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Belowground Parts

The main belowground parts of Scirpus maritimus and S. robustus are roots, rhizomes (horizontal stems often called stolons) and corms (enlarged stem bases often called tubers; Fig. 1). No rooting material of either species occurs more than 60 cm belowground. In nature, the bulk of the belowground biomass of S. maritimus probably occurs within 20 cm of the surface (Fiala et al. 1968), but corms can form at 30-40 cm depth in culture. McNease and Glasgow (1970) found more than 98% of the rhizome mass of S. robustus less than 11 cm belowground and none at depths exceeding 18 cm. Scirpus maritimus rhizomes are white and thin and seldom exceed 4% of the plant mass. Shoots, roots, and corms form at nodes (joints) along the rhizomes that are usually unbranched. The internodal length of S. maritimus is usually about 4-12 cm (rarely as long as 20 cm). Smaller corms are sometimes adnate to a larger mother corm. These thick (as wide as 3 cm), ligneous, and scaly structures are creamy white when young but soon turn brown and finally black. The dry weight of an S. maritimus corm may exceed 0.5 g and may account for 60% or more of the plant mass. Each corm has 1-5 buds that can grow or assume dormancy. Corm growth can lead to more corms or adventitious shoots (tillers). In S. maritimus, as many as 45 tillers may develop in one season from a single overwintered corm. Living roots of both bulrushes are tan to orange and rigid, whereas dead roots are brown or black and flaccid. The maximum root length of North Dakota specimens of S. maritimus is about 10 cm (personal observation).

Figure 1. Scirpus robustus. a. Habitat showing rhizome-forming corms and general apperarance of a mature plant, x 0.16; b. lens-shaped achene showing anthers, stigma, and bristles, x 7.6; c. cross section of achene, x 7.6; d. inflorescence, showing involucral leaves, x 2.0; e. awned scale, x 4.9. Appearance of S. maritimus is similar. (From Mason 1957; reproduced with permission of University of California Press, Berkeley.)

Aboveground Parts

Scirpus maritimus and S. robustus attain heights of about 1.5 m, but the average is much less and can be as little as 8 cm in dry or heavily grazed stands. During peak growth periods, stems elongate at about 1-3 cm/day. Reproductive stems can be twice as tall as vegetative stems. The dry weight of a mature stem is about 1-5 g. Reproductive stems usually compose about 20-75% of the total stems when maximum standing crop is attained.

Scirpus robustus has as many as nine stem leaves, S. maritimus as many as 13; leaves of both species are as much as 60 cm long and 1.5 cm wide. Leaves of S. robustus can grow 3.8 cm/day in the laboratory. As many as four leaflike bracts attach below the inflorescence. The inflorescence consists of one to several ovoid to lanceolate spikelets, 1-4 cm long, sessile and clustered or some on the tips of rays (Fig. 1). Dry S. maritimus inflorescences weigh as much as 1.03 g, usually less than 2% of the total plant mass. In S. maritimus (and probably S. robustus) the stigmas mature before the anthers. Seedlings of both bulrushes probably do not flower in the first year, at least not in temperate regions.

Spikelets of mature plants produce as many as 25 olive to black, one-seeded, indehiscent fruits called achenes. These are shiny, flattish to lens-shaped or with a low angle on the back. They are as much as 0.4 cm long and have an apiculate tip that represents the persistent style base. The volume of a single S. maritimus achene is about 0.004 ml; the air-dried weight is about 5.6 mg. Air-dried S. robustus achenes weigh about 3.3 mg and fresh achenes about 3.5 mg. Isely (1944), Koyama (1962), Schuyler (1975), Browning and Gordon-Gray (1992, 1993), and Browning et al. (1995) provide additional information on spikelet, achene, and embryo morphology of S. maritimus, S. robustus, and related species.

Phenology

Phases in the growth of established stands of S. maritimus, from the beginning to the end of the growing season, are as described by Dykyjova et al. (1972): (1) shoots use energy reserves stored in corms; (2) belowground parts grow after aboveground growth is well developed; (3) generative organs form when aboveground biomass is maximum; (4) organic substances translocate to belowground parts at senescence.

In northern California and in the Pacific Northwest, S. robustus and S. maritimus show winter dormancy, new shoot growth from corms in March and April, flowering in May, peak growth rate in June or July, and peak shoot mass in August or September. However, New York populations of S. robustus sometimes do not flower until mid-June, nearly 1 month later than S. maritimus. Louisiana populations of S. robustus also are dormant in winter, begin spring growth when soil temperatures exceed 16° C, flower in mid-April, and maturate achenes in late August. The initial growth of S. maritimus in Utah begins in early to mid-April; anthesis is in early May. In Kansas, S. maritimus flowers from May through July and fruits from June through August. Saskatchewan plants begin growth from corms in May, the first shoots flower in June, the second nonflowering shoots mature by late August, and all aboveground parts are dead by October. In southern France, S. maritimus is dormant from October to January, at which time the underground buds begin growth. Shoots appear in February, and in early March buds destined to become rhizomes appear. The underground network is well developed by the end of April, flowering starts in mid-May or early June, and fruiting begins at the end of June or in early July. In Czechoslovakian fish ponds, S. maritimus sprouts corms at the beginning of May, flowers by late June, and ripens achenes by the end of August; peak biomass occurs after 86-94 days of growth. In Baltic populations of S. maritimus, new stems are evident by the end of April; flowering occurs in late May, and stems are dead by late July or early August.