Natural History

Geographical Origin and History

Flowering rush is native to Eurasia. It is the only member of the family Butomaceae (Gleason and Cronquist 1991). It was probably introduced to North America in the late 1800s, being first documented in Quebec around 1897 (White et al. 1993). It was then reported from Michigan around 1918 (Anderson et al. 1974), and collected from there in 1930 (Voss 1972). It is now common along the St. Lawrence River Valley and Lake Champlain. In recent years it has spread westward from these areas, facilitated in part by its sale as a garden plant. While still relatively uncommon in the upper Great Lakes region, flowering rush is now established in scattered sites in Michigan, Minnesota, and Wisconsin, including one site in northern Oneida County, Wisconsin (Voss 1972, MINN 2004, WIS 2005).

Characteristics

Flowering rush can grow as an emergent plant, or submersed in water to several meters deep. It is strongly rhizomatous, potentially forming extensive patches. The leaves arise from the rhizome in two rows or ranks. Aerial leaves, produced in shallow water and on wet ground, are long, narrow, and stiff, without a blade, and can reach 1 m (3.3 ft) tall (Gleason and Cronquist 1991). Submersed leaves produced in deeper water and are limp and flexible. The stems and leaves are triangular in cross-section. The leaf tips tend to twist spirally.

Only plants growing in shallow water or on moist ground produce flowers. The flower stalks may reach 1.5 m (4.9 ft) tall or more (Gleason and Cronquist 1991), though they are usually shorter than this. Numerous attractive pinkish-white (rarely white or pinkish-brown) flowers about 2.0-2.5 cm (0.8-1.0 in) across are produced in a terminal head. Each flower is on its own stalk, with each stalk being about the same length and coming from about the same point, forming an umbel. Each flower has three shorter sepals alternating with three longer petals.

Similar Species

Flowering rush is a very distinctive and easily-recognized plant when in flower. Except for several members of the carrot family, or Apiaceae (e.g., water hemlock, Cicuta maculata L., and water parsnip, Sium suave Walt.), flowering rush is the our only aquatic plant that produces flowers in umbels. These carrot family plants differ from flowering rush in a number of characteristics, though, including their round stems, numerous small, white, 5-petaled flowers, and flat, dissected leaves. Each flowering rush umbel typically consists of 20-50 flowers (Wilder 1974). The flower stalks persist beyond the flowering time and can be helpful when searching for flowering rush in late summer or early fall.

When not in flower, flowering rush can be difficult to detect, as it resembles the sedges and bulrushes with which it often grows. Sedges and bulrushes also have stems that are more-or-less triangular in cross-section, but the leaves are generally flat or folded. Leaves of mannagrass (Glyceria spp.) and several other wetland grasses tend to be weakly triangular, but their stems are jointed and their flowers are constructed much differently, lack sepals and petals, and are not showy. Arrowheads (Sagittaria spp.) often have leaves that are triangular in cross-section, and appear very similar to flowering rush when not in flower, but their roots are clearly segmented while flowering rush roots are not. Most bur-reeds (Sparganium spp.), have leaves that are triangular in cross-section as well, but the seedheads resemble large burs.

Reproduction and Dispersal

Flowering rush is perennial, regenerating each spring from the underground rootstock. Dispersal is mainly vegetative, via small, pea-sized bulblets that form in large numbers along the rhizomes (Wilder 1974). These bulblets are easily detached, with each bulblet capable of growing into a new plant. Triploid plants (see below) sometimes produce bulblets in place of some of the flowers (Hroudova et al 1996, Eckert 2000). Rhizome fragments and bulblets can be dispersed by water, animals, or human activities, and form new plants. Rhizomes and attached bulblets are sometimes spread by muskrats, which use plant parts for their homes (MNDNR 1999).

Flowering takes place from July through August in the upper Great Lakes region. Only plants in shallow water or on wet ground produce flowers. The seeds float, and can live for many years. In their 19 eastern Canada populations, Eckert (2000) found an average seed production of 200 seeds / flower and 6148 seeds per umbel, though two populations had seed set rates of under 1%. Eckert (2000) and others have found seed viability as high as 64% under laboratory conditions. Survival of seedlings is apparently low in natural habitats, as the seedlings are poor competitors with existing vegetation (Hroudova et al 1996).

Two genetic types of flowering rush exist: diploid types (two types) with two sets of chromosomes, and a triploid type with three sets of chromosomes (Gleason and Cronquist 1991). Flowers of the diploid types are self-compatible, and capable of producing a significant number of viable seeds, while those of the triploid type are self-incompatible and rarely produce viable seed (Hroudova et al 1996). Only one of the approximately 12 known populations in Minnesota produces viable seed, a factor which may be responsible for the relatively low rate of spread of this plant in the state so far (MNDNR 1999). Diploids probably have an advantage in long-distance dispersal, but triploids produce more bulblets and spread vegetatively more rapidly (Hroudova et al 1996).

Habitat Preference and Tolerance

Flowering rush inhabits shores, riverbanks, marshes, ditches, shorelines, seasonally flooded fields, and other wet places (Hroudova et al 1996, MNDNR 1999). It typically grows as an emergent on wet ground or in shallow water, but can also grow as a submergent, forming persistent vegetative patches in fairly deep water. It tolerates constant water levels without difficulty (Hroudova et al 1996). In its native Europe it tends to be suppressed by shoreline sedges and other species, and often forms a band between the shoreline community and the waters edge when water levels fall (Hroudova et al 1996). It responds well to fluctuating water levels and quickly colonizes newly-exposed ground. Rivers, with their fluctuating water levels and dispersal of bulblets and seeds downstream, are quickly colonized (Hroudova et al 1996, Lavoie et al 2003).