Rare Species Guide

  Basis for Listing

Historically, Crotalaria sagittalis var. sagittalis (rattlebox) is known to have occurred in five counties in east central and southeast Minnesota (Chisago, Washington, Ramsey, Dakota, and Houston), but the only recent records are from Washington and Houston counties (Eastern Broadleaf Forest). Its native habitat appears to be sand prairies and sand savannas, which are in very short supply. These habitats were rare to begin with, but since settlement began (ca 1850) they have been nearly eliminated by agriculture, urban and suburban developments, and most recently by exurban development; only small fragments remain (Minnesota's Remaining Native Prairie).  It is unclear at this time if these small fragments can support viable populations of C. sagittalis var. sagittalis; this species was given the status of special concern in 2013.

  Description

Stems of C. sagittalis var. sagittalis are ± erect, simple or branched above, and 10-40 cm (4-16 in.) tall. They are covered with ascending or loosely spreading hairs. The leaves are sessile, or nearly so, lanceolate to linear in shape, 3-8 cm (1-3 in.) long, and up to 1.5 cm (0.6 in.) wide. The stipule of the leaf is sagittate, meaning that it is shaped like an arrowhead. The flowers are similar in structure to those of a pea, are yellow, and arranged in racemes of 2-4. The racemes alternate with the leaves on the stem. The fruit is an inflated pod, 1.5-3.0 cm (0.6-1.2 in.) long.

  Habitat

It appears that C. sagittalis var. sagittalis requires dry, loose and sandy soil and direct sunlight. It should be noted that in Minnesota this species does not occur in sand deposits that are not supporting a stable and healthy community of native plants. Such a community most closely resembles a prairie, or some type of sparsely vegetated grassland. Savannas also develop on sandy soil and could have been a habitat for this species in pre-settlement times.

  Biology / Life History

Although in the southern part of its range C. sagittalis var. sagittalis is reported to be a perennial, in Minnesota it is probably an annual (Senn 1939). As an annual which occurs in sandy soil, C. sagittalis var. sagittalis likely relies on periodic soil disturbance for reproduction. This would serve the dual purpose of exposing buried seeds and creating a small patch of exposed habitat to serve as a seed bed. Under suitable conditions, the seeds would germinate, grow to maturity, flower, and set seed before perennials recolonized the exposed patch of habitat. Suitable disturbance is probably provided by wind or water erosion or possibly by animal burrowing. Anthropogenic disturbance such as road building, sand extraction, or anything done with heavy equipment may mimic natural disturbances, at least temporarily, but may not provide the cyclical processes at a scale needed to support a viable population of C. sagittalis var. sagittalis. A full-functioning ecosystem, by its very definition, does provide these functions, which is why it is so important to preserve or restore these systems wherever possible. 

By all indications, pollination is accomplished by flying insects, probably large or midsize bees. Seeds are produced in a pod that splits open when dry, releasing the seeds to the force of gravity and, to a lesser extent, the forces of wind. No specialized dispersal agent is known, though the seeds are of a size (2-3 mm (0.08-0.12 in.)) that might attract small mammals or seed-eating birds, which could serve as inadvertent dispersal agents.

  Conservation / Management

The original habitat of C. sagittalis var. sagittalis in Minnesota is believed to have been sand prairies. The vegetation in this habitat type was apparently maintained by a complex interaction of forces including wind, wild fire, regional climate, and wild animal grazing/browsing/burrowing. Although wind and climate patterns have not changed appreciably since settlement, wild fires and most species of large animal have been eliminated from the landscape. This explains, in part, why full-functioning examples of this habitat type are no longer found in Minnesota. Regrettably, all we have left are small habitat fragments. Fire in the form of prescribed burns has been returned to a few of these fragments; however, there is no evidence that this is restoring conditions needed by C. sagittalis var. sagittalis. Perhaps all that can be done with these fragments is to protect them from encroachment and damage by motorized vehicles. 

  Best Time to Search

Crotalaria sagittalis var. sagittalis is a late season plant. It likely will not even be visible above ground until July and will not start flowering until mid-July. Pods develop in August and are held on the stem until the end of September or middle of October, though the leaves will have been shed by that time. The recommended time to search is from mid-July to early September.

  Authors/Revisions

Welby Smith (MNDNR), 2018

(Note: all content ©MNDNR)

  References and Additional Information

Flora of North America Editorial Committee, eds. 2022. Flora of North America North of Mexico, List of Online Accepted Names, exported March 30, 2022.

Minnesota Department of Natural Resources. 2005. Field guide to the native plant communities of Minnesota: the eastern broadleaf forest province. Ecological Land Classification Program, Minnesota County Biological Survey, and Natural Heritage and Nongame Research Program. Minnesota Department of Natural Resources, St. Paul, Minnesota. 394 pp.

Minnesota Department of Natural Resources. 2006. Tomorrow's habitat for the wild and rare: An action plan for Minnesota wildlife, comprehensive wildlife conservation strategy. Division of Ecological Services, Minnesota Department of Natural Resources. 297 pp. + appendices.

Minnesota Department of Natural Resources. 2009. Map of Minnesota's remaining native prairie 100 years after the public land survey. . Accessed 19 June 2009

NatureServe. 2009. NatureServe Explorer: An online encyclopedia of life [web application]. Version 7.1. NatureServe, Arlington, Virginia.

Senn, H. A. 1939. The North American species of Crotalaria. Rhodora 41:317-370.

Wisconsin Department of Natural Resources. 2007. Arrow-headed Rattle-box (Crotalaria sagittalis) factsheet. <http://dnr.wi.gov/topic/endangeredresources/Plants.asp>. Accessed 26 June 2009.