Bolboschoenus fluviatilis (River Bulrush)

Plant Info
Also known as: River Bulsedge
Genus:Bolboschoenus
Family:Cyperaceae (Sedge)
Life cycle:perennial
Origin:native
Habitat:part shade, sun; wet, often in shallow water; lake shores, ponds, river banks, marshes, swales
Fruiting season:July - September
Plant height:3 to 6 feet
Wetland Indicator Status:GP: OBL MW: OBL NCNE: OBL
MN county distribution (click map to enlarge):Minnesota county distribution map
National distribution (click map to enlarge):National distribution map

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Detailed Information

Spikes: Flower shape: irregular Cluster type: spike

[photo of spikelet clusters and bracts] Loose cluster at the top of the stem with 4 to 12 branches, each branch up to 4 inches long with 1 to 8 stalkless spikelets (flower clusters) at the branch tip, often 2 or 3, though some stalkless spikelets may be clumped at the base of the cluster. A papery sheath surrounds the base of each branch. Spikelets are 10 to 25 mm long (3/8 to 1 inch), narrowly egg-shaped to elliptic, pointed at the tip, orange-brown to straw-colored, with 25 or more florets spirally arranged. Florets are perfect (both male and female parts) each with a 3-parted style and subtended by a single scale. At the base of the cluster are 3 to 6 leaf-like bracts that are erect to spreading and longer than the branches, the longest up to 14 inches long and 4 to 15 mm wide.

Leaves and stems: Leaf attachment: alternate Leaf type: simple

[photo of leaves] Leaves are alternately arranged along the lower half or more of the stem, flat to V-shaped in cross-section, up to 20 inches long, the widest blades 7 to 22 mm (to 7/8 inch) wide, and may be rough along the edges and/or the midrib on the underside.

[photo of sheath (and lack of ligule), and stem cross-section] Sheaths are straight to convex and papery across the tip, the vertical veins virtually straight all the way to the tip. A distinct ligule (membrane where the leaf blade meets the sheath) is lacking. Stems are erect, sharply 3-sided in cross-section and mostly smooth, though may be slightly rough on the upper stem. Plants form colonies from stout rhizomes that terminate in a tuber (corm) from which new shoots emerge.

Fruit: Fruit type: seed without plume

[photo of scale and achenes] Fruit develops in summer, the mature achenes (seeds) dropping off individually. Scales are 7 to 10 mm long, 3 to 4 mm wide, orange-brown to straw-colored, generally elliptic, tapered or notched at the tip with a straight awn up to 3 mm long, the surface and awn covered in short hairs though may become hairless. Achenes are 3.8 to 5.5 mm long, 2 to 2.9 mm wide, 3-sided with rounded angles, urn-shaped in outline, gray-brown and smooth. Surrounding the base are 6 stout, barbed, whitish to light brown bristles that are all about the same length, about as long as the achene, and persist with the achene long after it drops.

Notes:

Bolboschoenus fluviatilis, formerly Scirpus fluviatilis then Schoenoplectus fluviatilis, is a common wetland species, often found in shallow water and may form large, dense stands of all or mostly vegetative shoots. It is one of two Minnesota Bolboschoenus species, the other is Bolboschoenus maritimus. Both share the common traits of a leafy and sharply 3-sided stem, sheaths that lack a distinct ligule, terminal clusters usually with 1 or more branches, 1 to several stalkless spikelets at branch tips, hairy floral scales, erect to spreading leaf-like bracts longer than the spikelet branches, and colony-forming with rhizomes that terminate in a corm.

B. fluviatilis is the more common and robust of the two, distinguished by veins on the sheath front that are straight all the way to the tip, all or most of the spikelets grouped 1 to 8 at branch tips, florets with 3-parted styles, 3-sided achenes with 6 bristles all about the same length that are about as long as the achene and persist with the shed achene. By comparison, B. maritimus is limited to our western counties, has a more membranous sheath front with veins that diverge near the tip, usually has less than half its spikelets on branch tips (most are stalkless at the cluster base), mostly 2-parted styles with lens-shaped achenes, and has weak bristles much shorter than the achene that usually do not persist.

Compare these with other Bulrush species, which may differ by their clump-forming habit, round stems, bracts that appear to be a continuation of the stem, less leafy stems, globular clusters of numerous small spikelets, hairless floral scales, rhizomes that lack corms, or other traits not as above.

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More photos

Photos by K. Chayka taken in Lake County. Photos courtesy Peter M. Dziuk taken in Lake and Winona counties.

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