment rather than control.
Mechanical: Mowing, burning, and flooding have been largely ineffective in controlling purple loosestrife. Cutting followed by flooding has show some success. Flooding alone may enhance seed germination, and burning may reduce competition from native vegetation. Hand removal of isolated individuals (1-2 years old) can be effective on small scales. Plants should be pulled prior to period when plants set seed, and care should be taken to avoid scattering seeds when pulling plants. The entire rootstock should be removed because plants can reproduce from root fragments.
Chemical: Moderately resistant to 2,4-D, but very resistant to many other chemicals. Glyphosate (Roundup or Rodeo) is commonly used to control purple loosestrife. Roundup cannot be used to control purple loosestrife over water. Rodeo has been cleared for use over water, but it is non-specific to loosestrife. Broadcast spraying with Rodeo kills all of the vegetation and may result in an increase in loosestrife density because of the removal of competing vegetation. As a result, it is recommended that Rodeo be directly applied to purple loosestrife. The safest method is to cut off all stems about 6 inches and then spray or drip glyphosate (20-30% solution) onto the cut surface.
Biological: Several biological control agents show potential for controlling purple loosestrife. The weevil (Hylobius transversovittatus) shows the most potential for control. Weevil larvae and adults seriously damage the root system of purple loosestrife. Field studies show that plants may be killed by the attack, have stunted growth, or reduced seed output. Up to 100% of the roots may be infested with these root weevils. Two leaf eating beetles also show potential as biological control agents. Galerucella calmariensis and G. pusilla feed on the buds, leaves and stems. The adult and larvae prevent normal growth of purple loosestrife by destroying the meristemetic regions. At high densities, these beetles can almost entirely defoliate the plants, and which ultimately prevents seed production.


Bender, J. 1988. Element stewardship abstract for purple loosestrife (Lythrum 
    salicaria). The Nature Conservancy, Minneapolis. 7 pp.

Heidorn, R., and B. Anderson. 1991. Vegetation management guideline: Purple 
    loosestrife (Lythrum salicaria L.) Natural Areas Journal I 1: 172- 173.

Hight, S.D. and J.J. Drea, Jr. 1991. Prospects for classical biological control 
    project against purple loosestrife (Lythrum salicaria L.). Natural Areas Journal 
    I 1: 151 - 157.

Kok, L.T., T.J. McAvoy, R.A. Malecki, S.D. Hight, JJ. Drea, and J.R. Coulson. 1992. 
    Host specificity test of Hylohius transversovittatus Goeze (Coleopera: 
    Curculionidae), A potential biological control agent of purple loosestrife, Lythrum 
    salicaria L. (Lythraceae). Biological Control 2: 1-8.

Kok, L.T., T.J. McAvoy, R.A. Malecki, S.D. Hight, J.J. Drea, and J.R. Coulson. 1992. 
    Host specificity tests of Galerucella calmareinsis (L.) and C. pusilla (Dutt.) 
    (Coleoptera: Chrysomelidae), potential biological control agents of purple 
    loosestrife, Lythrum salicaria L. (Lythraceae). Biological Control 2:282-290.

Mal, T.K., J. Lovett-Doust, L. Lovett-Doust, and G.A. Mulligan. 1992. The biology 
    of Canadian weeds. 100. Lythrum salicaria L. Canadian Journal of Plant Science 

Malecki, R.A., B. Blossey, S.D. Hight, D. Schroeder, L.T. Kok, and J.R. Coulson. 
    1993. Biological control of purple loosestrife. BioScience 43:680-686.

Stumpf, J.A. 1994. Lythrum salicaria L. pp. 98-105 In: An Assessment of Exotic 
    Plants at Scotts Bluff National Park and Effigy Mounds National Monument. 
    University of Nebraska, Lincoln Nebraska.

Thompson, D.Q. 1991. History of purple loosestrife (Lythrum salicaria L.) biological 
    control effects. Natural Areas Journal I 1: 148- 150.

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