Rare Species Guide

  Basis for Listing

Astragalus flexuosus var. flexuosus (slender milk-vetch) occurs throughout the western plains and grasslands of North America, but its occurrence in Minnesota is at the far eastern edge of the species' range. In Minnesota, it occurs in nine counties along the westernmost part of the Minnesota River valley and in the northern part of the Prairie Coteau. Historically, it may have occurred in several more adjoining counties, but apparently was never widespread in the state.

One of the main reasons A. flexuosus var. flexuosus is rare is loss of its prairie habitat. Of the original 7.2 million hectares (18 million acres) of prairie that occurred in Minnesota, less than 2% remains intact (Minnesota's Remaining Native Prairie), primarily due to conversion to agricultural production. Remnant prairies in Minnesota are threatened by overgrazing, invasion by exotic species, herbicide application, and continued land conversion, especially for gravel quarrying. Although there are over 50 recorded locations for this species in the state, most populations are small, and many of them are concentrated in geographically limited areas. This makes the plant vulnerable to localized threats. Because of its limited geographic distribution, small population sizes, and threats to its habitat, A. flexuosus var. flexuosus was listed as a special concern species in Minnesota in 1984.

  Description

The overall impression of A. flexuosus var. flexuosus is that of a low, sprawling plant (less than knee-high), that appears almost viny. It is a perennial legume that can form clumps nearly 1 m (3.3 ft.) in diameter, with several to many stems giving rise to the clump from a branched crown. Though they tend to recline along the ground, the tips of the stems turn up. The color of the leaves and stems is greenish or grayish because of the hairs on the plant. The leaves have 11-25 leaflets arranged along 2 sides of the leaf axis, with a leaflet at the tip. Leaflets are linear to narrowly oblong-lanceolate; leaflet tips are blunt, squared-off, or with a small notch at the tip. At the base of the leaf are 2 leafy stipules, which are 1.5-7 mm (0.06-0.28 in.) long and connected at their bases; their tips are separate.

Astragalus flexuosus var. flexuosus tends to have many racemes of flowers, and the pea-shaped flowers are pale purple. They are arranged in a cluster of 12-26 flowers at the tip of the flower stalk (peduncle). Peduncles arise from the base of the leaves and are 2.5-6 cm (0.98-2.36 in) long. At the base of each flower is the calyx tube, which is approximately 2.5 mm (0.10 in.) long with teeth about 1 mm (0.04 in.) long. The fruits are sessile (unstalked) pods 1.2-2.4 cm (0.47-0.94 in.) long and they have short, sharp, stiff, appressed hairs.

There are 12 species of Astragalus in Minnesota, but only 6 that overlap in range with A. flexuosus var. flexuosus. Astragalus flexuosus var. flexuosus can be distinguished from the others by the following combination of characters: length of the calyx tube as noted above; flower banner 7-9 mm (0.28-0.35 in); and leaf stipules connate at the base. Superficially, A. flexuosus var. flexuosus might be confused with A. crassicarpus var. crassicarpus (ground plum) in terms of height, appearance of leaves, and their tendency to form large, sprawling clumps. However, A. crassicarpus var. crassicarpus flowers earlier in the season, its flowers are clustered in the center of the plant as opposed to being distributed all along the sprawling stems as in A. flexuosus var. flexuosus, and its clumps are smaller than those characteristic of A. flexuosus var. flexuosus. The fruits of A. crassicarpus var. crassicarpus are also very distinctive in that they appear to be fleshy, cherry-size spheres clustered on the ground in the center of the clump. These turn red as they mature, hence the common name for this species (ground plum). In contrast, A. flexuosus var. flexuosus forms narrow, elongated, tubular fruits at the ends of elongated stems.

  Habitat

In Minnesota, A. flexuosus var. flexuosus is found in dry prairies and in mesic prairies, and most commonly in hill prairies, which are frequently dry-mesic in nature. Both types of prairie are dominated by grasses, but in dry prairie the midheight and shortgrass species are prominent, while in mesic prairie, tallgrasses dominate. Astragalus flexuosus var. flexuosus seems to be found across quite a range of moisture and topographic conditions, from dry hill prairie to rolling mesic prairie. It can be found on level terrain, such as on flat uplands at the top of blufflines, or on moderate and steep slopes, and it can be found mid-slope or at the crest of slopes. It grows on all slope aspects (west-, south-, east-, and north-facing). Soils where the species is found are typically loams, including sandy loam, clay loam, and gravelly clay loam.

Most of the recorded locations of A. flexuosus var. flexuosus in Minnesota are in the Minnesota River Prairie subsection, with just a few in the Coteau Moraines subsection. Population concentrations are found on blufftops above Lake Traverse and Big Stone Lake, and on glacial river terraces of the Minnesota River. Other populations are scattered over various glacial landforms, including one site where A. flexuosus var. flexuosus occurs on dry gravel prairie on a glacial kame.

Prairie plants often growing in association with A. flexuosus var. flexuosus include Bouteloua curtipendula var. curtipendula (side-oats grama), Schizachyrium scoparium var. scoparium (little bluestem), Hesperostipa spartea (porcupine grass), Astragalus crassicarpus var. crassicarpus (ground plum) and Amorpha canescens (leadplant). It can also co-occur with two rare species, Solidago mollis (soft goldenrod) and Astragalus missouriensis var. missouriensis (Missouri milk-vetch).

  Biology / Life History

Like most legumes, A. flexuosus var. flexuosus is insect pollinated. Many species of Astragalus are somewhat toxic to livestock. They can have either nitrotoxins or toxic alkaloids in their tissues, and some species accumulate selenium. All of these substances can cause illness in livestock with prolonged grazing. However, many Astragalus species also have high forage value and cattle will graze on them, especially in the early spring when other forage is scarce. Astragalus flexuosus var. flexuosus does contain nitrotoxins, but does not contain toxic alkaloids or accumulate selenium (Williams and Barneby 1977; Ralphs et al. 2002). Based on available literature, it does not appear that A. flexuosus var. flexuosus is an especially problematic species for grazing livestock.

  Conservation / Management

The remnant habitats for A. flexuosus var. flexuosus are mostly in grazed landscapes (either currently grazed or grazed in the past). Given this setting, the species faces several threats. Overgrazing causes erosion and compaction, which may degrade prairie habitats. The common practice of managing active pastures with herbicide to control Cirsium spp. (thistles) further threatens prairie forbs (Minnesota County Biological Survey 2007). Many pastures are also heavily invaded by non-native cool-season grasses such as Poa pratensis (Kentucky blue grass) and Bromus inermis (smooth brome), which outcompete native species for light, space, nutrients, and/or water. Other non-native herbs such as Euphorbia virgata (leafy spurge) can also be a problem. Lastly, hilly habitats of A. flexuosus var. flexuosus are threatened by habitat conversion. While not as big a concern as it is for Astragalus missouriensis var. missouriensis, aggregate quarrying may be a threat where A. flexuosus var. flexuosus populations are located on gravelly sites. Several programs and resources are available to land managers and landowners who want to help protect and manage remaining prairie parcels, including the Native Prairie Bank Program, the Native Prairie Tax Exemption Program, and a prairie restoration handbook.

Prescribed fire is recommended as a habitat management tool to reduce the dominance of non-native cool-season grasses. It can also be used to target certain invasive species, and if woody cover is increasing at the site, it can keep invading shrubs and trees in check. Targeted use of herbicide may be necessary to control some exotic species.

  Best Time to Search

The best time to search for A. flexuosus var. flexuosus is when the plants are flowering or fruiting, from about June through early September.

  Conservation Efforts in Minnesota

Over 1/3 of the existing populations of A. flexuosus var. flexuosus are on lands owned by state or federal agencies or private conservation organizations. This affords the species some level of protection from development, and some of these areas are being actively managed to perpetuate the prairie habitats. However, no known attempt has been made to specifically assess the conservation needs of A. flexuosus var. flexuosus within these areas or to determine the effects of current management practices on its status.

  References and Additional Information

Minnesota County Biological Survey. 2007. Native plant communities and rare species of the Minnesota River Valley counties. Division of Ecological Resources, Minnesota Department of Natural Resources, St. Paul. 153 pp.

Minnesota Department of Natural Resources. 2005. Field guide to the native plant communities of Minnesota: the prairie parkland and tallgrass aspen parklands provinces. Ecological Land Classification Program, Minnesota County Biological Survey, and Natural Heritage and Nongame Research Program. Minnesota Department of Natural Resources, St. Paul, Minnesota. 362 pp.

Montana Plant Life. 2010. Flexile milk-vetch (Astragalus flexuosus). . Accessed 16 February 2010.

Ralphs, M. H., J. D. Graham, and L. F. James. 2002. A close look at locoweed poisoning on shortgrass prairies. Rangelands 24(2):30-34.

The Great Plains Flora Association. 1986. Flora of the Great Plains. University Press of Kansas, Lawrence. 1,402 pp.

Williams, M. C., and R. C. Barneby. 1977. The occurrence of nitro-toxins in North American Astragalus (Fabaceae). Brittonia 29(3):310-326.