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.
Distribution
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.
Reproduction
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.
Competition
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.
Control
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.
References
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
72:1305-1330.
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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