Flowering-rush | Butomus umbellatus

California Pest Rating for
Flowering-rush | Butomus umbellatus
Pest Rating: B  |  Proposed Seed Rating: R


Initiating Event:

This plant was recently added to the Washington and Oregon noxious weed lists and it seems to be spreading.

History & Status:

Flowering rush is a perennial aquatic plant in the monotypic family, Butomaceae. First detected in North America in the 19th century along the St. Laurence River, it has spread into the Great Lake Region and begun to spread across the Northern United States and Southern Canada. Its habitat is lake shorelines and slow moving waters to a depth of around 2 meters. It is especially well adapted to the fluctuating water levels found in reservoirs, a habitat to which few other plants are adapted, but that has increased under anthropogenic conditions. Where it occurs, flowering rush densities can vary from scattered clumps to populations exceeding 50% cover. It has been documented in Idaho and Montana, but populations in Western North American are still limited. There are no infestations identified in California. The plant is spread via horticulture and water and it still is occasionally available from nurseries that sell pond plants.

California Distribution:  Flowering rush has not yet been detected in California.

California Interceptions: None.

United States Distribution: Flowering rush is distributed across the northern tier of states, including Washington.

World Distribution: This weed is native to Eurasia

Consequences of Introduction: 

1)  Climate/Host Interaction: Evaluate if the pest would have suitable hosts and climate to establish in California. Score: 2

Low (1) Not likely to establish in California; or likely to establish in very limited areas.
– Medium (2) may be able to establish in a larger but limited part of California.
High (3) likely to establish a widespread distribution in California.

Risk is medium (2), as the plant could occur in wetlands such as montane lakes, as well as irrigation canals and watering ponds in northern CA and at higher elevations.

2)  Known Pest Host Range: Evaluate the host range of the pest. Score: 3

Low (1) has a very limited host range.
Medium (2) has a moderate host range.
High (3) has a wide host range.

Risk is High (3) as weeds do not require any one host, but grow wherever ecological conditions are favorable.

3)  Pest Dispersal Potential: Evaluate the natural and artificial dispersal potential of the pest. Score: 2

Low (1) does not have high reproductive or dispersal potential.
Medium (2) has either high reproductive or dispersal potential.
High (3) has both high reproduction and dispersal potential.

Risk is Medium (2). The plant can spread in water and on boats and equipment via seeds and rhizomes. It is also grown as a pond plant.

4)  Economic Impact: Evaluate the likely economic impacts of the pest to California using the criteria below. Score: 1

A. The pest could lower crop yield.
B. The pest could lower crop value (includes increasing crop production costs).
C. The pest could trigger the loss of markets (includes quarantines).
D. The pest could negatively change normal cultural practices.
E. The pest can vector, or is vectored, by another pestiferous organism.
F. The organism is injurious or poisonous to agriculturally important animals.
G. The organism can interfere with the delivery or supply of water for agricultural uses.

Low (1) causes 0 or 1 of these impacts.
Medium (2) causes 2 of these impacts.
High (3) causes 3 or more of these impacts.

Risk is Low (1) as the plant can impede water flows in unlined canals.

5)  Environmental Impact: Evaluate the environmental impact of the pest on California using the criteria below.

A. The pest could have a significant environmental impact such as lowering biodiversity, disrupting natural communities, or changing ecosystem processes
B.  The pest could directly affect threatened or endangered species
C.  The pest could impact threatened or endangered species by disrupting critical habitats
D.  The pest could trigger additional official or private treatment programs
E.  Significantly impacting cultural practices, home/urban gardening or ornamental plantings.

Score the pest for Environmental Impact. Score: 2

Low (1) causes none of the above to occur.
Medium (2) causes one of the above to occur.
High (3) causes two or more of the above to occur.

Risk is Medium (2) as the plant could invade the water systems of California, disrupt natural wetland communities and potentially lower biodiversity by invading wetlands. This dense growth impedes water movement, blocks the growth of native plants, and reduces available habitat for water birds and native fish.

Consequences of Introduction to California for Water-primrose:

Add up the total score and include it here. (10)

Low = 5-8 points
Medium = 9-12 points
High = 13-15 points

6)  Post Entry Distribution and Survey Information: Evaluate the known distribution in California. Only official records identified by a taxonomic expert and supported by voucher specimens deposited in natural history collections should be considered. Pest incursions that have been eradicated, are under eradication, or have been delimited with no further detections should not be included. Score: 0

-Not established (0) Pest never detected in California, or known only from incursions.
-Low (-1) Pest has a localized distribution in California, or is established in one suitable climate/host area (region).
-Medium (-2) Pest is widespread in California but not fully established in the endangered area, or pest established in two contiguous suitable climate/host areas.
-High (-3) Pest has fully established in the endangered area, or pest is reported in more than two contiguous or non-contiguous suitable climate/host areas.

Final Score:

7) The final score is the consequences of introduction score minus the post entry distribution and survey information score: (10)


Medium. The plant has established in other states, but the extent of its adaptability to California unknown.

Conclusion and Rating Justification:

A potentially troublesome weed of wetlands, especially in northern and montane regions of California.  Deserves a B rating as it has proven weedy elsewhere, but its eventual spread in California may be limited as current infestations are in climates with cold winters.


Consortium of California Herbaria: ucjeps.berkeley.edu/consortium/

Flora of North America Editorial Committee, eds.  1993+.  Flora of North America North of Mexico.  16+ vols.  New York and Oxford.

Hroudová Z., A. Krahulková, P. Zákravsky, & V. Jarolimová. 1996. The Biology of Butomus umbellatus in shallow waters with fluctuating water level. Hydrobiologia 340: 1-3.

Invasive Plants of Wisconsin: Butomus umbellatus, flowering-rush, www.uwgb.edu/biodiversity/ herbarium/invasive_species/butumb01.htm

Kings County, WA Noxious Weeds. Butomus umbellatus. Accessed 3/12/2015: http://www.kingcounty.gov/environment/animalsAndPlants/noxious-weeds/weed-identification/flowering-rush.aspx

Miller, G. 2011. Oregon Risk Assessment of Butomus umbellatus. Accessed 3/12/2015: http://www.oregon.gov/ODA/shared/Documents/Publications/Weeds/PlantPestRiskAssessmentFloweringRush2013.pdf

Lavoie C., Jean M., Delisle F., Letourneau G. 2003. Exotic plant species of the St. Lawrence River wetlands: a spatial and historical analysis. Journal of Biogeography 30: 537-549

Wisconsin Department of Natural Resources – Invasive Plant Species – Flowering Rush (Butomus umbellatus). Accessed 3/12/2015: dnr.wi.gov/invasives/fact/rush_flowering.htm

USDA Plants. Butomus umbellatus. Accessed 3/12/2015: http://plants.usda.gov/java/profile?symbol=BUUM

Rice P., Dupuis V. 2008 Flowering rush: An invasive aquatic macrophyte infesting the headwaters of the Columbia River system. Northern Interior Columbia Basin Invasive Aquatic Plant Summit. 10/21/2008

Responsible Party:

Dean G. Kelch, Primary Botanist; California Department of Food and Agriculture; 1220 N Street, Sacramento, CA 95814; Tel. (916) 654-0312; plant.health[@]cdfa.ca.gov.

Comment Period:  CLOSED

The 45-day comment period opened on Wednesday,  April 8, 2015 and closed on May 23, 2015.

Pest Rating: B  |  Proposed Seed Rating: R

Posted by ls