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Spread, Impact, and Control of Purple Loosestrife (Lythrum salicaria) in North American Wetlands

Containment Strategy and Tactics

If preliminary work on a biological control program were to continue, it would be 5 or 6 years under the most favorable circumstances before a release program could be started. With a few delays, it could be 8 or 10 years before relief could be expected from local purple loosestrife infestations. In the interim, containing the spread of existing infestations is obviously the best approach. As a background for containment strategy, we have constructed a map (Fig. 19) that displays the extent and intensity of purple loosestrife infestations across North America in 1980. We have defined "infestation" as one or more mature plants in each wetland unit or drainage basin. Within the past 15 years, the senior author has traversed sections of most of the States and Provinces within the boundaries of infestation during L. salicaria's period of bloom. A log of roadside sightings of purple loosestrife was kept; field observations and photographs were made of infestations of special interest. These observations, correspondence with other field observers, and botanical records compiled by Stuckey (1980) are the basis for the infestation map. The north and south boundaries of spread are intended to define the approximate limits within which L. salicaria can be expected to expand. There are already collection records or reports outside of these limits, but the rate of spread from these isolated sites has been slow. The probabilities of continued spread northward in British Columbia and southward in the lower Central Valley of California are particularly difficult to judge.

A useful perspective can be gained by noting the inverse relation between L. salicaria infestations (Fig. 19) and the distribution of wetlands in the northeastern and midwestern United States (Table 3). The four wetland types listed in this table are of prime importance in waterfowl and shorebird production. Totals for the Atlantic Flyway North versus Mississippi Flyway North are 37,400 and 1,881,400 ha, respectively; these estimates are adequate to underscore the need for prompt action in checking the westward expansion of purple loosestrife. The northeastern United States wetlands that are now almost completely colonized represent but a small fraction of the wetland resource at risk in the north-central States. The impact of L. salicaria on one of our major waterfowl areas (Mississippi Flyway North) will be determined within the next 10-20 years. The Central and Pacific flyways are as yet unaffected by purple loosestrife.

Minimizing Spread and Impact

The concept of classes or levels of infestation is useful in evaluating containment or control tactics. Table 7 characterizes three stages in the development of a local infestation. Obviously, these stages vary with the dynamics of each site. It is wise to keep the logistic curve in mind when judging the age of an infestation. The slow growth of biomass in Class I gives the landowner or manager the best opportunity for control or eradication. Class II infestations are possible to control, but the investment in effort and chemicals has roughly trebled. Class III infestations may be in a phase of exponential increase and are the most difficult and expensive to control.

Table 7. Purple loosestrife infestation classes.
Class Age (years) Growth form Percent of biomass
I 1-3 Scattered small plants with 1-5 flowering stems per rootstock. <5
II 3-5 Mature plants with 10 or more flowering stems per rootstock. Clumps sometimes coalescing and forming aggregate masses. 10-20
III 5 or more Aggregates closing to form large monospecific patches or strands >30

Early Detection

Although a stubborn and aggressive weed, L. salicaria has two characteristics that can be readily exploited to slow its spread and impact. First, its tall, showy, floral stalks and prolonged period of bloom immediately identify an established plant. Second, the rate of spread of purple loosestrife in undisturbed habitats is slow. Seeds or propagules falling from the parent plant need moving water (or surface drift) to expand into surrounding habitat. They also need to find a patch of moist soil that is exposed to sunlight to establish themselves as seedlings. Moreover, these conditions must prevail for about 50 days to allow the seedlings to grow to sufficient size to survive over winter. If an isolated adult plant has somehow become established in an otherwise healthy wetland, its seeds will fall in a 10-m-long ellipse on the downslope or downwind side of the plant, where they will remain viable for at least 3 years. If the emergent vegetation around the invader remains vigorous and undisturbed, the spread of these seedlings will be very slow. Managers whose units are within the limits of purple loosestrife distribution would do well to include an annual search for purple loosestrife in their work schedules. The search need not be highly organized or exclusively pursued, but it is important that it remain among each summer's plans. The blooms on mature plants are brightest and most obvious during July; however, the surveillance should be continued into the last half of the bloom season (August) when year-old plants or exceptionally vigorous seedlings of the current growing season may come into bloom. A 20-power spotting scope is useful for identifying floral stalks in large reaches of open marsh. The most likely wetland sites for the appearance of purple loosestrife are along water supply entrances, or on any disturbed area within the upper flood contour. Muskrat houses, old tree stumps, exposed bottom deposits, new dike or road fills, boat ramps, and duck blinds are likely sites for a drifting or transported loosestrife propagule to set roots. If there are infested pools or waterways adjacent to the management unit, crossings or pullovers by muskrats or snapping turtles are also possible routes of entry for purple loosestrife seeds.

Local Eradication

Early detection of the arrival of purple loosestrife is essential to successful local control. If the wetland management unit is completely circumscribed by dikes, roads, or other barriers to drift of local seed sources, the manager can successfully follow a program of local eradication. The first appearance of invading L. salicaria is usually a few scattered plants. A small field crew should be sent to these sites with instructions to dig or hand pull all purple loosestrife plants, including the root crown. Since fragments of purple loosestrife can reproduce vegetatively, it is important to carry all pulled material out of the marsh basin. If the plants are several years old, they will be too large and deep-rooted to be removed. In this event, a hand sprayer should be used to apply an herbicide directly on the leaves. Great care should be taken to avoid drift onto the weed's nearest neighbors; these plants are needed to close in the space occupied by the dying loosestrife clump. Spraying with glyphosate can be done any time after loosestrife foliage is well developed; however, Rawinski (1982) obtained best results with August sprays. If seedlings are established in the vicinity of the parent plants, herbicides should be applied and the area marked for continued treatment. With diligent followup and continued surveillance, local eradication is feasible. L. S. Smith (personal communication) was successful in eradicating L. salicaria from the Iroquois NWR in the early 1960's.

Coping with Infested Areas

If L. salicaria is well established within the management unit and several years of seed production have occurred, the opportunity to attempt local eradication has probably been lost. The unit is vulnerable to heavy infestation and wetlands managers must be cautious in exercising their options. If the area of infestation is concentrated and funds are available, the manager could embark on a salvage program. Spraying with glyphosate from air or by boat is the only effective means of delivery that does not physically damage the habitat; however, many pitfalls must be avoided in an aggressive control program. The high cost of aerial spraying with glyphosate makes chemical control over large marsh units impractical, especially if loosestrife clumps are scattered among areas of undisturbed native cover. Spraying of purple loosestrife interspersed in willow, dogwood, or other marsh shrubs should be avoided since these woody plants have the ability to shade out purple loosestrife as their canopies close. Carefully handled, and in concert with intensive work by field crews, spraying with glyphosate or other chemicals can be a successful tactic in the local control of purple loosestrife. This tactic, however, does nothing for the vast areas of purple loosestrife that are not under intensive management. These remain a constant seed source that can only be checked with a national strategy, that is, a biological control program.

Until a biological control program can be implemented, the key to coping with established L. salicaria is to avoid any manipulations or actions that might stress the native vegetation and allow L. salicaria seedlings to spring up from dormant seed stocks. The standard waterfowl management practice of early drawdown to encourage smartweed (Polygonum spp.) and millet seedlings on shallow impoundment margins is an open invitation to purple loosestrife dominance. Shallow midsummer flooding to provide dabbling duck foraging will often not be sufficiently deep to suppress young loosestrife seedlings. If reflooding is delayed by multiple stage coverage, seedlings may grow tall enough to survive as emergent aquatic plants.

If, for example, a water control structure must be repaired, drawdown should be delayed until mid-July. The peak of the growing season will have passed and L. salicaria seedlings will not have had sufficient time to become well established. If an early drawdown cannot be avoided, reflooding with at least 70 cm of water will kill most first-year plants.

Last, any form of disturbance or stress to the native plant community should be avoided. As suggested earlier, the shaded understories of wetland hardwood forests or shrub-carr stands will exclude L. salicaria. However, if these stands are flooded for more than half of their normal growing season, they will begin to die and purple loosestrife will become established as openings appear in the forest canopy. Similarly, the use of heavy equipment in open marsh that has been colonized by purple loosestrife will greatly favor the dominance of the weed. L. salicaria has the ability to reproduce from adventitious buds growing from crushed stem segments, whereas many of its native associates (Typha spp., Scirpus spp., and Phalaris arundinacea) do not survive crushing.

Moist Soil Management

Manipulating wetland edge soils is an effective habitat enhancement technique in southeastern Missouri (Fredrickson and Taylor 1982). Moist soil management involves discing or scarifying the topsoil to encourage the establishment of seedlings or propagules of desirable food or cover plants. The site is then flooded for part or all of the growing season and later exposed with a gradual drawdown. Depending on tactical goals, drawdowns could begin in April, or be delayed until August, with corresponding adjustments in the time of drawdown. Although moist soil management is usually practiced on sites that can be worked with power equipment—and therefore can be cultivated to control purple loosestrife—great caution should be exercised in using this technique in the presence of purple loosestrife. Moist soil regimens that use early drawdowns (first half of the growing season) will favor the establishment of purple loosestrife; on these sites the cost of L. salicaria control will probably outweigh the benefits of enhancement.

Control in Pastures, Roadsides, and Ditches

The perennial growth pattern of L. salicaria makes it susceptible to control by plowing or discing wherever these methods are feasible and cost effective. Spraying with glyphosate is also an effective control measure on ditchbanks or along roadsides that can be reached with power equipment.

Weed Control Law

Plant Quarantine

For the wildlife manager, the basic problem with weed control laws is that they are aimed at agricultural pests and are enforced by State and Federal agencies whose mission (as it should be) is protection of food and fiber crops. Purple loosestrife is not the first weed that primarily impacts fish and wildlife values (water hyacinth and alligatorweed are among previous examples in the subtropics), but it is surely the first weed that has threatened to dominate millions of hectares of temperate marsh and open riparian habitats across the continent. We are caught without effective laws or institutions to cope with this weed. Although Federal plant quarantine laws are of little use once an immigrant plant has become naturalized, State laws and enforcement could be very effective in shutting down the shipment of wild-type purple loosestrife as horticultural stock. These laws may be seen as a threat to wholesale suppliers, but if their stocks are truly infertile hybrids, they have nothing to lose but competition from less scrupulous suppliers. Amendments to existing State quarantine regulations should exempt infertile stock from quarantine. The identification of nursery stock may be an enforcement problem, but the key point should be bona fide infertility. This would have to be tested in trial plantings from time to time. Some sort of cooperative enforcement agreements between fish and wildlife enforcement officers and State plant quarantine authorities would need to be set up; similar arrangements are already in place to screen importations of wild animals at international air terminals.

Transportation and Sale of Seeds

The Federal Seed Act of 1939 (Robbins et al. 1942) regulates the transport and sale of seeds in foreign and interstate commerce. Although there is little likelihood of purple loosestrife occurring in grain or forage seed shipments, Wade (1983) showed that it is present in seed mixes offered by various wild plant suppliers. These sources of impure seed would need both Federal and State regulations to control. Pure seed regulations would also be a useful means of curtailing the sale of purple loosestrife seeds by apicultural suppliers.

Noxious Weed Status for Purple Loosestrife

Declaring purple loosestrife a noxious weed is a State-level strategy that is often highly effective. This action has already been taken by California, Idaho, and Ohio; three other States (Minnesota, Illinois, and Wisconsin) are considering similar action. Noxious status usually places the species under the State's pure seed surveillance and plant quarantine. In addition, most States have enabling legislation in place that provides for the establishment of weed control districts by landowners. Herein lies a potential conflict between agricultural and wildlife interests. The local weed board has the power to issue warnings to the managers of private and public lands that they must control noxious weeds on their land. If action is not taken by the landowner, the weed board has the right to send in their own spray crews and bill the landowners for the cost. The stress of systemic herbicides on natural vegetation plus the ruts and crushed plants caused by off-road spray rigs could have disastrous effects on the health and productivity of a wetland habitat. The Nebraska Game and Parks Commission estimates that it pays $45,000 each year for unwanted spraying (mostly for Canada thistle [Cirsium arvense] and musk-thistle [Carduus nutans]) on their Fish and Wildlife Management Areas (C. McLain, personal communication, 17 December 1982). The Nebraska Game and Parks Commission is well aware that in addition to obligatory spraying costs, their greatest loss may be the unintended impact of spraying on native wildlife food and cover plants. Perhaps an early understanding between local conservation officials and the district weed board could avoid these misdirected and unfortunate efforts. Nevertheless, we believe far more is to be gained by seeking noxious weed status for purple loosestrife than by avoiding possible conflicts and thereby losing the chance to enter the plant into existing weed control enforcement programs.

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