Written by: Wesley L. Gerrin, Sarah F. McNair, James L. Shelton
Asian Marshweed (Limnophila sessiliflora) is an invasive species of emergent aquatic plant that has both submerged and emergent leaves. Submerged leaves are feathery and grow in an opposite arrangement around the stems, and emergent leaves are sword-shaped with serrated edges and grow in whorls of 5-8 around the stems. Flowers are lavender-colored, and stems grow upright and protrude out of the water. Asian Marshweed can form thick mats that cover the surface of ponds or lakes.
Important Characteristics
Submerged leaves opposite and feathery
Emergent leaves sword-shaped and serrated
Lavender-colored flowers
Similar Species
None
Mechanical Control Methods: physical removal of plant material from the pond
Mechanical control of Asian Marshweed is possible, but not feasible as a standalone control method. It can re-establish if any fragments are left behind including stems, roots, and tubers. Gear should be cleaned thoroughly afterwards to ensure that no remnants are spread or reintroduced after removal. Discard the removed plant material in a location away from other bodies of water to ensure that it does not spread.
Biological Control Methods: use of a biological agent that consumes plant material to control vegetation
Grass Carp are not an effective biological control mechanism for this plant.
Chemical Control Methods: use of herbicides or algaecides to control vegetation
The first step when using an aquatic herbicide is to always read the entire label. Herbicide labels are legal documents that provide important information on how to use the product, including which sites can be treated, treatment rates, and precautionary statements. The label is the law! Consult your county cooperative extension agriculture and natural resources agent, professional consultants, or other qualified authorities for help if something is unclear. Click the linked trade names of the herbicides in this document to see their product labels. Trade names are included for example purposes only, and their inclusion in this document does not constitute endorsement of any product.
Always use herbicides in accordance with the directions on the label. The treatment of aquatic plants with herbicide is most effective when the plant is actively growing. Results of treatment are dependent upon many factors including dose, time of year, stage of plant growth, plant susceptibility, method of application, and water movement.
If the weeds are localized to small areas of the pond, all may be treated simultaneously. If most of the pond is affected, spot treatments that reduce the amount of dead vegetation are suggested. Mass amounts of dead vegetation can deplete oxygen and result in a fish kill. On large infestations, spot treating no more than 1/3 of the total pond is prudent.
Modern treatment plans include practices to reduce the likelihood of developing herbicide-resistant aquatic plants. Strategies may include rotating herbicides with different modes of action, using integrated pest management, applying multiple herbicides concurrently, avoiding repeating treatments too frequently, and consulting with aquatic plant professionals.
Choosing between systemic and contact herbicides depends on the type of aquatic plant, their growth stage, and treatment goals. Systemic herbicides are absorbed and move through the plant’s entire system. Contact herbicides act only on the parts of the plant they touch, so may require multiple treatments to achieve long-term control. Systemic herbicides often act more slowly than contact herbicides. Due to the rapid post-treatment decomposition of plant material, treating large areas with contact herbicides should be avoided to prevent oxygen depletion events and subsequent fish kills.
For more detailed information regarding chemical selection, chemical application, and calculating chemical treatment rates refer to Integrating Chemicals into Pond Management Plans.
Aquatic herbicides have been evaluated and rated either poor (<70% control), fair (70-79% control), good (80-89% control), or excellent (90-100% control) for controlling certain aquatic plants. The herbicides within the following section were all rated excellent for control of this plant. To see the full herbicide rating table, which includes all available aquatic herbicides, refer to the Aquatic Environments section of the Georgia Pest Management Handbook.
2,4-D is a systemic herbicide that kills plants by causing uncontrolled cell division in the plants’ vascular tissue. 2,4-D is often used as a whole-system treatment because of its ability to quickly spread through an entire pond or lake.
Weedar® and Navigate® are examples of trade names of products that use 2,4-D as an active ingredient.