NRCS−Montana−Technical Note−Invasive Species−MT-33 1 

United States Department of Agriculture

NATURAL RESOURCES CONSERVATION SERVICE

Invasive Species Technical Note No. MT-33

September 2011

 Ecology and Management of Flowering Rush (Butomus umbellatus L.) 

By

Jim Jacobs, Plant Materials Specialist, NRCS, Bozeman, MT 

Jane Mangold, Extension Invasive Plant Specialist, Montana State University, Bozeman, MT 

Hilary Parkinson, Research Associate, Montana State University, Bozeman, MT 

Virgil Dupuis, Salish Kootenai College, Pablo, MT 

Peter Rice, Research Ecologist, University of Montana, Missoula, MT 

Figure 1. Flowering rush restricting flow in an irrigation ditch in Lake County, Montana. Photo by 

Alvin Mitchell, Salish Kootenai College, Pablo, Montana.

Abstract

Flowering Rush is an invasive Eurasian aquatic plant resembling a large sedge with emerged and 

fully submerged forms and umbrella-shaped clusters of 20 to 50 light-pink to rose-colored flowers. 

The fleshy rhizomatous roots fragment by minor disturbances and spread populations long distance 

by floating on water currents. It grows along lake shores and slow moving bodies of water. In 

Montana it is currently reported along the shores of Flathead Lake, portions of the Flathead River, 

Thompson Falls Reservoir, Noxon Reservoir, Cabinet Gorge and the Clark Fork River. Dense 

populations growing within irrigation ditches reduce water availability and flow (see Figure 1), and 

dense populations along lake shores inhibit boating, fishing, and swimming. Control methods are 

currently limited to hand and mechanical digging and herbicide application after water drawdown

(check local regulations before applying these managements). Chemical control method studies are 

ongoing. Prevention, early detection and rapid response to eradicate new populations are the 

management priorities for Montana. Consult your county weed coordinator for the best management
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options for your area. New populations should be reported to county Extension agents or weed 

coordinators. 

Plant Biology 

Figure 2. Rhizomes of flowering rush. Photo by Peter Rice, University of Montana, Missoula, 

Montana.

Identification

Flowering rush has triangular leaves like a sedge (Cyperaceae) and round flowering stems like a 

rush (Juncaceae), but it is neither. It is a Butomaceae, and it is the only representative species of this 

taxonomic family. It may grow either in shallow waters up to 10 feet deep (3 meters) as an 

emergent plant with upright foliage, or in deeper waters from 10 to 20 feet deep (3 to 6.1 meters) as 

a submerged plant with flexible leaves suspended in the water column. Both forms have fleshy, 

rhizomatous roots (see Figure 2). The narrow leaves are triangular in cross section (see Figure 3); 

spongy and compressible, and emerged leaves are twisted spirally toward the leaf tip. 
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Figure 3. Cross sections of flowering rush leaves. Photo by Gary Fewless, University of Wisconsin, 

Green Bay, Wisconsin.

The flowers (see Figure 4) are three-quarters to one inch wide (2.0 to 2.5 centimeters) with three

small, slightly-greenish sepals, six pink to rose-colored petals, nine stamens in two whorls (the outer 

whorl has six and the inner whorl has three), and six pistils that can produce about 200 seeds each. 

Twenty to 50 flowers are clustered in a round, umbrella-shaped inflorescence (see Figure 4) atop a 

stalk that is round in cross section. There are two flowering types; one that flowers regularly and 

produces viable seed, and the other that flowers occasionally but produces sterile flowers. The two 

reproductive types are distinguished also by their genetics, the fertile type is diploid (two sets of 

chromosomes) and the sterile type is triploid (three sets of chromosomes). A study of one 

population in Montana found only one plant in 1,000 flowered. Another population flowered 

profusely but produced no viable seed and very few bulbils (a small bulb-like vegetative 

reproductive structure similar to a bulblet). Thus, populations in Montana are believed to be the 

sterile type and do not reproduce sexually. 
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Figure 4. A flowering rush inflorescence and flower. Photo by Alvin Mitchell, Salish Kootenai College, 

Pablo, Montana.

Life History

Flowering rush is a perennial growing from a reproductive rhizome. Rhizomes develop structurally 

weak constrictions between vegetative buds formed along the rhizome. Minor disturbances such as 

moving water, waves, passing boats, or waterfowl break the rhizomes at the constrictions. The 

rhizome fragments disperse to form new populations. For the sterile type of flowering rush found in 

Montana, this is the only means of reproduction. The fertile type can reproduce by rhizome 

fragments as well as seed and small bulb-like structures (bulbils) formed on the rhizomes and on 

flowers.

Initiation of vegetative growth from rhizomes is somewhat variable. On Flathead Lake in 

northwestern Montana, emergence dates ranged from February 26 to April 15 over a three-year 

period. This is typically earlier in the spring than most native aquatic plants. Vegetative growth is 

continuous throughout the season and into fall. Plants flower from early summer to mid-fall. Fall 

frosts cause leaves to collapse as opposed to remaining upright through the winter like cattails.
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Figure 5. The shallow and slow moving waters of the mouth of Dayton Creek provide optimum habitat 

for flowering rush. Photo by Alvin Mitchell, Salish Kootenai College, Pablo, Montana.

Habitat 

Flowering rush habitat includes lake shores, slow moving waterways, irrigation ditches, and 

wetlands. It is typically found in shallow waters, but can survive and grow across a range of water 

depths. It has been observed in very clear water at depths up to 20 feet (6.1 meters) in Flathead 

Lake. At depths of about 10 feet (3 meters) or greater, submersed leaves persist, but become limp 

and more ribbon-like than triangular in shape. 

Fluctuating water levels promote flowering rush establishment and population expansion. As water 

levels decrease, un-vegetated or sparsely-vegetated substrata are exposed to rhizome establishment 

and the shallow waters or exposed sediments warm quickly promoting vegetative sprouting and 

accelerated growth. On the Flathead Lake before water levels were regulated by Kerr Dam, low 

water levels were typically during mid- to late summer and native emergent vegetation dominated 

low water zones. Since dam operations began, lake levels have been held at full pool through the 

summer and are at low pool in the late winter to early spring. This favors flowering rush because it 

is adapted to emerge and grow during this time, whereas the native plant species are not.

Competitive plants provide a barrier to flowering rush establishment. In the Czech Republic under 

stable water levels, flowering rush established in a band around reeds but did not advance into the 

reed patch. Currently, it is not known how North American native plants interact with flowering 

rush, but they may prevent population establishment and growth. 

Spread

The sterile type of flowering rush found in Montana only spreads by rhizome fragments which are 

buoyant and a storehouse of carbohydrates. The floating fragments disperse on water currents, 

sometimes over long distances. Sparsely vegetated or un-vegetated silty substrata where water is 
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shallow and currents have slowed to less than two miles per hour (mph) are ideal for rhizomatous 

establishment and vegetative growth fueled by the carbohydrate reserves.

First reported in Peaceful Bay on the northwest shore of Flathead Lake in 1964, flowering rush has 

spread through the Kerr Dam, along the lower Flathead river, and down the Clark Fork River to the 

delta and through Lake Pend Oreille, Idaho into the Pend Oreille River in Washington by 2010. 

Populations are in Flathead Lake (2,000 acres), Thompson Falls Reservoir (28 acres) and Noxon 

Reservoir (46 acres), and the Cabinet Gorge Reservoir. It occurs in sloughs, backwater eddies, low 

flow areas, and near boat launches along the Flathead and Clark Fork Rivers. 

Flowering rush has been used ornamentally in water gardens. To avoid accidentally introducing 

non-native or invasive plants to surrounding water bodies, water gardens should never be placed 

near or allowed to overflow into wetlands, streams, or rivers. Non-native water garden plants should 

never be dumped into natural water bodies. 

Impacts

Flowering rush growing prolifically in irrigation ditches reduces water flow and distribution, and 

increases ditch maintenance costs (see Figure 1). Plants interfere with boat propellers, swimming, 

and fishing thus reducing recreational opportunities along rivers and lake shores. Flowering rush 

supports habitat for the great pond snail that hosts parasites that cause swimmers' itch.

Fish habitat is affected where flowering rush forms dense stands in previously un-vegetated or 

sparsely-vegetated aquatic environments. This is a disadvantage for native cutthroat and bull trout 

that require open water to spawn, and an advantage to introduced fish like largemouth bass, yellow 

perch, and northern pike that spawn in vegetated substrata. Ambush piscivors (fish-eating fish) such 

as largemouth bass and northern pike hide in flowering rush vegetation. Northern pike are 

significantly depredating cutthroat and bull trout in the Flathead Lake and impairing their recovery. 

Management Alternatives

Prevention

Prevention is the most important management option for flowering rush in Montana. Mapping, 

monitoring, early detection and eradication are critical to prevention. If a new infestation is found, a 

specimen should be saved, and the infestation reported to your county Extension Agent or the 

Montana Department of Fish, Wildlife and Parks at 

http://fwp.mt.gov/fishing/fishingmontana/ans/default.html. Inspection and sanitation of recreational 

equipment will prevent spread to un-infested water bodies. Any aquatic plant debris on boats, 

trailers, live wells, boat bilges, and fishing equipment should be disposed of away from lakes, ponds 

and rivers. Establishing washing stations with sanitation instructions at water-based recreational 

sites is recommended.

Mechanical Control

Methods of mechanical control include hand digging, raking, cutting, and bottom barriers. Hand 

digging may be a feasible management for very small infestations, especially when water levels are 

low. However, at a boat mooring on Flathead Lake, hand digging increased flowering rush density, 

most likely because of an increased number of rhizome fragments after digging. For hand digging to 

be successful, rhizome fragments will need to be diligently removed and repeated digging will 

probably be required. Raking is commonly used to remove aquatic vegetation from ponds. 

However, raking is not recommended for management of flowering rush because it has little effect 
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on the population and it may disturb the root system and creates rhizome fragments. Similarly, 

cutting flowering rush below the water surface is not recommended because it will not kill the plant 

and the disturbance may fragment rhizomes. Bottom barriers are like underwater weed cloth used 

below boat moorings and ramps, and when properly installed and maintained according to 

recommendations and regulations, they can effectively restrict flowering rush growth in these small 

areas. It is advisable to check with state and tribal permitting authorities before applying any 

disturbance or bottom barriers in water bodies.

Ten years of mechanical harvesting in the Detroit Lakes (Minnesota) system only made the problem 

worse and at great expense. Mechanical control methods that disturb lake or river beds, and bottom 

barriers, may require permits from state, tribal, or federal agencies. 

Cultural Control 

Observational information suggests flowering rush establishes and quickly fills in areas devoid of 

aquatic vegetation, but it appears to invade areas with existing vegetation more slowly. It stands to 

reason that culturing desirable aquatic vegetation will retard flowering rush invasion and spread. 

Native sedges, rushes and reeds can be planted in areas susceptible to flowering rush invasion. In 

addition, managing water levels where possible to reduce winter and early spring draw downs that 

favor flowering rush, to more natural late summer draw downs, may favor native species over 

flowering rush.

Chemical Control 

Application of herbicides directly to waters in Montana requires a 308 permit from the Montana 

Department of Environmental Quality. This includes private ponds and lakes that drain into 

irrigation ditches, creeks, rivers or other public waterways. Waters on tribal land are under the 

jurisdiction of the tribe. In addition, exposure of aquatic and riparian habitats to herbicides has high 

environmental risks. Contact you county weed coordinator prior to managing flowering rush with 

herbicides. 

Herbicidal1/management of flowering rush is being developed. Research at the University of 

Montana and the Salish Kootenai College has investigated a number of herbicides applied at high 

and low water levels of Flathead Lake. Preliminary results suggest spraying during the spring drawdown period on Flathead Lake after five to seven inches (12.7 to 17.8 centimeters) of leaves had 

emerged from the exposed lake bed was most effective. Imazapyr (Habitat®) and imazomox 

(Clearcast®) suppressed flowering rush for one season but did not kill rhizomes. The effect on 

rhizomes of injecting herbicides in the water column is being tested.

1/ Herbicides mentioned here are still being researched and are not listed as recommendations. Check 

www.greenbook.net for herbicide label updates. When herbicides do become available, note that a 308 permit from 

Montana Department of Environmental Quality is required before applying aquatic herbicides to water.

Biological Control

No biological control agents are currently available for the management of flowering rush. 

Integrated Pest Management (IPM)

Prevention of further spread and education are the main priorities for flowering rush. Thoroughly 

wash all water recreational equipment. Dispose of plant material away from shores. Learn to 

identify flowering rush and report any findings to the Montana Department of Agriculture; Montana 
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Fish Wildlife and Parks; or your county Extension Agent or weed coordinator. Map and monitor 

current infestations and scout for new populations. Remove small populations by digging and 

follow-up with monitoring and repeated removal. Use bottom barriers in high use recreational areas 

with flowering rush. Develop a herbicide management strategy to target high priority areas and to 

minimize environmental consequences. 

References

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