Cardamine impatiens L. (Brassicaceae) in New Jersey

Journal of the Torrey Botanical Society, Jul-Sep 2004 by Glenn, Steven D, Barringer, Kerry

GLENN, STEVEN D. AND KERRY BARRINGER (Brooklyn Botanic Garden, 1000 Washington Avenue, Brooklyn, NY 11225-1099, U.S.A.). Cardamine impatiens (Brassicaceae) in New Jersey. J. Torrey Bot. Soc. 131: 257260. 2004.-Cardamine impatiens L. is reported for New Jersey for the first time. Its life history is reviewed, and hydrochory is hypothesized to be a major component of seed dispersal. An efficient dispersal system combined with prolific reproductive traits suggests invasive behavior, and warrants continued monitoring of this species.

Key words: Cardamine impatiens, New Jersey, invasive plants, seed dispersal.

Canlamine impatiens L. (Brassicaceae), the narrowleaf bittercress, is an annual or biennial herb native to Eurasia that has become naturalized in many parts of the eastern United States (Rollins 1993, USDA 2003). This wide-ranging species is native from Scotland eastward into northern and central Asia to Japan (Ohwi 1984, Schul/. 1903, Stace 1997), occurring in wet woods, along rivers and streams, and in wet meadows, often on calcareous substrates.

The first published record of Cardamine impatiens in the northeastern United States appears to be the eighth edition of Gray's Manual of Botany (Fernald 1950), where it was said to occur in southern New Hampshire and eastern Pennsylvania. It has been recently reported in Connecticut, Delaware, Massachusetts, New York, and Pennsylvania (Mehrhoff et al. 2003, USDA 2003, Weldy et al. 2003, Wherry et al. 1979). It is listed as invasive by the Connecticut Invasive Plant Working Group (2003), and the Invasive Plant Atlas of New England (Mehrhoff, et al. 2003). It has not been previously reported foiNew Jersey (Hough 1983, Rollins 1993, USDA 2003), but recent fieldwork and herbarium studies show that the species grows in northern New Jersey and is spreading.

Materials and Methods. Cardamine impatiens was observed and collected during the 2000 through the 2003 field seasons. The appearance and growth of the plants was noted throughout the year. Additional specimens were sought at BKL, CHRB, PH, and NY.

Results and Discussion. The first known collection of C. impatiens from New Jersey was made in Morris Township, Morris County, in 1990 (L. J. Mehrhoff 14175, BKL, CONN). No other collections have been found in regional collections from before 2000. During ficldwork in 2000-2003 plants were found in Essex, Hunterclon, Mercer, Monmouth, Morris, Passaic, and Somerset Counties (Figure 1). Populations of C. impatiens were found in floodplains along the Musconetcong River, the North and South branches of the Raritan River, the Black River, and the Delaware and Raritan Canal. They were also found along smaller streams feeding these rivers. Occasionally, isolated populations were observed on mesic sites in shaded woods, roadsides, footpaths and lawns, well removed from any riparian system.

New Jersey collections of Cardamine impatiens are essentially glabrous herbs with a yellow-green color. The first year, a rosette of basal leaves is formed. The basal leaves can be more than 10 cm long and have three to eleven leaflets along their rachis. The leaflets are usually 3lobed, and the lobes are rounded. Rarely, first year plants will flower and set fruit in the fall. Over winter, the leaves of the basal rosette usually die. In the second year, upright flowering stems begin to grow from the old rosettes in April. These stems can grow to be almost a meter tall, but are usually about half that. The upright stems have many, alternate, pinnately compound leaves with sharply toothed or lacerate leaflets.

The leaves are distinctive among our cardamines in having auriculate leaf bases. Cardamine impatiens stems do not branch near the base, only above in the inflorescence. There they are borne upright and have flowers along their length. Flowers are small, with white petals up to 2.5 mm or often apetalous, and last only a short time. The first flowers appear in May, but flowering plants may be found until September under the right conditions. The fruits are thin siliques that are borne upright on short pedicels.

When the fruit is mature, the two valves dry and contract, setting up a mechanical tension. The valves of the siliques suddenly coil upward, shooting the seed a short distance from the plant. Dehiscence can be caused by the slightest touch, reminiscent of Impatiens species or Cardamine hirsuta. Plants can bear flowers and fruits at the same time, and die after fruiting. In the Northeast, this species would most likely be confused with Cardamine pensylvanica Muhl., and their differences are summarized in Table 1.

Investigations of Cardamine impatiens in Asia have found prolific reproductive traits. One study found self-pollination and fructification rates of over 99 percent (Kimata 1983). Each fruit can produce 10-26 seeds and each plant can produce copious fruits; one plant yielded over 5,500 seeds (Kimata 1983). Reported seed weights range from 0.086 mg (Kimata 1983) to 0.193 mg (Nakanishi 1988). Mature seeds from New Jersey specimens are light orange-brown, small, obliquely-rectangular to ovoid approximately 1.5 � 1 mm.