USGS Home
Contact USGS
Search USGS

Sexual Reproduction

I assume that alkali and saltmarsh bulrushes are wind-pollinated. Scirpus maritimus pollen is eaten by some syrphid flies, but it is unknown whether these flies are effective pollinators (Leereveld et al. 1981; Leereveld 1984).

Achene germination and seedling growth are relatively rare events in mature, natural stands of S. maritimus and S. robustus; sexual reproduction mostly serves to spread plants to new areas. In general, plants shift energy to achene production when water becomes deeper and salt concentrations decrease. Achene production may depend on energy stored in corms during prior years. Achenes of both species initially float and eventually sink and overwinter on or under submersed sediments. Scirpus maritimus achenes sometimes overwinter in the spikelets and shed gradually until spring.

The achenes of both bulrushes float for longer periods than do propagules of many other hydrophytes. Scirpus maritimus achenes float for as long as 4 weeks and those of S. robustus for more than 4 months. The floating achenes of both species disperse, primarily from winds or currents, to shores or shallow vegetated areas. Dispersal by birds probably is common. Feeding trials show that the viability of S. maritimus achenes is enhanced in the intestinal tracts of killdeers (Charadrius vociferus) and mallards (Anas platyrhynchos), where retention times average about 4 and 2 days, respectively.

After sinking, the achenes are viable for at least 2 years. Achenes can be abundant in sediments supporting submersed hydrophytes (Grillas et al. 1993), but few occur in the deeper areas of large wetlands. Nearly all achenes are in the upper 6 cm of bottom sediments. Achenes from previous growing seasons can be resuspended by wave action (Foote 1988). Achenes sometimes are well represented in seed banks, even when plants do not occur in existing vegetation. Achenes germinate under conditions of relatively high illumination and temperature and low salinity in shallow water or on exposed saturated bottoms. Scirpus robustus achenes can germinate while floating. Roots emerge only when seedlings are in contact with bottom substrates.

The germination requirements of S. maritimus and S. robustus achenes are well known (see later section on propagation). Scirpus maritimus achenes germinate poorly at maturity, but 97% germination can be obtained 2 months later. Scirpus robustus achenes collected late in the growing season and stored in air-tight containers for more than 5 years readily germinate. Regardless of age, achenes held at a constant 2-4° C germinate at higher rates. Such after-ripening and stratification occur naturally in wetlands during the winter after production. Ninety percent germination of S. robustus achenes can be obtained in commercial germinators set for a daily cycle of 14 h of light at 35° C and 10 h of dark at 20° C. Germination begins in 7 days and can continue for 30 days. Pericarp injury by mechanical means or by acid treatments increases germination. Germination requires light yet seems little affected by photoperiod length. Darkness may induce a secondary dormancy in S. robustus achenes that may prevent their germination when buried or when exposed in heavy shade.

Germination of achenes of both bulrushes increases with increasing daytime temperatures and when temperatures fluctuate. Regardless of storage conditions, S. robustus achenes subject to alternate freezing and thawing or constant freezing germinate better than those stored at room temperature. Optimum temperatures for S. robustus germination are 35° C during day and 25° C at night. For S. maritimus, optimums are 25-35° C during the day and at least 5° C lower at night.

Under proper light and temperature cycles, S. robustus achenes show high germination rates on paper moistened with distilled water. Even small amounts of salts depress germination and seedling growth. Germination is reduced by about half when the paper is moistened with water of 9-15 g/L salinity and is nil when salinity is more than 21 g/L. Scirpus maritimus achenes may continue to germinate at slightly higher salinity levels. Salinity probably reduces germination by reducing water absorption by the achenes. Salinity reduces the speed as well as the percentage germination of S. maritimus achenes. No chemicals found in other hydrophytes are known to significantly depress achene germination in either of the two bulrushes.

Achenes of the two bulrushes may differ in moisture required for germination. As little as 2.5 cm of water over S. robustus achenes held in pure quartz sand stifles germination for at least 25 days, but the same achenes germinate normally when water levels are lowered to the sand surface. However, more S. maritimus seedlings emerge when core samples of bottom substrate are held under 4-5 cm of water than when they are merely kept moist, although the germination rate is highest when these conditions alternate. For this species, water depths of less than 10 cm have little effect on germination of achenes buried 2 cm or less in sediments. In summary, both bulrushes have adaptations for achene production, dissemination, and germination in unstable environments where water levels and salinities fluctuate and bottom sediments are exposed.

Asexual Reproduction

The common mode of expansion of clones of Scirpus maritimus and S. robustus is by a network of creeping rhizomes and corms that are concentrated at the periphery. After the dormant period, shoots and tillers initially grow from the old corms; several weeks later buds destined to become rhizomes appear. Rhizomes grow in a linear pattern and bear new shoots at regular intervals. Little, if any, seedling production occurs inside an established network of rhizomes. Clones of S. maritimus can expand vegetatively as fast as 2 m/year and attain a diameter of 24 m. The vegetative life cycle of this species is about 130 days. The central portions of older annular clones often die and expose corms, roots, and rhizomes. If competition from other plants is not severe, rhizomes of S. robustus can gain 5.2 g/day live weight, grow 2.1 cm/day, and add 0.2 nodes/day during peak growing periods. This species also reproduces from rhizome fragments if adequate growing conditions occur within 5 months; whether S. maritimus has this ability is unknown.

A substantial amount of the current-year growth of both bulrushes depends on the carbohydrate-rich corms that can remain dormant under conditions of low water levels and high salinity for at least 2 years. Germination of S. maritimus corms is inhibited when moisture content falls below 16% or sediment surfaces remain dry for about 1 month (Cao 1974, cited in Vergara et al. 1977). Scirpus maritimus exhibits bud dormancy whereby some of the usually 1-5 buds can remain dormant when the corms first germinate. Like achene germination, corm sprouting and growth are negatively affected by increased water salinity. The upper salinity limit for sprouting of S. robustus corms is about 21-30 g/L.