3.1 Asclepias meadii Torrey (Mead's Milkweed)

3.1.1 Past Restoration Efforts

Several restoration efforts have been made to restore Asclepias meadii to suitable habitats. Restoration efforts in the Shawnee National Forest in southern Illinois focused on introducing juvenile plants within sites containing wild stock. However, in 1991 the site was vandalized, resulting in the removal and theft of all specimens (Table 3-2). Since that time a restoration effort has been under way at seven Illinois sites and one Indiana site. A total of 686 seeds and 339 one-year-old juveniles were planted. A status report from 1995 showed that 178 seedlings (representing 20 genotypes) and 197 juveniles (13 genotypes) had survived (Bowles et al., 1995). The greatest number of plants surviving at a site was 79, representing 13 genotypes. The largest success rate occurred with juvenile plants which were better able to survive heat and drought stress. In support of this restoration effort the Morton Arboretum has established a garden for the production of Asclepias meadii, which includes stock from diverse genotypes (Bowles, 1997).

3.1.2 Site Selection and Evaluation

No sites were found that are suitable for restoration of Asclepias meadii in the near term. This species has been shown to be vulnerable to competition and is more suited to dry-mesic, late successional prairie habitats. In addition, while Asclepias meadii does not require an association with mycorrhizae, it does benefit from the association.

None of the potential candidate restoration sites identified demonstrated such features. Soil types having the greatest suitability were Varna, Warsaw, Symerton, Beecher, and Channahon (Table 3-1). However, all sites on which these soils occur are in row crops, hayfields, or pasture. Suitable mycorrhizal associations are difficult to establish under such conditions. Additionally, many sites were characterized by a dominance of aggressive weeds or grasses (e.g., Melilotus alba, Bromus inermis) that can be a threat to Asclepias meadii restoration efforts.

In the long term, MNTP may be moderately suitable for restoration with Asclepias meadii within selected areas in the eastern portion of the site. Areas having a Varna or Symerton soil type may be the most suitable. However, the introduction of Asclepias meadii into such sites is likely limited to a time in which the prairie has developed a late successional character. Such sites should consist of the prevalence of bunch grasses (e.g. Schizachyrium scoparium, Koeleria cristata, Sporobolus heterolepis) that allow for gaps within the ground layer which are suitable for colonization and establishment by Asclepias meadii seedlings. As has been suggested by Bowles et al. (in press), Asclepias meadii is most vulnerable to competition by aggressive turf or sod-forming grasses in its early life stages. In addition, the restoration of a mycorrhizal association within such soils may be an important prerequisite for reintroduction in order to allow for natural recruitment of seedlings.

3.1.3 Reintroduction Method

When reintroduction is appropriate, restoration of Asclepias meadii is best carried out using juveniles. Since many sites at MNTP have been shown to be unsuitable in the near term, initiation of restoration is best begun on sites that have a restored mycorrhizal association to promote seedling survival. Juveniles may be greenhouse raised by either germinating seeds or by culturing tissues from axillary buds taken from actively growing shoots as has been reported by Bowles et al. (1991). According to Bowles et al. (1995) plants that are sown as seed are highly dependent upon precipitation for germination. In years of low rainfall, as little as 4.8 percent of seeds may germinate. This compares to the 28.5 percent survival in a year with above average rainfall (200% above normal) and a 54 percent survival of transplanted plants. Because Asclepias meadii is an obligate out-crosser that has been demonstrated to have a high degree of genetic variability within its range (Tecic et al., in press), seedlings established within a given site should represent at least 25 genetically different populations in order to promote successful fertilization of the adults (Bowles et al., 1995). After germination, seedlings should be grown for one year prior to planting (Table 3-3). The established procedure for the germination and transplanting of Asclepias meadii according to Betz (1989), Bowles (1995), and Bowles et al. (in press) is as follows:

Moist stratify seed on wet filter paper at 5°C for 10 weeks. Then transfer to a calcareous silt (pH 5.7-6.7) loam soil. The soil should be mixed in a ratio of 1 to 3 with sand to allow for drainage. These seedlings should then be allowed to grow in a greenhouse for one year. While growing in greenhouse conditions plants need to be monitored for predation by aphids and thrips, which need to be removed by hand. In May while the plants are still dormant, place the tubers into a zip-lock bag filled with sphagnum for transportation. Finally, place the tubers in a 10 cm deep incision in the soil made with a tile spade with the bud 3 cm below the surface. Plants should be planted into gaps between late successional tallgrass. Water the plants after transplanting. Supplemental irrigation of plantings may be advantageous during years of initial plant establishment to artificially create above average rainfall conditions.