Sparganium identification key and species descriptions
By Josh Sulman
University of Wisconsin-Madison
Department of Botany
Updated March, 2016
Identifying bur-reeds in the field presents a special challenge to botanists, wetland scientists, and anyone interested in aquatic plants. The group has its own set of specialized terms. Often, at least until mid-summer, it is necessary to be familiar with vegetative characteristics of the different species. Most fruits are not ripe until late summer, so understanding the features of the flowers and inflorescences is important for spring or summer field surveys. Additionally, there is remarkable phenotypic plasticity within species. Plants of some species are typically limp and floating, but erect and emergent during periods of low water levels. Also, typically erect and emergent species are occasionally found limp and floating in deep water or flood conditions.
The treatment of Sparganium by Robert Kaul in the Flora of North America (1997) is a thorough and reliable guide to the genus.
The worldwide monograph (Cook and Nicholls, 1986, 1987) is the most comprehensive taxonomic treatment, and is now available online:
A key is presented below to the 8 species found in Wisconsin. Detailed descriptions of the plants will follow.
General description and ID tips:
The flowers are borne in globose heads, the female flowers borne in separate heads from the male flowers. The male heads are borne at the tips of the inflorescence axis and branches, and the whole male head is deciduous. After it has fallen, you can tell where it was attached by the scar that is left on the stem. A branch bears 2 or more heads; a “branch” bearing a single head is a peduncle.
The female flowers are packed into spiky heads of their own. Each female flower will bear one or two stigmas, depending on the species. The stigma remains prominent in fruit, developing into a sharp-pointed beak. The beak may be straight, curved or hooked and may function in dispersing the fruits. The bur refers to the spiky appearance of the fruit heads.
How the heads are positioned in relation to the leaf tends to be one of the most reliable characters for ID, but is also a source of confusion! The terms axillary and supra-axillary are applied to the position of the female heads of Sparganium:
If the head is borne in the axil of the leaf below it, it is termed axillary.
If the head is attached to the stem somewhat above the leaf, it is termed supra-axillary.
(Actually, supra-axillary heads have a stalk that is fused with the stem, so that it appears attached somewhat above the subtending leaf.)
A supra-axillary head may be sessile or stalked. Such a stalk (or peduncle) is NOT considered a branch. A branch must bear at least two heads (male or female), and terminates in a male (staminate) head.
In some individuals, the subtending leaf may be absent. This is often the case in branched inflorescences, on the upper female heads, as in Fig. 1. Also, male heads usually are not subtended by a leaf.
Even within a single plant, some heads may be axillary while others are supra-axillary: in this case, it should be keyed as supra-axillary.
A. stigmas 2, mature fruits flat-topped or slightly domed. Common in eutrophic habitats, throughout Wis.; up to 6 ft. tall . . . . . . 1.S. eurycarpum
AA.stigmas 1, fruits tapered acutely to a pointed beak. Plants in meso- to oligotrophic habitats, mostly central and northern, occasionally SE Wis.; under 3 ft. tall . . . . . . . . . . B.
B. plants limp and floating . . . . . . . . . . . . . . . . . . . . . . . . . . . . C.
C. Plants mostly under 30 cm, often completely submerged; female heads <1cm across. . . . . . . . . . . . . . . . . . . 2. S. natans
CC. Plants longer, with long leaves floating on the surface, female heads over 1 cm across. . . . . . . . . . . . . . . . . . . . . . . H.
H. Leaves white beneath, inflorescence branched. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.S. fluctuans
HH. Leaves green beneath, inflorescence not branched, though heads often stalked . . . . . . . . . . . . . 4. S. angustifolium
BB.Plants erect and emergent . . . . . . . . . . . . . . . . . . . . . . . . . . . . D.
D. Leaves narrow, usually <8mm across, some heads supra-axillary, in an unbranched inflorescence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . F.
F. pistillate heads clustered at top of stem, staminate head 1; NW Wis., NE Minn, rare . . . . . . . . . . . . . . . . 5.S. glomeratum
FF. pistillate heads spaced along stem, staminate heads 3 or more; common, widespread. . . . . . . . . . . . . . . . . . . 6.S. acaule
[previously treated as S. emersum]
DD. Leaves up to 2cm wide, heads axillary in a simple or branched inflorescence . . . . . G.
G. mature fruits glossy, with beak 5-6mm. . . . . . . . . . . . . . . . . . 7.S. androcladum
GG. mature fruits dull, with beak 2-5mm. . . . . . . . . . . . . . . . 8.S. americanum
The following are based on field work over two summers in Wisconsin and neighboring regions of the U.P. of Michigan, and northern Minnesota. Associates lists were compiled from a matrix of 134 species observed at 75 Sparganium habitat localities: associates for each species below are listed in descending order of frequency, to a minimum of two observed localities (Sulman, 2010).
1. Sparganium eurycarpum Engelm.
Leaves cat-tail-like, up to 6 ft. tall, but keeled abaxially along the middle, trigonous toward base. Leaves usually dark green, turning brown from the tips in late summer. Female heads axillary or without subtending leaf; main axis and lateral branches bear numerous male heads. Female flowers and immature fruits conical, tapered to sharp beak. Mature fruits truncate above, with abruptly pointed beak, bearing (1)-2 stigmas. This is the most common bur-reed throughout the southern two-thirds of the state. This is the only bur-reed to have 2-stigmas/flower; in vegetative material it can be identified by the leaves that are flattened, but strongly keeled all the way to the tip. Grows in extensive colonies, in water 0 to 3 ft. deep. Typical habitats include ponds, marshes, lakeshores, streambanks, wet spots in sedge meadows, and stagnant to slow-moving water generally. Flowering may be rare in some colonies, especially in deep water. Found in mesotrophic to hypertrophic conditions. It can be aggressive in its native habitats, and is considered a nuisance in some situations, as in waterfowl habitat along the Mississippi R. at Alma, Wisconsin, where this species has invaded open water visited by tundra swans during the migration (see http://www.nps.gov/miss/naturescience/tundraswan.htm). S. eurycarpum can be displaced by invasive Typha X glauca, as along the Minocqua Thoroughfare in Oneida County, and the shore of University Bay, on the UW-Madison campus. Here, it is a minor component of an 8ft. tall Typha stand. Formerly, S. eurycarpum constituted a major component of the marsh vegetation of University Bay (Heddle, 1910), forming a 10ft. wide “Sparganium zone” or a Sparganium-Typha latifolia zone.
Frequent associates in Wisconsin include Sagittaria latifolia, Typha latifolia, Lemna minor, Eleocharis palustris, Schoenoplectus tabernaemontani, Ceratophyllum demersum, Nymphaea, Phalaris arundinacea, Polygonum amphibium, Alisma, Carex lacustris, Elodea canadensis, Nuphar, Sagittaria rigida, Sparganium americanum, Utricularia spp, Zizania, Asclepias incarnata, Bidens sp., Bolboschoenus fluviatilis, Leersia oryzoides, Sium suave, Acorus calamus, Calamagrostis canadensis, Calla palustris, Carex comosa, Carex utriculata, Cicuta bulbifera, Eleocharis sp., Glyceria grandis, Lycopus americanus, Lycopus uniflorus, Polygonum sp., Potamogeton richardsonii, Spirodella polyrrhiza, Wollfia.
2. Spaganium natans L.
Leaves flat, limp, often entirely submerged, or floating at the tips; a few small upper leaves may extend above the surface. Leaves green on both sides, 2—5mm diameter; often forming dense underwater lawns. Flowering stems leafy, elongating to the water surface: in deep water, producing 6in. internodes, and leaves up to 2 ft. long. Grows in clear water, 6in. to 3 ft. deep, often on a soft, organic substrate; across northern Wisconsin, though not common. Rare in calcareous lakes in southeastern Wisconsin. This species is short in stature and easily overlooked. Typical habitats in northern Wisconsin and Minnesota are marshy to boggy shallow lakes, in protected bays, inlets, and wild rice beds: best observed by canoe. The species has a wide range of conductivity tolerance, varying from 686 μS in the calcareous WI lake to 42μS in an acidic thermal pond in Yellowstone NP. It appears to thrive in areas of low competition, but may grow amongst much taller vegetation. This is a circumboreal species, and is at the southern limit of its range in Wisconsin. Frequent associates include Nuphar, Nymphaea, Utricularia spp, Calla palustris, Chara, Myriophyllum sibiricum, Potamogeton natans, Sparganium acaule, Typha latifolia, Zizania palustris.
3. Sparganium fluctuans (Morong) Robinson
Leaves floating, conspicuously parallel-veined on lower surface, with fainter cross-partitions, especially visible in dead tissue. Leaves white beneath, with layers of non-photosynthetic cells. Leaves flat, ribbonlike; max leaf width per plant 5-7mm. Inflorescence consistently compound, each branch topped with one or more male heads, typically several in tight clusters. The female flowers have curved stigmas. Fruits with strongly curved beaks, the body turning reddish at maturity. Fruits may be uncommon; in Shearer Lake, Taylor County in 2008, 1% of plants had flowers or fruit. Can be confused with S. angustifolium; S. fluctuans leaves are truly flat, not rounded on top as in S. angustifolium; S. angustifolium leaves are green beneath, and much narrower. Found in oligotrophic to mesotrophic lakes in northern Wisconsin, in water 0.5-5ft. deep; often in a zone of floating aquatics bordered on the shallow side with a zone of emergents. Often rooted in sandy substrate. Many habitats have dark-stained water and relatively low (10 to 83μS, mean=63) conductivity; generally not found in the clearest, lowest-conductivity waters. Frequent associates include Nuphar, Brasenia schreberi, Nymphaea, Carex lacustris, Carex lasiocarpa, Dulichium arundinaceum, Eleocharis palustris, Eriocaulon aquaticum, Glyceria borealis, Potamogeton pectinatus, Potamogeton robbinsii, Potentilla palustris, Sparganium americanum, Sparganium acaule, Utricularia spp., Zizania palustris.
4. S. angustifolium Michx.
Leaves floating, linear, the widest usually 2-4mm across; green beneath, slightly rounded above. The inflorescences are simple, often with a lower pistillate head on a stalk; one or more pistillate heads supra-axillary; female flowering heads often held up above surface; fruits floating at or just below surface. Staminate heads few and clustered at stem tip. Found in clear, oligotrophic lakes and occasionally, in streams; frequent in lakes in the sandy regions of northern Wisconsin, south to Portage County. Usually in sandy substrate, in water depths from 0 to 2ft. Prefers very low to low (10 to 115μS, mean=40) conductivity water with low vegetation density. Frequent associates include Isoetes, Lobelia dortmanna, Eriocaulon aquaticum, Carex utriculata, Nymphaea, Potamogeton natans, Equisetum fluviatile, Littorella uniflora, Nuphar, Potamogeton richardsonii, Sagittaria cuneata, Sagittaria rigida, Typha latifolia.
Leaves keeled or triangular in cross section, floating or emergent; pistillate heads supra-axillary, aggregated into a tight cluster at top of stem, sometimes with 1 or 2 lower heads on peduncles. Mature fruits shiny, football-shaped, narrowed to a short beak. Staminate heads 1—2, adjacent to uppermost pistillate head, soon deciduous, and then leaving no visible trace. Grows in ditches, marshes and black ash swamps. Locally common near Superior, Wis. and Duluth, Minn., where first reported by Olga Lakela in the 1930’s; collected recently in other parts of northern Minnesota and northwestern Wisconsin (Walton, 1999). This species is listed as Threatened in Wisconsin. It was observed in a ditch along an old railroad grade, on peat growing with Salix lucida, S. exigua, Typha latifolia, Equisetum fluviatile, Sium suave, Alisma subcordatum, and Bidens sp.
Leaves usually emergent, triangular in cross-section, or flat and keeled above. Leaves typically much exceed the inflorescence. Height ranging from 0.5-2ft. Female heads (at least one) supra-axillary, inflorescence unbranched, lower heads often stalked. The mature fruits have a curved beak, and are somewhat constricted around the midsection; they tend to be red-dotted on the lower surface. Male heads usually three or more, spread out along the upper portion of the stem. This species is easily confused with S. americanum, (especially in flowering or early-fruiting specimens!) S. acaule can usually be identified based on the presence of supra-axillary heads, and a smooth, shiny fruit surface. However, rare individuals have branched inflorescences (see U. Müller-Doblies, 1969; Voss & Reznicek, 2012), or have all pistillate heads axillary. S. acaule may produce floating leaves in deep or fast-moving water, where it rarely flowers. These plants can be differentiated from S. angustifolium by the keeled leaf. S. acaule occurs frequently across the northern half of Wisconsin, in shallow water (up to ≈6in. deep) at lake margins, in bog moats, marshes, in sluggish streams and ditches, and in beaver ponds. It occurs sporadically in southern Wisconsin in marshes, lakes, bogs, fens, streams and river bottoms. It occurs in low and high conductivity water (60 to 650, mean=176μS). Frequent associates include Nuphar variegata, Nymphaea odorata, Zizania palustris, Brasenia schreberi, Sparganium americanum, Carex lacustris, Ceratophyllum demersum, Elodea canadensis, Potamogeton natans, Sagittaria latifolia, Calla palustris, Chara, Dulichium arundinaceum, Eleocharissp., Leersia oryzoides, Lemna minor, Myriophyllum sibiricum, Scirpus cyperinus, Sparganium fluctuans, Sparganium natans, Spirodella polyrrhiza, Utricularia spp.
7. Sparganium androcladum (Engelm.) Morong
Robust, emergent plants to 3 ft. tall, leaves keeled or triangular in cross section, inflorescences branched, or simple with axillary heads. The lateral inflorescence branches usually bear staminate heads only. The pistillate heads are large and few; the mature fruits are glossy, with beaks 4-7mm, straight and often hooked at the tip. This species is often confused with S. americanum, especially in flower or immature fruit. Their ranges overlap in central and northwestern Wisconsin. In Wisconsin, S. androcladum is sporadic across southern and western Wisconsin. It is locally abundant in the bed of Glacial Lake Wisconsin, in Jackson, Monroe, Juneau and Wood counties; in a flowage in Taylor County (Fields, 2003); and may be found as far north as the shore of Lake Superior. Typical habitats are margins of flowages, in mesotrophic, moderate to low conductivity water (from 27 to 82, mean=52μS), 0 to 3 feet deep, on silty, clayey or sandy substrates. It was observed at four localities in Wisconsin. Frequent associates include Brasenia schreberi, Dulichium arundinaceum, Nymphaea, Sagittaria latifolia, Bidens sp., Eleocharis palustris, Phalaris arundinacea, Scirpus cyperinus.
8. Sparganium americanum Nutt.
Robust, emergent plants to 3 ft. tall, leaves triangular in cross section below and keeled toward the tip, light green. In deeper water this species may produce floating leaves, which are keeled. The inflorescence may be branched or simple, with axillary female heads (except where no leaf is present). The tepals of the flowers may or may not have a dark spot. The fruits have a dull, non-glossy surface, beaks 2-5mm long, straight or slightly curved. Typical habitats are peaty to sandy lakeshores, ditches (in central WI), beaver ponds, in sluggish streams, and floating vegetation of boggy lakes, in water 0 to 3 ft. deep. The plants may form large colonies. This species is very common in northern and central Wisconsin, in oligotrophic to mesotrophic water, from low to moderately high conductivity (15 to 200, mean=67μS). Frequent associates in Wisconsin include Eleocharis palustris, Nymphaea, Brasenia schreberi, Nuphar variegata, Sagittaria latifolia, Utricularia spp, Typha latifolia, Dulichium arundinaceum, Elodea canadensis, Zizania, Glyceria grandis, Potamogeton epihydrus, Potamogeton natans, Schoenoplectus tabernaemontani, Scirpus cyperinus, Sparganium acaule, Sparganium eurycarpum, Calamagrostis canadensis, Calla palustris, Carex lacustris, Sagittaria rigida, Acorus calamus, Carex comosa, Glyceria borealis, Glyceria canadensis, Leersia oryzoides, Lemna minor, Potamogeton amplifolius, Potamogeton sp., Puccinellia, Sparganium fluctuans.
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Cook, CDK and MS Nicholls. 1987. A Monographic Study of the Genus Sparganium. Part 2: Subgenus Sparganium. Bot. Helv. 97:1
Fields, DM 2003. The Vascular Plants of Taylor County, Wisconsin. The Michigan Botanist 42(4): 171-282.
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Voss EG and AA Reznicek 2012. Field Manual of Michigan Flora. Ann Arbor: University of Michigan Press.
Walton GB 1999. Report for the 1994-1995 status survey for Sparganium glomeratum in Minnesota. Prepared for the Minnesota Department of Natural Resources Natural Heritage Program. Submitted Dec. 29, 1995.