Weed of the Week: Flowering Rush

Flowering rush (Butomus umbellatus) is a perennial aquatic invader that resembles a large sedge, and flourishes along shorelines and as a submersed plant in lakes and rivers. Dense stands interfere with recreation, crowd out native plants, and can be harmful to fish and wildlife.

Flowering rush is regarded as one of five invasive alien plants having a major ecological impact on natural ecosystems in Canada, and is considered a high priority species for eradication in parts of Ontario, mainly the Great Lakes. It is rare in southwestern BC, and is an emerging invasive to watch for and report.

Flowering rush is easiest to identify when in flower, reaching about three feet in height. Each umbrella-shaped cluster has whitish pink pedals, with green stems that resemble bulrushes. Flowering rush is very difficult to identify, as it closely resembles many native plants, especially common bulrush. Its extensive root system can break into new plants if disturbed, spreading quickly to new areas.

Dense stands of flowering rush often occur in wetlands with purple loosestrife(Lythrum salicaria); flowering rush dominates shallow areas while purple loosestrife dominates open water areas.

Spread of flowering rush is largely due to its popularity in water gardens, and boaters can transport it between bodies of water if it becomes tangled in equipment. Once in a watershed, flowering rush spreads locally by rhizomes and root pieces, as well as muskrats, forming new plants. To help prevent this, please ‘clean-drain-dry’ your boat and equipment before leaving an area.

If flowering rush establishes in a lake, its growth varies due to changes in seasonal weather and annual changes in temperature and water clarity. A change in water level can allow this plant to easily invade newly exposed areas. Once established, flowering rush can displace native vegetation, reducing the overall biological diversity of an ecosystem.

One way to protect the shoreline and restrict the movement of flowering rush is to protect native plants and limit disturbance. For effective control (with proper permits), hand-cut flowering rush plants below the water surface. Cutting will not kill the plant, but it will decrease the abundance, and repetitive cutting may be needed throughout the growing season. All cut plant parts must be removed from the water.

Once removed from the water, flowering rush can still grow and spread, mainly from the root-stalk. Thoroughly dry all plant and plant pieces, turning the pile over—ideally somewhere away from the shoreline. Aquatic plants make excellent compost!

Sources: Alberta Invasive Plant Council Fact Sheet; Invasive Species Council of Manitoba Fact Sheet; Wikipedia Montana State University Extension; King County Noxious Weeds; Eflora.

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