1. Nasturtium officinale R. Br. (watercress)

Sisymbrium nasturtium-aquaticum L.

Rorippa nasturtium-aquaticum (L.) Hayek

Nasturtium officinale R. Br. var. siifolium (Rchb.) W.D.J. Koch

Pl. 325 i–k; Map 1373

Plants perennial herbs, with rhizomes, usually emergent or floating aquatics, glabrous. Emergent, submerged, and/or floating stems 10–65(–200) cm long, rooting at most nodes. Leaves alternate (basal leaves absent except in seedlings), 2–10 cm long, petiolate, the bases usually clasping the stem with small, rounded auricles, pinnately compound with 3–9(–13) leaflets or less commonly simple, especially when plants occur in relatively deep water, the leaflets linear to irregularly ovate or nearly circular, the margins entire, wavy, or with few, shallow, blunt teeth. Sepals 1.5–2.5(–3.5) mm long. Petals 2.5–4.8(–6.0) mm long, white. Styles absent or 0.1–0.5 mm long. Fruits 10–15(–20) mm long, straight or slightly arched upward. Seeds mostly 20–50 per fruit, in 2 rows in each locule, 0.9–1.3 mm long, nearly circular in outline, the surface with a coarse, netlike or honeycomb-like pattern of 25–60 ridges and pits on each side, reddish orange to reddish brown. 2n=32. April–October.

Introduced, scattered, mostly in the Ozark and Ozark Border Divisions (native of Europe, Asia; widely naturalized in the U.S. and adjacent Canada). Emergent, submerged, and/or floating aquatic in spring branches and streams, less commonly terrestrial on banks of streams or stranded by a receding waterline, and occasionally fens and marshes; also ditches.

This species is a conspicuous member of the aquatic flora in spring branches and streams in the Ozarks. Plants often are encountered as large, nonflowering, submerged colonies. Pieces from these are easily dispersed by water currents, and much of the species’ distribution in southern Missouri is probably due to vegetative reproduction. Steyermark (1963) suggested that watercress was native in Missouri, based upon its occurrence in relatively remote spring branches in the Ozarks. Unfortunately, these localities are somewhat less pristine than Steyermark thought, and the native range of N. officinale is certainly confined to the Old World. There are relatively few Missouri specimens in herbaria that were collected prior to 1890, and the labels on these indicate that the plants collected then were introduced at the collection sites. Voss (1985) discussed a similar situation in Michigan and also concluded that watercress is not native there.

Watercress is maintained as a separate genus, Nasturtium, based on extensive molecular data (Les, 1994; Bleeker et al., 1999, 2002; Sweeney and Price, 2000) and critical morphological comparison with Rorippa (Al-Shehbaz and Price, 1998), in which it has been maintained previously (Al-Shehbaz, 1988b). These studies demonstrate that Nasturtium is much more closely related to Cardamine than to Rorippa.

All of the Missouri specimens examined thus far are referable to the diploid (2n=32) cytotype, N. officinale, which is characterized by slightly broader and shorter fruits with seeds in 2 rows in each locule, as well as seeds with a coarser pattern of reticulation. The tetraploid (2n=64) cytotype is known as N. microphyllum Boenn. ex Rchb.; it also is widely naturalized in the United States and Canada, but it tends to have a slightly more northern distribution. It has been reported from as far south as Nebraska and Kentucky (Barker, 1986; Rollins, 1993), but because most collections do not include mature fruits, its actual distribution in this country is not known well. This taxon should be looked for in Missouri.

Watercress has a long history of use as a salad green. It currently is cultivated to a limited extent in the United States for the gourmet food market. Plants collected from the wild should be washed carefully prior to consumption to avoid accidental ingestion of microscopic parasites, such as the protozoan Giardia, that may be present in untreated water.