It is estimated that about 12,000 acres of Nebraska’s wetlands are infested with purple loosestrife (Lythrum salicoria), mostly along the main rivers. To limit the further spread of purple loosestrife — one of the state’s seven official “noxious weeds” — landowners will need to work together to implement integrated weed management strategies.
Chemical control
Herbicides alone can not provide economical control of this weed; however, they are a necessary part of an integrated approach to stop the expansion of currently infested acres. Herbicides are especially important for spot spraying and control along roadsides and ditches.
Recommended herbicides include Rodeo (4-6 pts), Habitat (1 pt/a) Garlon 3A (3-5 pts), Escort (2-4 oz/ac), and 2, 4-D (3-5 pts/ac), and Arsenal under special circumstances (see further). Before using any herbicide, check the label carefully for recommended rates, appropriate additives and plant species sensitivity. Each of the recommended herbicides has benefits and risks associated with its use. The question may still remain as to “Which herbicide to use?”
Garlon or 2.4-D, or the mix of the two will prevent seed production and provide short-term suppression, which means yearly applications for at least several years, but annual expenses will be low. Longer-term control, which means spraying once in several years, can be achieved with Rodeo (and other aquatic glyphosates), Habitat, Escort, Arsenal and a mixture of Escort and 2,4-D. Arsenal should be used for specifically targeted and controlled sites. Due to non selective nature of these herbicides, they should be used as part of an integrated and site specific approach. Furthermore, these non-selective herbicides should not be used at the same site continuously for more than two to three years in order to allow the native vegetation to regrow .
Four aquatic herbicides (glyphosate, aquatic 2,4-D, aquatic triclopyr (Garlon) and Habitat) are currently registered in Nebraska for use in aquatic sites (those continuously under water). Do not use non-aquatic glyphosates (Roundup products,and generic glyphosates registered for use in Roundup-Ready soybeans) for spraying aquatic sites. They are toxic to aquatic wildlife (eg. fish, frogs, etc).
The best time to apply herbicide is at the start of the flowering stage, which usually occurs from mid June to late July. Early flowering is the preferred time for application because: a) landowners can more easily identify the plant due to its purple flowers, and b) flowering is one of the most vulnerable stages for chemical control of perennial species. Herbicide solutions can be applied using a backpack sprayer or a tractor-mounted or pulled sprayer. Areas can be accessed by boat or plane (aerial applications should apply 10-20 gallons of solution per acre).
If you're using a backpack sprayer, for example use 1 pint of Rodeo + 3 oz NIS per 3 gallons of water. The spray solution volume is on a spray to wet basis. The product is nonselective, therefore, avoid injuring native vegetation. As always, read and follow the product label directions.
Biological control
Biological control using insects does not eliminate the target weeds, but in general, it can suppress weed population to a non-significant level. Bio-control agents alone can not provide long-term control of purple loosestrife; however, they are a necessary part of an integrated approach to stop the expansion of currently infested acres. Several insect species were introduced from Europe, where purple loosestrife originated. The list of insects includes: root weevil (Hylobius sp.), two beetles (Galerucella sp.), and two flower-feeding weevils (Nanophyes sp.). They are highly host specific to purple loosestrife, defoliating the plant as both adults and larvae. These insects, in combination with other natural competitors, have historically kept loosestrife under control in Europe.
It is believed that insects alone can not provide adequate control of purple loosestrife in North America. Experience from other parts of the United States and Canada suggest that it takes 7-15 years to observe some effect of insect feeding. For more information on rearing bio-control agents, see the UNL Extension NebGuide, Rearing and Releasing Galerucella Beetles to Control Purple Loosestrife (G1436).
Rearing and releasing insects however is just one step in the process of bio-control. Monitoring insect establishment, spread, and impact is crucial for the success of the biological control program. Monitoring programs will determine the effectiveness of Galerucella beetles throughout Nebraska to optimize the beetle release program. For more information see the UNL Extension publication, Biological Control of Purple Loosestrife: Monitoring Galerucella Establishment and Impact (EC175) .
Monitoring
Once you've tackled the problem and appear to have eliminated an infestation of purple loosestrife, be sure to monitor the site for several years. New shoots may come up from plant remnants and will be easier to control when the plant is small.
Taking action
Once sold as a landscaping plant in Nebraska, purple loosestrife is an aggressive weed that can quickly develop into a serious threat to wetland habitat. Stop the spread of purple loosestrife by working with your neighbors to implement an integrated weed management strategy. If the spread of purple loosestrife is not stopped, the economic burden to taxpayers will only increase as our vital waterways and wetlands become choked .
In local plantings and flower gardens, you can do your part by pulling and burning any plant, root and seed material. Discourage any new purple loosestrife plantings. Landscapers and gardeners have a wide variety of alternative perennial plants that pose no threat to the environment. These include spiked speedwell, lilies, Siberian iris, spiked gayfeather and garden sage. Check with your local extension office or horticulture organization for information on alternative landscape species.
Summary
Purple loosestrife is a serious, perennial weed found in wetlands of the Prairie states, including Nebraska. Due to major characteristics of the loosestrife habitat (e.g. marshy land), it is believed that neither herbicides nor bio-control agents used alone can provide long-term control. When used with other weed management methods, the long-term and cost effective control goal may be achieved.
Stevan Knezevic
Integrated Weed Management Specialist
Haskell Ag Lab, NEREC
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