NPWRC :: Species Abstracts of Highly Disruptive Exotic Plants

Northern Prairie Wildlife Research Center

Taxonomic Description:
Purple loosestrife is an erect perennial forb (up to 12 dm tall) with a strongly developed taproot. Its four-angled stem is glabrous to pubescent. Leaves are sessile, opposite or whorled, lanceolate (2-10 cm long and 5-15 mm wide), with rounded to cordate bases. Margins are entire. Leaf surfaces are pubescent. The inflorescence is spike-like (1-4 dm long), with numerous spikes per plant. The floral tube is cylindrical (4-6 mm long), greenish, and eight- to twelve- nerved. Calyx lobes number six. The corolla (up to 2 cm across) is rose-purple and consists of five to seven petals. The flowers are trimorphic with regard to the relative lengths of the stamens (mostly 12) and style. The fruit is a capsule (4-7 mm in diameter) with many small, ovoid seeds (< 1 mm long). Two species, winged loosestrife (L. alatum Pursh) and California loosestrife (L. californicum T. & G.), are native species similar in appearance. They differ from purple loosestrife by having solitary or paired flowers in the leaf axils, rather than many flowers in terminal spikes, and six to eight stamens rather than twelve.

Biology/Ecology:
Purple loosestrife is a highly aggressive, perennial wetland plant native to Eurasia. It was introduced to the northeast coast of North America in the early 1800's as a contaminant of European ship ballast and as a medicinal herb. It has subsequently spread to most of the temperate parts of the United States and Canada and now occurs in dense stands throughout the northeastern United States, southeastern Canada, the Midwest, and in scattered locations in the western United States and southwestern Canada. Purple loosestrife is found in wetlands such as cattail marshes, sedge meadows, and bogs. It occurs along stream and river banks and lake shores. Purple loosestrife is also found in ditches and other disturbed wet soil areas. In addition, the use of purple loosestrife as an ornamental has contributed to its' spread. However, a number of sterile varieties are now available.

Purple loosestrife prefers moist, highly organic soils but can tolerate a wide range of environments. It is found growing on calcareous and acidic soils, can withstand shallow flooding, and tolerates up to 50% shade. Purple loosestrife can also withstand poor mineral nutrient conditions, as it has low nutrient requirements and a high capacity for nutrient uptake. It has been shown that under poor nutrient conditions the root/shoot ratio increases providing purple loosestrife with a natural competitive advantage over other wetland species. Purple loosestrife requires warm temperatures (15-20°C) and at least a 13-hour photoperiod in order to germinate. Since seeds are small, weighing about 0.06 mg each, and carry little food reserves, germination must occur under conditions in which photosynthesis can occur quickly.

Purple loosestrife begins to bloom in July and continues until September or October. Flowering occurs 8-10 weeks after germination and starts at the bottom of the inflorescence. The lowest capsules on the inflorescence will ripen and disperse their seeds while flowering is still occurring further up the spike. Each capsule contains approximately 120 seeds with an average plant having approximately 900 capsules. Purple loosestrife seeds are mostly dispersed by water but are also dispersed by wind, and in mud adhering to aquatic wildlife, livestock, and people. Seed can remain viable underwater for several years.

Germination occurs with the onset of favorable conditions, usually in late spring. Spring-germinated seedlings have a higher survival rate than summer-germinated seedlings. Seedlings that germinate in the spring grow rapidly and will produce a floral shoot up to 30 cm in length the first year, while summer-germinated seedlings develop only five or six pairs of leaves before the end of the growing season.

The taproot is strongly developed in the seedling stage and persists throughout the life of the plant. In mature plants, the taproot and major root branches become thick and woody. The semi-woody aerial shoots die in the fall but persist for one to two years making stands of purple loosestrife very dense. New shoots arise the following spring from buds at the top of the rootstocks. Purple loosestrife can spread vegetatively by resprouting from cut stems and regenerating from pieces of root stock.

Distribution:
An intermediate number of populations of purple loosestrife occur at Effigy Mounds National Monument (EFMO) combining to cover an area of 5 to 10 hectares. These plants are found in regularly disturbed sites. Currently, populations occur in the North Unit along the Mississippi River. Purple loosestrife has the potential to invade and replace native communities and poses an endangerment to the areas' primary resources. It has a major visual impact on the vegetation of EFMO.

Control:
Several control methods have been used on purple loosestrife with varying levels of success. An important consideration in controlling this plant is that it is a prolific seed producer. These seeds are readily dispersed by water and have the potential to remain viable for several years. Also, this plant can spread vegetatively by resprouting from cut stems and regenerating from pieces of root stock. Most control methods have a detrimental effect on other plants and result in disturbances that favor reinvasion by purple loosestrife or other exotic species. Once established, purple loosestrife displaces native vegetation. In addition, it is important to note that plants of similar appearance are present in the area and should not be subjected to control. At a distance purple loosestrife may resemble plants of the mint family or Liatris spp.. Proper identification is important.

Prior to applying control, natural area managers must determine their objectives. It may be more feasible to contain a population of purple loosestrife than control it. Large populations extending over one hectare or more will be difficult to completely eradicate. It may be more feasible to contain large populations at their present position. This can be accomplished through hand-pulling new plants along the periphery or applying herbicides on plants extending beyond the main population. If the loosestrife cannot be controlled, then efforts should be concentrated on keeping it out of the highest quality areas.

Mowing, burning, and flooding have been found to be largely ineffective. Cutting followed by flooding so that cut plant stalks are completely immersed has shown some success. However, flooding may encourage the spread of purple loosestrife from seed present in the soil. Cut plant parts may also root. Mature plants can withstand short-term immersions. Burning has been ineffective, as there is no good time during the year for burning. Burning also stresses native plants and may subsequently enhance loosestrife spread.

Hand removal is recommended for small populations and isolated stems. Younger plants (one to two years old) can be hand pulled. Plants should be pulled, prior to seed set, with minimal disturbance to the soil. Pulling after seed set will scatter seed. The entire rootstock must be pulled out since regeneration from root fragments is possible. Older plants, especially those in bogs or in deep organic soils, can be dug out. Roots of older plants can be "teased" loose with a hand cultivator. It is important to remove uprooted plants and broken stems from the area since they can resprout. Plants should be bagged and removed from the site. Failure to bag could result in spreading the plants along the exit route. Follow-up treatments are recommended for three years after plants are removed. If seeds were present, clothing and equipment should be cleaned. Replacement control has been attempted in several wildlife refuges. Research has shown that Japanese millet (Echinochloa frumentacea Link) seedlings outcompeted purple loosestrife seedlings. The millet must be planted immediately after marsh drawdown and replanted each year, as it does not regenerate well. Replacement treatment has also been attempted using native seed. Pale smartweed (Polygonum lapathifolium L.) was seeded into, and outcompeted purple loosestrife. However, purple loosestrife begins to grow first in the spring. Replacement methods have a limited application in natural areas but may be useful to control or contain loosestrife populations on bordering property.

Various chemical treatments have been used on purple loosestrife with varying success. Many herbicides are not specific to purple loosestrife and may not be specifically licensed for the uses discussed here. It is important to read and follow all label directions. In areas with populations exceeding 100 plants (up to 1.6 ha in size), spot application of a glyphosate herbicide to individual purple loosestrife plants is the recommended treatment where hand-pulling is not feasible. Glyphosate is available under the trade names Roundup and Rodeo. Rodeo is registered for use over open water and is the most commonly used herbicide to control purple loosestrife. Glyphosate is nonselective and should be applied carefully. Excessive application, allowing the herbicide to drip, can kill desirable plants under the loosestrife. Application to the tops of plants alone can be effective and limit exposure of nontarget species.

Herbicide treatment should be conducted as early as possible during the manufacturer's recommended time of application in order to kill the plants and prevent seed production. Application is most effective when plants have just begun flowering. Timing is important because seed set can occur if plants are in mid- to late flower. Where possible, the flower heads should be cut, bagged, and removed from the site prior to application to prevent seed set. Rodeo should be applied as a 1.5% solution (2 oz. Rodeo/gallon clean water) with the addition of a wetting agent, as specified on the label.

Another option, which may be more effective, is to apply glyphosate twice during the growing season. The plants should be sprayed as described above when flowering has just started and a second time two to three weeks later.

Application of ghyphosate from a vehicle-mounted sprayer is usually necessary in areas with extensive stands of purple loosestrife. The most effective control can be achieved by beginning treatment at the periphery of large patches and working toward the center in successive years. This technique allows native vegetation to reinvade the treated area as the loosestrife in eliminated.

A combination of 2,4-D and Banvel (dicamba) has been used on a limited basis. This formulation is broadleaf specific and apparently would not hurt the dominants if sprayed in a cattail marsh. Once the loosestrife has reached 10-15% of its mature growth, it can be sprayed with good results. Treatment should be repeated once during the growing season.

Several biological control agents have the potential to aid in the control of purple loosestrife. Of 120 species of phytophagous insects associated with purple loosestrife in Europe, 14 species were considered host-specific to the target plant. From this group, six species were selected as the most promising for biological control. These species were a root-mining weevil, Hylobius transversovittatus Goeze, which attacks the main storage tissue of purple loosestrife; two leaf-eating beetles, Galerucella calmariensis L., and Galerucella pusilla Duftschmid, which are capable of completely defoliating the plant; two flower-feeding beetles, Nanophyes marmoratus Goeze and Nanophyes brevis Boheman, which severely reduce seed production; and a gall midge, Bayeriola salicariae Kieffer, which similarly reduces seed production by attacking the flower buds.

The most promising insect appears to be the root-mining weevil, H. transversovittatus Goeze. Preliminary studies have shown that weevil larvae and adults seriously damage the root system of purple loosestrife. Plants may be killed by the attack or stunted and produce fewer seeds. Field studies have shown that nearly 100% of roots may be infested.

The two leaf-eating beetles, G. calmariensis L. and G. pusilla Duftschmid also appear to be promising. These beetles defoliate plants and destroy the flowers. Preliminary studies have shown that feeding by the leaf beetles had a substantial impact on the growth and survival of purple loosestrife. At high densities of attack (200 larvae/plant), plants were entirely stripped of all green tissue, leaving only whitish skeletons and thereby preventing seed production. At lower populations of the beetles, adult and early larval feeding prevented normal growth of purple loosestrife by destroying meristematic regions. In outbreak densities the beetles killed seedlings, completely defoliated mature plants, and destroyed or prevented the formation of flower spikes.

In June 1992, H. transversovittatus, G. calmariensis, and G. pusilla were approved by USDA-APHIS for introduction in the United States. The insects were released in New York, Pennsylvania, Maryland, Virginia, Minnesota, Oregon, and Washington. Stocks of the three beetles were also sent to Canada, where their release was also approved.

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Species Abstracts of Highly Disruptive Exotic Plants

at Effigy Mounds National Monument

Lythrum salicaria