Scirpus hattorianus (Mosquito Bulrush)

Plant Info
Also known as: Smooth Bulsedge
Family:Cyperaceae (Sedge)
Life cycle:perennial
Habitat:part shade, sun; moist to wet often disturbed soil; shores, ditches, open forest
Fruiting season:July - September
Plant height:2 to 5 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: indistinct Cluster type: panicle Cluster type: round Cluster type: spike

[photo of spikelet clusters] Stiff, open branching cluster at the top of the stem, the main branches up to 5 inches long and more or less radiating in all directions from a central point, usually with shorter, divergent branchlets at each branch tip. At the tip of each branchlet is a hemispheric to round head of 4 to 55 stalkless spikelets (flower clusters). Spikelets are 2 to 3.5 mm (to ~1/8 inch) long, oval to egg-shaped, blunt at the tip, dark brown to blackish when mature, with the florets spirally arranged. Florets are perfect (both male and female parts) each with a 3-parted style and subtended by a single scale.

[photo of late season sprouting bulblet] Bulblets may form on one or more branches and sprout late in the season. At the base of the cluster are 3 or more leaf-like bracts, green at the base though may be blackish or spotted reddish-brown along the edge. Bract blades are shorter than to longer than the cluster branches. Bracts at the base of auxiliary branches are more scale-like.

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

[photo of inconspicuous cross-partions on lower sheaths] 3 to 9 leaves are alternately arranged along the stem, 8 to 14 inches long and 5 to 15 mm (to ~½ inch) wide. Lower leaf sheaths are green to light brown with at least a few inconspicuous cross-partitions. Stems are single or a few from the base, erect, smooth and 3-sided in cross-section with rounded angles. Plants form loose clumps from short, fibrous rhizomes.

Fruit: Fruit type: seed without plume

[photo of spikelet, scales and achenes] Fruit develops in early to mid-summer, the mature achenes (seeds) dropping off individually and the scales soon after, leaving behind naked branchlets. Scales are 1 to 1.4(2) mm long, oval-elliptic with an abrupt taper to a minute point at the tip or the midrib extending not more than .2 mm. Scale color is dark brown to black, the midrib initially green turning light brown. Achenes are .6 to 1.1 mm long, .3 to .5 mm wide, plumply to weakly 3-sided in cross-section, elliptic to urn-shaped in outline, yellowish turning pale brown to creamy colored at maturity. Surrounding the base are 5 or 6 barbed, pale bristles that are usually about half or so as long as the achene.


Scirpus hattorianus is an occasional species of shores, ditches, marshes, depressions in forest openings and meadows, often in somewhat disturbed soils and sometimes where it may only be seasonally wet. We've encountered it at the woodline of trails and gravel roads where you don't normally expect to see a wetland species. It reaches the western edge of its range in Minnesota and is mostly found in the northeastern quadrant of the state. It closely resembles 4 other Minnesota Scirpus species: Scirpus georgianus, Scirpus atrovirens, Scirpus microcarpus, and Scirpus pallidus. These share the common traits of a weakly 3-sided stem, 3 or more stem leaves, 3 or more leaf-like bracts that are shorter than to longer than the cluster branches, and a terminal cluster with fairly stiff erect to spreading branches that end in dense, hemispheric to round heads of several to many spikelets.

Scirpus hattorianus is distinguished by the inconspicuous cross-partitions on sheaths (check especially on lower sheaths), floral scales usually blackish at maturity with the tip merely pointed or the midrib extending not more than .2mm, 3-parted styles, 5 or 6 weak bristles usually all about half or so as long as the achene, and usually some plants in a population with bulblets on cluster branches that sprout late in the season. It is the most slender of this group and has the smallest heads, scales and achenes, though there are overlaps between this and the other species.

The most readily distinguished of this group is S. microcarpus, which is rarely taller than 2 or 3 feet, has purplish-red sheaths, 2-parted styles, and at most 18 spikelets in a head. The other 3 can be very difficult to distinguish from each other and it may be necessary to examine several plants in a population to get a consensus on some traits. S. georgianus, which has only been recorded twice in Minnesota and not since 1983 (probably now extirpated), has at most 3 bristles all shorter than the achene. S. atrovirens is generally a stouter plant with slightly larger heads, longer spikelets, slightly larger floral scales that may have an awn up to .4mm long, 5 or 6 bristles all more or less as long as the achene, and have rather conspicuous cross-partitions especially noticeable on basal sheaths. S. pallidus is a more western species, has larger heads usually in a more compact cluster, may or may not have conspicuous cross-sections, lacks any bulblets, its floral scales are larger with an awn at least .4mm long, and bristles are about as long as the achene. Most references put some reliance on scale and achene colors, but I have found these too subjective and not consistently distinct. Scales on all can be streaked with black and mature achenes can all be creamy colored. And hybrids exist which complicates matters.

Compare these with other Bulrush species, which may differ by their round or sharply 3-sided stems, erect bracts that appear to be a continuation of the stem, less leafy stems, nodding clusters, some or all spikelets single at branch tips, or other traits not as above.

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More photos

Photos by K. Chayka taken in Carlton, Cook, Crow Wing and Lake counties. Photos courtesy Peter M. Dziuk taken in Cook and St. Louis counties.


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