Purple loosestrife
Family: Lythraceae (Loosestrife)
Other Names: purple lythrum
Legal Status: Colorado Noxious List A (general weeds)
Growth form: Perennial forb or woody sub-shrub.
Flower: Flowers are purple with 5-7 petals arranged in long vertical racemes.
Seeds/Fruit: Fruits are many-seeded capsules, seeds are small and ovoid.
Leaves: Leaves are simple, entire, and opposite or whorled (Whitson et al. 1996).
Stems: Annual stems arise from a perennial rootstock (Mal et al. 1992). Stems are erect, and often grow 6-8 feet tall. Plants become taller and bushier over the years as the rootstock matures.
Roots: Short rhizomes and taproot.
Seedling: No information available.
Similar Species
Exotics: Sometimes confused with Dame’s rocket (Hesperis matronalis), an exotic mustard.
Natives: Sometimes confused with fireweed (Epilobium spp.), which has 4-petaled flowers.
Agricultural: No information available.
Ecological: Purple loosestrife is an ornamental species that often escapes to sites such as streambanks or shallow ponds. The invasion of purple loosestrife leads to a loss of plant diversity, which also leads to a loss of wildlife diversity (Bender and Randall 1987). Purple loosestrife germinates at such high densities that it outcompetes native seedlings. Dense infestations can impede water flow in canals and ditches. When placed under moisture stress, purple loosestrife may produce additional roots, which may give it a competitive advantage over other species. It is an aggressive invader of wetlands. Once it is established, it often displaces native wetland species and degrades wildlife habitat. If purple loosestrife is left unchecked, the wetland eventually becomes a monoculture of loosestrife (Bender and Randall 1987). Where purple loosestrife is competing with cattails, it is favored by fluctuating water levels because marsh drawdown aids in seedling establishment (FEIS 1996). However, where water levels remain constant and relatively deep, cattails may be able to outcompete purple loosestrife (FEIS 1996).
Human: No information available.
Habitat and Distribution
General requirements: Purple loosestrife usually occurs in marshes, wet meadows, stream margins, shores of lakes and wetlands. A few of its most common associates include cattail, reed canarygrass (Phalaris arundinacea), sedge (Carex spp.), bulrush (Scirpus spp.), rush (Juncus spp.), and willow (Salix spp.). Purple loosestrife can tolerate a wide range of conditions (up to 50% shade), can grow on calcareous and acidic soils (Rutledge and McLendon) and will even grow in standing water.
Distribution: In Colorado, purple loosestrife is known to occur in the Denver/Boulder area and along the South Platte River, in Mesa County along the Colorado River, in Montrose County near Nucla, in Otero County near the Arkansas River, and in Colorado Springs. It is not known if purple loosestrife has upper elevational limits, but since it grows successfully in Canada, it should be considered a threat at higher elevations in Colorado.
Historical: Purple loosestrife is a native of Eurasia and was first recorded in America in 1814 (Bender and Randall 1987).
Life cycle: Purple loosestrife begins its growth about a week to 10 days after cattail and reed canarygrass. Spring established seedlings grow rapidly and produce flowers 8 to 10 weeks after germination. After flowering, each stem supports a dense spiraling row of dark-brown seed capsules.
Mode of reproduction: Purple loosestrife is a perennial that reproduces by seeds and rhizomes. The rootstalk of purple loosestrife is the main organ of local propagation; therefore, wide vegetative spread is unlikely. However, detached root or stem fragments can take root and develop into flowering stems (FEIS 1996).
Seed production: A single flowering stalk can produce 300,000 seeds, and densities as high as 80,000 stalks per acre have been recorded (FEIS 1996).
Seed bank: Purple loosestrife seeds may remain viable for up to 20 years.
Dispersal: Seeds are mainly distributed by water, but can also be dispersed by animals and humans. Seeds do not drop from capsules until the air temperature becomes cold in the early fall.
Hybridization: No information available.
Keys to Control:

  • Prevent new seeds from being added to the seed bank.
  • Maintain a healthy cover of perennial plants.
  • Any control effort should be followed up the same growing season and for several years afterwards.

Biocontrol: There are several biological control agents that show potential for controlling purple loosestrife (Rutledge and McLendon). The root-boring weevil (Hylobius transversovittatus), can seriously damage the root system of purple loosestrife, stunt growth, and reduce seed production (Rutledge and McLendon). This species is being reared at the Division of Plant Industry’s Insectary, but is currently unavailable for general redistribution. Two leaf-eating weevils Galerucella calmariensis and G. pusilla also show potential as biological control agents (Rutledge and McLendon). Experimental releases of Galerucella in the Denver area by the Bureau of Reclamation have become established and appear to be providing effective control of purple loosestrife (D. Weber, pers. comm.). These two species may be available for redistribution upon request.
Mechanical: Hand removal of isolated individuals can be effective on a small scale. Pulling should be conducted prior to seed set. It is important to remove the entire rootstalk of the plant to avoid regrowth from root fragments.
Fire: No information available.
Herbicides: Purple loosestrife is found in very wet soils, thus great care should be used when using herbicides as these may endanger other water plants (Rutledge and McLendon). Glyphosate (in an aquatic formulation such as Rodeo) is commonly used to control purple loosestrife (Rutledge and McLendon). A non-ionic surfactant must be mixed with the Rodeo prior to spray application. The safest method is to cut off all stems about 6 inches from the bottom of the plant and then spray or drip glyphosate (20-30% solution) onto the cut surface (Rutledge and McLendon), however, it is more effective to spray individual plants using a backpack sprayer (D. Weber, pers. comm.). Colorado research by the Bureau of Reclamation has shown that loosestrife plants sprayed with glyphosate will still produce viable seeds if the flowers are 50% or more developed on the stalk when spraying occurs. Therefore, flower heads must be cut and hauled away to prevent seed spread if they are mature when sprayed (D. Weber, pers. comm.).

Colorado Riparian Association