Introduction

Flowering Rush or Butomus umbellatus is an aquatic perennial from continental Eurasia and Africa. Despite its common name, it is not actually a rush which is part of the Juncaceae family in the Poales Order of Monocot plants. Flowering Rush is a member of the Butomaceae family of the Alismatales Order of Monocot plants. It is an invasive species in many parts of the world including North America, and once established removal of Flowering Rush can be challenging to say the least.

Butomus umbellatus has been spread around the world because of its use as an ornamental aquatic plant. It then escapes cultivation and wreaks havoc in native wetlands where it chokes out other vegetation. Please do not buy Flowering Rush for your garden as invasive aquatic plants are notoriously difficult to control once they become established. The best course of action would be to research what aquatic ornamentals are native to your area and plant those instead. There are also other non-native aquatic ornamentals that are non-invasive but again do your research for your specific region. Sometimes plants that are invasive in one area are not invasive in others due to differences in climate and other local factors.

Description of Butomus umbellatus

Leaves & Stems

Flowering Rush is an emergent aquatic perennial from stout rhizomes with bulbils that spontaneously break off and float away allowing it to reproduce vegetatively. In appearance it superficially looks quite similar to native rushes and sedges. The narrow linear leaves grow to 100 cm long, have no teeth, are parallel-veined, and often appear twisted. It has a round flower stem (scape) that can grow from 100 – 150 cm tall, which, without flowers makes it look similar to native rushes. The leaves are three-angled in cross-section, which can also make them easily confused with native sedges.

However, the umbel of pink flowers that appear on top will help you differentiate it from both native rushes and sedges. If you are not certain of the identification, wait for June to August when they are in flower to identify. Then you can mark it with a colorful flagging tape if you need to return to remove it in a season when it is not flowering.

Flowers & Fruits

Butomus umbellatus produces large clusters of pink flowers in umbels on top of the 100 – 150 cm long scape. Each bisexual flower is from 2 – 3 cm across and is light pink in color with darker veins. It has tepals rather than petals and sepals. Its outer tepals (sepals) are elliptic in shape and are 6 – 7.5 mm long while the inner tepals (petals) are oblanceolate and 9 – 11.5 mm long.

It has 6 – 9 stamens that have 3 – 4.5 mm long filaments topped with 1 mm long anthers. It has 6-9 partially connate (united) carpels crowned with a persistent style. The fruit is a 1 mm long follicle, though it primarily reproduces vegetatively through spontaneous fragmentation of rhizome buds known as bulblets or bulbils.

It flowers from June to August though it produces little if any seeds.

Native Distribution

Flowering Rush or Butomus umbellatus is native to continental Africa and Eurasia including Afghanistan, Albania, Algeria, Armenia, Austria, Azerbaijan, Belarus, Belgium, Bosnia and Herzegovina, Bulgaria, China, Croatia, Czech Republic, Denmark, Estonia, Finland, France, Georgia, Germany, Greece, Hungary, Iran, Iraq, Ireland, Israel, Italy, Jordan, Kazakhstan, Latvia, Lebanon, Lithuania, Moldova, Mongolia, Morocco, Netherlands, Norway, Poland, Portugal, Romania, Russia, Serbia, Spain, Sweden, Switzerland, Syria, Turkey, Turkmenistan, Ukraine, United Kingdom, and Uzbekistan.

It is considered an endangered species in Israel where it is native to but has been threatened by habitat loss.

Human Uses of Flowering Rush

Flowering rush is often used as an aquatic ornamental plant in ponds and water gardens.

Sometimes it is also used as a food source as the bulbets contain more than 50% starch. They are peeled then steamed and eaten. They can also be dried and ground and used as a thickener for soups or ground with grains into flour for breads and other uses.

It has occasionally been used medicinally as an antimicrobial agent. However, recent studies on its use for this did not show positive results.

Habitat Types Where Flowering Rush is Found

Flowering rush is an emergent aquatic found in marshes, lakes, rivers, riparian zones, and wetlands in water up to 3 m deep. It grows to the deepest range that emergent aquatic plants can be found. It can also inhabit storm-water retention ponds, wet gravel pits, and roadside ditches. Sometimes it also grows as a submerged plant in rivers and lakes. It is an aquatic plant that requires access to water year-round but it has tolerance to variable depths of water, making it a suitable invader for disrupted wetlands. It, however, does not tolerate brackish water or salinity so it will not invade estuaries or inland brackish lakes.

Flowering Rush requires sunlight and will not grow in shade. It also prefers cool to warm temperate climates and will not grow in tropical climates. It grows in most soil types including sandy, loamy, and clay and tolerates acid, neutral, and basic pH levels.

Distribution of Butomus umbellatus in North America

The species was first observed in North America in 1897 in the St. Lawrence River area and has spread throughout eastern North America. It has since then continued its spread westward through long-distance dispersal primarily from boaters and intentional plantings from gardeners. From there, once introduced by humans to a new area they will spread vegetatively. Humans are the primary means of long-distance dispersal. This makes wetlands and lake shores located near human settlements at the highest risk of invasion. The bulbets frequently escape a garden pond during high water or from humans or wildlife transporting them.

In Canada, Butomus umbellatus has been recorded in most of eastern Canada including New Brunswick, Nova Scotia, Prince Edward Island, Quebec, Ontario, and Manitoba. It is particularly problematic throughout the Great Lakes region. In western Canada it is found in both BC and Alberta. So far in BC, however, it has only been found in the Lower Mainland. It currently is not found in Saskatchewan, Newfoundland, or any of the northern territories.

In the USA, Flowering Rush has been found in Maine, Connecticut, Vermont, New York, Pennsylvania, Ohio, Michigan, Wisconsin, Indiana, Illinois, Minnesota, North Dakota, South Dakota, Nebraska, Montana, and Idaho. It is particularly problematic in the Great Lakes Region. It is illegal to buy or sell Butomus umbellatus in several of the eastern states. So far it has not been reported in any of the southern states.

In Mexico, so far, Butomus umbellatus has only been reported in one location. In 2017 it was found in Puebla Nuevo, Durango, Mexico on the eastern edge of the Sierra Madres.

How Flowering Rush Spreads

It is primarily dispersed vegetatively through bulblets that are carried by water currents or wildlife to new locations. The little bulbils float in water and are easily transported this way. Muskrats, waterfowl, and water currents are believed to be the primary method of local dispersal.

Humans are believed to be the primary source of long-distance dispersal. Anthropogenic dispersal is primarily done through gardeners planting them in their water gardens. This is likely how it was been introduced to North America in the first place. Boaters are another human source of dispersal where the small bulbils get carried from one location to another.

Habitats at Risk of Invasion in North America

The habitats most at risk of invasion in BC are marshes, lake shores, wetlands, and any aquatic habitat including roadside ditches. Since it is hardy from zones 3-10 it has a high potential for invasion throughout most of North America, excluding the most northern parts of Canada, desert areas throughout the southern USA and northern Mexico, and the most southern parts of Mexico where the climate becomes tropical.

Worst case scenario is that the flowering rush will invade lake shores, marshes, rivers, streams, and wetlands throughout the continent. They would displace our native wetland vegetation in the process and significantly reduce biodiversity in the affected areas. The associated economic costs will be enormous if it gets out of control since aquatic weeds are notoriously difficult to control. Since it has been seen to compete with even willows and cattails, it is expected to be highly invasive in any aquatic habitat that it gains a foothold in.

Impacts of Invasion

Studies in Canada have shown that in 40% of the areas where it was found Flowering Rush made up more than 50% of the total vegetation coverage. This suggests it has a high potential for being invasive and displacing native vegetation reducing biodiversity in the process. Light and nutrient levels are also affected where it is found because of its ability to form dense mats of vegetation. This would further increase competition with native species which would then get out-competed. Flowering Rush has been found to increase sedimentation in our waterways as well as reduce water flow through the dense mats they form. Given the other major threats all our wetlands, waterways, and riparian areas already face (habitat loss, increased nutrient input, altered water regime, pollution, other invasive species, etc) this is all of great concern.

Butomus umbellatus has already been shown to negatively affect wild rice (Zizania aquatica) populations. Flowering Rush reduces wild rice biomass which reduces their potential food source to both wildlife and the indigenous peoples who still use the rice as a food source.

Flowering Rush has the potential to clog irrigation channels reducing water delivery for agriculture. It also cogs other waterways impeding the flow of water, boats, and other industrial uses. Butomus umbellatus also reduces open water which affects recreational uses such as boating, swimming, and fishing.

Flowering Rush has also been shown to provide an ideal habitat for Lymnaea stagnalis, the great pond snail. This snail is an intermediate host for Trichobilharzia ocellata which is the trematode responsible for swimmer’s itch.

Potential Benefits of Invasion

Muskrats and waterfowl have been found to feed on the bulbils, providing food for native animals. However, the native vegetation that it displaces would have also provided that same benefit. It may also provide structure for fish that require vegetation for spawning. However, the increased sedimentation and reduced water flow that result from their invasion would counter that apparent benefit as well.

Removal of Flowering Rush

As always prevention is the preferred method of control. Do not buy or transport any of the bulbils. If you acquire them somehow do not simply throw them in the trash. Instead, dry them thoroughly first or burn them to ensure they do not survive to propagate. If you see them being sold online or in your local garden stores please inform them of their invasive status and ask them to do their part and cease selling them.

Physical Control Methods

Once already established, however, aquatic invasives are notoriously difficult to control. Physical control is the most effective means. This can be challenging, however, depending on water depth. Choose a time of the year when the water is at its lowest point. If necessary, mark the the patch with flagging tape when in flower so that you can return to it when the water is lower and they are not in flower.

When removing single isolated Flowering Rush plants hand pulling can be a very effective means of control to prevent them from getting a foothold in a new environment. Simply dig out the plants, removing all pieces and taking care to remove any floating pieces of bulbs as well. This is the most effective time to control them once they are found in a new environment before they become a dominant plant in the community.

If you are working on a large patch of Flowering Rush it would be a good idea to use an aquatic containment net around the area you are removing Flowering Rush from to keep all the pieces contained. (See Making an Aquatic Containment Net Below). When removing a large patch you can use your hands or a narrow-headed shovel if necessary, but be careful to remove all the rhizomes in the soil so dig around the edge of the patch a little bit to ensure you get all the rhizomes.

How to Make an Aquatic Containment Net

Purchase an appropriate length of fine-mesh netting. Netting can come in all different size meshes, heights and lengths. You want fine-mesh with holes that are less than 5 mm, large netting more than that will allow pieces to flow through. They also come in different depths which you will need to determine based on the depth of your water. You want it to sit on the soil in the water and reach the surface of the water at least.

To make your containment net you need to weigh down the bottom edge of the net. Every 20-30 cm sew, zap-strap or otherwise attach weights or rocks to the bottom edge of the net only. I have folded over the bottom and sewn rocks into nets in the field before and it worked great. This keeps the bottom of the net in the water and prevents it from floating. Then you can use posts pounded into the soil to hold the net in place, again zap straps work good in a pinch for attaching the nets, just make sure you use the reusable ones for this part. Then strain the water in the enclosed area with a net to be sure you remove all the plant pieces before removing the containment net and moving onto the next patch.

Alternative Physical Control Method

Note that simply cutting the plant below the water surface will not kill the plant. And since they spread almost exclusively by vegetative means preventing them from flowering also will not reduce their population. However, sometimes this method can be effective, particularly if deep water levels make it difficult to dig them out. You would need to return to the area and cut them down repeatedly throughout the growing season. This can reduce the area covered by Butomus umbellatus, allowing room and resources for other plants to grow in their place. This could weaken the plants eventually to the point they will be eradicated if this process is repeated annually. This is because they will not be able to put enough energy into their rhizomes to promote their growth for the following year(s). This is less labor-intensive in the short-term than digging them out but is much more labor-intensive in the long term.

Ongoing Monitoring

In all cases of physical removal ongoing monitoring is absolutely essential. Yearly monitoring programs need to be put in place to ensure that any surviving individuals are removed so that the population is not able to recover.

Chemical Control Methods

Chemical applications are almost never an ideal method of control for any invasive species. That is because chemical alteration of the environment often makes the environment more suitable for invasive species than native species. Furthermore there are no chemical control methods that currently target only Butomus umbellatus. Since the environment is aquatic it is also exceedingly difficult to keep the chemical control method contained so that it does not affect any native species that are there.

Biological Control Methods

Biological control methods are extremely risky and should only be carried out by professionals after years of rigorous study. This involves the use of a predator, herbivore, disease, or some other agent to control an invasive species once it is established in the environment. The problem with biological control is that the agent used must be entirely specific to only the target organism before releasing it into the environment. This is often difficult to determine since the agent of control is also not native to the environment and could behave differently when released there. Take the example of the mongoose and the rat. The mongoose was released in Hawaii in the late 1800’s to help control the rat. To this day there are still rats in Hawaii but the mongoose has helped to decimate many native bird populations.

There are currently no effective biological control methods in use for Butomus umbellatus. However, CABI is working on test studies with a semi-aquatic weevil, a fly, and a fungal pathogen. All are showing some promise but may need to be further studied before put into wide-scale use.

References and Resources

Butomus umbellatus photo from Wikipedia By Ivar Leidus – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=27558951

CABI Biological Control of Flowering Rush https://www.cabi.org/projects/biological-control-of-flowering-rush/

Canadensys Plant Search https://data.canadensys.net/vascan/search

Dictionary of Botanical Terms – Lyrae’s Nature Blog Dictionary of Botanical Terms

Eflora Plants of North America http://www.efloras.org/browse.aspx?flora_id=1

Great Lakes Aquatic Nonindigenous Species Information Systems by NOAA https://nas.er.usgs.gov/queries/greatlakes/FactSheet.aspx?Species_ID=1100&Potential=N&Type=0

iNaturalist Plant Search https://www.inaturalist.org/home

US Fish & Wildlife Services Flowering Rush (Butomus umbellatus) Ecological Screening Summary https://www.fws.gov/fisheries/ans/erss/highrisk/ERSS-Butomus-umbellatus-FINAL.pdf

USDA Plants Database https://plants.sc.egov.usda.gov/home

Willis, Lyrae (2022).  Plant Families of North America. Not yet published.

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