Plant Species Biology Summary for Meads Milkweed
Illinois Department of Natural Resources
Division of Natural Heritage

The following information is derived largely from monitoring records of the Illinois Department of Natural Resources for the species in Illinois but some comes from the literature and other sources.

Species: Asclepias meadii, Meads Milkweed

Compiler: John Schwegman, Illinois Department of Natural Resources

Date of Most Recent Update: February 9, 1990

Range of Natural Communities: Mesic Prairie in central Illinois and Dry Barrens in southern Illinois.

Range of Plant Communities (based on most abundant species in its immediate habitat): Sporobolus heterolepis-Dalea purpurea-Helianthus rigidus in central Illinois, and Schizachyrium scoparium-Danthonia spicata-Helianthus divaricatus, Danthonia spicata-Liatris squarrosa, and Quercus stellata-Quercus marilandica-Vaccinium arboreum in Southern Illinois.

Range of Soil and Substrate: Silt loam over glacial till in central Illinois and silt loam 8cm to 27cm deep over sandstone in southern Illinois.

Range of Slope and Aspect: Nearly level in central Illinois and 12 to 15 degrees slope and 240 to 270 degrees aspect in southern Illinois.

Range of Shading or Crown Cover: Crown cover is 0% in Central Illinois and about 20% at two populations and 80% at one population in southern Illinois. Shading at one southern Illinois population was reduced to 0 by physical tree removal.

Flowering Dates in Illinois: 6-2 to 6-22 in central Illinois and 5-30 to 6-16 in southern Illinois.

Fruiting Dates in Illinois: No mature fruits ever observed in Illinois this century.

Known Phenotypic Variation: Plants in central Illinois have larger flowers with more yellowish sepals and thicker and stiffer pedicels than the more green flowered plants of southern Illinois. Central Illinois plants also seem to average fewer flowers per umbel than southern Illinois plants. These differences seem to be genetic. Southern Illinois populations have numerous small, weak sterile plants that have not been observed in central Illinois. These plants are interpreted as dwarfs resulting from moisture stress and shading, not genetic differences.

Known Pollinators in Illinois: A variety of insects have been observed visiting this species fornectar which are deemed incapable of pollinating it. These include small green metallic bees, ants, and skipper butterflies. The only visitors observed that are deemed capable of pollination in southern Illinois are small bumble bees. These bees were observed apparently following a scent trail to the flowers late in the evening in mid June. Some pollenia are observed to have been removed by pollinators almost every year in southern Illinois. No effective pollinators have been observed in central Illinois but Bowles and Betz report natural pollination occurring there in 1989.

Reproductive Mode: No seed production has been observed at any of the currently known Illinois populations since their discovery (earliest discovery was in 1959). Any reproduction that is currently going on must be asexual. Plants come up in new locations in the demographic plot almost every year. These are interpreted as arising from rhizomes or roots scattered through the stand. Scattered plants in southern Illinois (flowering and sterile) have been observed to be wilting from moisture stress while others only 20 cm away were not moisture stressed. This indicates that if the plants arose asexually from the same rhizome they had become disconnected. In 1977 2 of 4 flowering plants in one southern Illinois population initiated pod formation (but reportedly set no seed) indicating both that more than one genet may be present at the site and that natural sexual reproduction may be possible in Illinois under ideal conditions.

Known Conditions for Sexual and Asexual Reproduction: No observation of seed production or seedling establishment has been made in Illinois. The one observation of pod initiation indicates that several vigorous plants in close proximity and probably representing 2 genets are necessary for natural pollination and seed formation. Various observations in southern Illinois indicate vigor is related to moisture stress and freedom from excessive shading. The apparent conditions for seed production in southern Illinois seems to be freedom from shade and above average moisture through spring and early summer. On the deeper soils of central Illinois average moisture may be adequate. Lack of severe stress the previous growing season may also be important. Conditions necessary for asexual reproduction are unknown.

Known Diseases: Broad-leaved but sterile plants under drought stress in southern Illinois have been observed with leaves with large areas of leaf tissue mottled pale yellowish. This is a rare condition only observed once. Smaller drought stressed plants frequently have yellowed to dead leaf tips which may be disease related. None of the observed diseases seem to be serious problems for the species in Illinois.

Known Grazers and Parasites: No evidence of insect grazing has been noted in southern Illinois except for one small plant that may have been grazed at 7cm height in late April or early May. Marlin Bowles and Robert Betz reported that milkweed weevils (Rhyssematus sp.) cut off and toppled the inflorescences of 4 plants after pollination in 1989 at the central Illinois site.

Known Problems with Exotic Species: The barrens habitats of this species in southern Illinois are being invaded by tall fescue (Festuca elatior) that is being introduced into the habitat by droppings of saddle horses which utilize a trail through the area. This is potentially a serious threat to the habitat of this species in southern Illinois.

Known Response to Fire: In southern Illinois one population that had probably not burned for decades increased from 4 sterile to 8 sterile plants after a March 13 fire in 1987. In 1988 after a second spring fire and shade removal it increased to 9 plants and produced the first flowering plant since 1983 in spite of severe drought. Following a third consecutive spring burn in 1989 the population declined to 6 plants, but the same plant flowered again and more vigorously as measured by number of flowers in the umbel. Another population declined from 9 to 7 plants after its first spring burn in 1989 but increased from no flowering plants to 2. A third southern Illinois population remained stable at 1 sterile plant after its first spring burn in 1988 and did not emerge after a second spring burn in 1989. This plant had severe shading problems with 80% canopy coverage and suffered from possible grazing and severe drought in 1988 which is considered the reason for its loss. Bowles and Betz found 2 flowering culms in 1988 following a spring burn and 4 in 1989 after a second spring burn in central Illinois. Although not censused annually, the last plant observed at the site prior to these burns was a single flowering plant in 1984. Spring fire seems to benefit the species by stimulation of growth and flowering and by shade removal.

Known Mortality Factors: One plant in southern Illinois that may have been observed dying appears to have died from shading aided by insect grazing and severe drought. Another southern Illinois population declined drastically after severe soil disturbance of unknown origin. It may have been due to severe mole tunneling, horseback riders or hikers during times when the soil was soft and muddy. It was impossible to determine what had caused the disturbance when observed in early June. Small sterile marked plants have been observed standing dry and apparently dead from drought one June and were absent the following year. Central Illinois plants were slightly "curled" in 1988 suggesting damage from herbicide drift from the adjacent crop field.

Known or Suspected Responses to Environmental Stress: Drought stress can keep plants in a continual dwarf, sterile condition in Southern Illinois. The evidence for this is soil depth measurements made at one population which occurs in a buried bedrock trough. In the center of the trough where soil is deeper and moisture more available the plants almost invariably flower but plants less than a meter away in shallower soil have been watched for 9 years and they have never flowered or gotten larger. Shade can reduce vigor and flowering as evidenced by a population that flowered for the first time in 5 years in 1988 after significant afternoon shading was removed. The flowering was in spite of the most severe drought in decades.

Known "Resting" (of Perennial Herbs): Thirteen marked plants in southern Illinois have been followed for 4 years. In this time 4 have disappeared and have had the opportunity to return but none have returned.

Summary of Apparent Factors Regulation Size and Health of Populations:

Past fire suppression in southern Illinois was a major problem for this species. It resulted in reduced acreage of grassy barrens habitat due to vegetative succession to woody vegetation and to increased shading of the relatively few surviving remnants of habitat. Succession has apparently been observed to have caused the extirpation of one plant in southern Illinois and doubtless has killed off others in the past.

The low population density and scattered nature of surviving plants (populations) makes natural sexual reproduction very difficult and has effectively stopped known sexual reproduction of the species in Illinois.

The shallow soil depth in much of the southern Illinois apparent habitat renders it unsuitable for the maintenance of vigorous flowering plants on a regular basis because of its inadequate moisture holding ability.

The lack of suitable habitat for expansion of the central Illinois population, because of its location along a railroad, limits its potential viability and makes it vulnerable to herbicide drift from adjacent cropland.