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BUL 943

Flowering Rush

Danielle Gunn, Don Morishita, Tim Prather, Terrell Sorenson

Idaho green thumb how-to's


Flowering rush (Butomus umbellatus) is an invasive, aquatic, perennial weed (Figure 1). It originated from Eurasia and was first discovered in Idaho in 1949 in the Snake River near Idaho Falls. It has since become a serious problem. Currently, seven major infestations have been identified in northern and southeastern Idaho. It is listed as “noxious” by the ISDA. One criterion that defines a weed as noxious is its potential to cause injury to public health, crops, livestock, land, or other property as deemed appropriate by Idaho law. Homeowners, landowners, and natural resource managers are required to control, reduce, eliminate, and prevent flowering rush infestations on their property.

Flowering rush
Figure 1. Flowering rush.

Plant Identification

Flowering rush roots in waterbody bottoms. It has triangular stems but should not be confused with another group of plants with triangular stems, the sedges.

Flowering rush leaves are dark green, narrow, and upright with triangular edges (Figure 2). They are spongy and rebound when squeezed. Leaves emerged from water can grow up to 3 ft long and twist in a spiral at their tips. Flowering rush can also grow submersed in water. Submersed leaves grow up to 10 ft long. The leaves collapse after seasonal fall freezes and when water levels in canals and water bodies are drawn down.

Its roots are fleshy and rhizomatous and grow approximately 6 inches deep. Rhizomes (Figure 3) break off easily when disturbed and are buoyant. This feature allows them to travel long distances in moving water to begin new plant colonies. Rhizome fragments establish best in shallow, sparsely vegetated silt in water currents less than 2 mi/hr.

Triangular leaves
Figure 2. Leaves are triangular in cross section.
Flowering rush rhizomes
Figure 3. Flowering rush rhizomes. Photo courtesy of Enoch Houtz.

Flowering occurs in early summer to midfall. Flowers (Figure 4) are 3-petaled and have 3 petal-like sepals. Petals are ¾–1 inch wide and are light pink in color. Flowers develop in umbrella-shaped clusters with 20–50 flowers per cluster. Approximately 200 seeds can be produced per flower. Seeds are brown and 1 mm in length.

Flowering rush flower
Figure 4. Flowering rush flower. Photo courtesy of Enoch Houtz.

There are two genetic types of flowering rush: diploid (contains 2 sets of chromosomes) and triploid (contains 3 sets of chromosomes). The diploid type flowers prolifically. It reproduces by seed, vegetative rhizome bulbils (small, bulblike structures that separate from the parent plant, can reproduce, and can be found on rhizomes and flowers), vegetative flower bulbils, and rhizome fragmentation. The triploid type only flowers occasionally and produces sterile seeds. It reproduces mainly by rhizome fragmentation.

Plant Habitat and Characteristics

Flowering rush grows on lakeshores, wetlands, and in slow-moving water bodies. It also grows prolifically in irrigation canals and thus can impede irrigation water distribution. Although typically found in shallow water of 2–3 ft, flowering rush adapts to various water depths. It has been identified on shorelines in as little as 6 inches of water (Steve Howser, Aberdeen Springfield Canal Company, email message to author, July 2, 2019) and submersed up to 20 ft depths. Submersed leaves persist but become limp. Idaho flowering rush populations stay viable at temperatures below 0°F to above 90°F (Cole D. Morrison, Idaho State Department of Agriculture, email message to author, June 19, 2019). Warm substrate temperatures above 32°F promote sprouting and accelerated rhizome growth. Flowering rush can initiate growth as early as late February. The earlier growth and seasonal irrigation patterns in Idaho enable this plant to become well established, allowing them to outcompete the later spring growth patterns of native plants.

Flowering rush can form impenetrable monocultures (growth of a single plant species that encompasses large areas), alter native habitats, use large amounts of water, increase water temperature, increase sedimentation, and transfer nutrients from soil to water. These characteristics negatively impact native plant and fish habitat, natural and man-made water systems, and surrounding ecosystems. This plant also provides ideal habitat for piscivorous fish that decrease native fish populations. Flowering rush also affects farmers and ranchers, natural resource managers, and outdoor enthusiasts. Left unmanaged, flowering rush infestations can destroy native ecosystems and make many irrigation systems necessary for food production ineffective.

Control Options

Imazapyr sold as Habitat, Imazamox sold as Clearcast, and Glyphosate sold as Roundup (among others) are labeled herbicides that have shown promise for season-long vegetative control by decreasing root biomass and foliage. Refer to the labels for information on application timing and rates. Furthermore, it is important to understand that many herbicides cannot be used due to irrigation and water-use restrictions. For example, irrigation water from herbicide-treated canals can affect sensitive crops.

Mechanical methods of removing flowering rush currently provide the best control. Small infestations can be removed by hand digging. Care must be used to remove all rhizome fragments. If done incorrectly, hand digging can break off and spread rhizome fragments, resulting in increased plant density. Bottom barriers (sheets of geotextile material anchored to water bottoms) in small areas such as boat docks can also be used to prevent rhizome movement and subsequent establishment of new infestations. Barriers suppress growth but must extend beyond the edge of existing plant growth. For large infestations, mechanical aquatic vegetation rakes ( are very effective. Rakes should have teeth 8–10 inches long to reach the proper digging depth of at least 6 inches. Always dispose of excavated plant material away from the water line to prevent further plant establishment. This method can provide effective control for up to five years.

Healthy native plant growth, such as reeds, should be encouraged to provide barriers to flowering rush establishment. At the time of this publication, biological controls, like insects and microorganisms, are being researched. However, currently none have been approved for use.

Integrated Weed Management

An integrated weed management program is critical for controlling and preventing the spread of flowering rush. To protect Idaho’s natural resources, educational programs about this weed should include identifying, monitoring, mapping, and managing flowering rush infestations. Recreational equipment, trailers, and tow vehicles should be cleaned of all plant parts and debris, which should be disposed of away from the shores, canals, and other water sources.

Monitoring for flowering rush infestations will help reduce the negative impact of this plant. Best practices include controlling small infestations immediately; encouraging and protecting native plant establishment and growth when possible to provide competition against flowering rush; and reporting plants that resemble flowering rush to your local Extension office and/or county weed superintendent.

About the Authors

Danielle Gunn—Agricultural Extension Educator, University of Idaho Extension

Don Morishita—Extension Weed Specialist (retired), University of Idaho Extension

Tim Prather—Extension Weed Specialist, University of Idaho Department of Plant Sciences

Terrell Sorenson—Extension Educator, University of Idaho Extension


Funding for this project was provided, in part, by USDA-NIFA FRTEP Grant Funds.


ALWAYS read and follow the instructions printed on the pesticide label. The pesticide recommendations in this UI publication do not substitute for instructions on the label. Pesticide laws and labels change frequently and may have changed since this publication was written. Some pesticides may have been withdrawn or had certain uses prohibited. Use pesticides with care. Do not use a pesticide unless the specific plant, animal, or other application site is specifically listed on the label. Store pesticides in their original containers and keep them out of the reach of children, pets, and livestock.

Trade Names—To simplify information, trade names have been used. No endorsement of named products is intended nor is criticism implied of similar products not mentioned.

Groundwater—To protect groundwater, when there is a choice of pesticides, the applicator should use the product least likely to leach.

Issued in furtherance of cooperative extension work in agriculture and home economics, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Barbara Petty, Director of University of Idaho Extension, University of Idaho, Moscow, Idaho 83844. The University of Idaho has a policy of nondiscrimination on the basis of race, color, religion, national origin, sex, sexual orientation, gender identity/expression, age, disability or status as a Vietnam-era veteran.

University of Idaho Extension


Physical Address:
E. J. Iddings Agricultural Science Laboratory, Room 10
606 S Rayburn St

Mailing Address:
875 Perimeter Drive MS 2332
Moscow, ID 83844-2332

Phone: 208-885-7982

Fax: 208-885-9046



Barbara Petty