Milkweed (Asclepias spp.) taxon

-Milkweed spp. Outline
-Milkweed spp. Bibliography
a. common milkweed (Asclepias syriaca)
b. swamp milkweeed (Asclepias incarnata)
c. butterfly milkweed (Asclepias tuberosa)
d. eastern whorled milkweed (Asclepias verticillata)
e. taxa: complex of associated species or demography


Agronomy 517: Weed Biology and Ecology

Species Paper-Milkweed Family
Asclepias spp. (Asclepiadaceae)
Prepared by: Todd DeGooyer

Note: the majority of the information contained in this report pertains to the common milkweed.

I. Description of milkweeds

A. Asclepias spp.
1. this genus was named for the Greek herbalist Aescupalius, chief among physicians
B. Name
1. common milkweed (Asclepias syriaca L.) (Linneaus believed it was from Syria)
a. other common names: silkweed, cotton weed, wild cotton
2. swamp milkweed (Asclepias incarnata)
3. butterfly milkweed (Asclepias tuberosa)
4. eastern whorled milkweed (Asclepias verticillata)
B. Characteristics of common milkweed (Bhowmik and Bandeen 1976)
1. perennial, with creeping underground rootstock
2. stems
a. simple
b. stout and erect, hairy
c. 60-120 cm high
d. typically, several stems together
3. leaves
a. opposite, oblong and entire
b. width: 5-15 cm
c. length: 7-20 cm
d. prominently veined
e. lower surface hairy
f. milky juice in all parts of plant
4. flowers
a. in umbels at top of stem or in axils of upper leaves.
b. individual flowers long, weak pedicel
1. 0.95 cm in diameter
2. corolla lobes 6-9 mm long
3. hoods 3-4 mm high, ovate, obtuse
c. purplish to pink, rarely white
5. ovaries
a. two, styles united to form flesh disk
b. five stigmatic areas (enclosed in a chamber) on undersurface of fleshy disk

6. stamens
a. pollen contained in sacs (pollinia)
b. pollinia attached in pairs, two per stamen

7. seed pods
a. grey (follicles)
b. slenderly ovoid
c. hairy, covered with soft projections
d. 7-10 cm long; 2.5 cm broad
8. seeds
a. brown
b. flat and oval
c. tuft of silky white hair (plume)
d. 0.77-0.79 cm long

II. Geographical Distribution of Common Milkweed

A. Native to North America (A. syriaca); introduced to southern Europe
B. Geographical boundary (Bhowmik and Bandeen 1976)
1. 35° and 50° north latitude (Doyon 1958)
2. 60° and 103° west longitude (Doyon 1958)
3. found in all Canadian provinces
4. found in midwestern, northcentral, and southeastern United States
C. Distribution of common milkweed by crop in north central region (13 states) (Evetts 1977):
1. corn - 12 million acres infested
2. soybean - 6.1 million acres infested
3. small grains - 2.5 million acres infested
4. sorghum - 1.3 million acres infested
5. in general, 26 million acres infested

III. Habitat of Common Milkweed (Bhowmik 1978)

A. Cultivated fields
1. serious common milkweed infestations have increased with the use of reduced tillage and of soil-applied herbicides, fertilizers and irrigation (Cramer and Burnside 1981)
2. common milkweed can be found on soil-textural group that is well drained (REF- see Cramer and Burnside 1981)
3. A. syriaca is not limited by soil pH or altitude; it easily adapts to moist, fertile soils
(Evetts 1977)
B. Pastures
a. 1.6 million acres infested in north central region (Evetts 1977)
C. Fence rows
D. Waste lands
a. 403 thousand acres infested (Evetts 1977)

E. Roadsides
a. 1.3 million acres infested (Evetts 1977)

IV. Biology of Common Milkweeds

A. Germination of seed
1. germination of seeds occur in spring after temperature has risen above 15°C, following an after-ripening period (Cramer 1977)
2. optimal germination occurs by alternating 20 to 30°C temperatures (Cramer 1977)
3. however, germination will occur between 14 and 35°C (Evetts and Burnside 1973c)
4. germination of seed is low one month after seen collection and was highest after 11 months of storage at 21°C (Bhowmik 1978)
5. best seedling emergence if planted at 0.5 or 1 cm depth (Bhowmik 1978)
a. but common milkweed can emerge from depths of 6 cm in silty clay loam (Evetts and Burnside 1972)
6.Groh (1943) reported at least 71 % germination after one year storage
7. up to 68% of common milkweed seeds germinated after 7 years of storage in glass jars
(Minhall 1977)
B. Perennation (Bhowmik and Bandeen 1970)
1. perennating activity of common milkweed is renewed annually from adventitous root buds
a. buds are located near the base of the plant (or main root) and on lateral roots
2. buds sprout in the spring and most remain viable in the winter (Bhowmik and Bandeen 1976)
3. the parent root survives for 2 or more growing season depending on tillage soil conditions (Bhowmik and Bandeen 1976)
B. Dormancy
1. 90% of freshly-harvested seeds were dormant when germinated at 26°C (Cramer 1977)
2. dormancy can be broken by removing the seed coat (scarification), gibberellic acid or kinetin (Oegema and Fletcher 1972, Evetts and Burnside 1972); also sodium
hypochorite followed by gibberellic acid (Campbell 1985)
3. seed dormancy can be broken by a few days of moist, low temperature after ripening
(Jeffrey and Robison 1971)
C. Phenology (Bhowmik and Bandeen 1976)
1. aerial shoots emerge from the ground in April-May from root buds
2. aerial growth continues as weather warms
3. root extension starts in July-August; new shoots do not reach the surface until the following spring
4. root growth terminates during mid-August to early September
5. seedlings do not flower during the first year of growth, but can produce new aerial shoots from root buds 18-22 days after germination (Bhowmik and Bandeen 1970)
6. newly developed shoots from the same root system flower the next growing season
7. flowering occurs in late June-July or early August
8. pods begin to form in August-September and may remain on stem until November
9. pods open during September-October and seed are carried away by the wind

V. Seed Reproduction by Common Milkweed and Seed (Pod) Characteristics
(Bhowmik and Bandeen 1970)

A. Flower biology
1.common milkweed is an outcrossing species (Kephart 1981)
a. when self-pollinations were conducted for common milkweed, only 4% of the pollination was successful
2. only 1 to 3 % of flowers produce a mature pod; 80% of flowers drop soon after flowering and pods abort before maturity (Moore 1946)
3. pollination
a. retinacula of pollinia becomes entangled on legs of insects
1. pollinium described (Eisikowitch et al. 1986)
a. special pollen dispersal unit of A. syriaca
b. contains hundreds of pollen grains
c. behaves similar to pollen grain; upon germination, pollen tubes protrude through a special ostiole
d. protects pollen from weather, predation, etc.
e. each flower produces only 5 pairs of pollinia (which restrict the chances of pollination) (Moore 1946a, Bookman 1984)
b. transported by insects (e.g. bees) and pollinia are inserted into the stigmatic
chambers of the flower (Bhowmik and Bandeen 1976)
4. seed production
a. following successful pollination, one both ovaries enlarge (Moore 1947) b. however, 50 to 75 percent of the ovaries abort (Moore 1947)
c. seed pods mature and split open in the early fall (September-October)
B. Characteristics of seeds (pods)
1. number of pods
a. most stalks of common milkweed have between 4 to 6 pods (Bhowmik and Bandeen 1973)
2. number of seeds
a. Bhowmik and Burnside (1976) found between 150 and 425 seeds per pod and Evetts and Burnside (1973c) estimated (on average) 220 viable seeds per pod
3. weight of seeds
a. seed weight for 100 seeds ranged from 42 mg (Stevens 1932) to 43.4 to 73.1
mg (Bhowmik and Bandeen 1973)
4.dispersal of seeds
a. the flat seed with attached floss facilitate the movement of the seed great distances (Evetts 1977)
b. milkweed seeds also float and are dispersed by water (Evetts and Burnside
c. seeds of A. incarnata are dispersed primarily by water
5. survival of seeds
a. seed of A. syriaca can survive at least 3 years in the soil (Cramer 1977) and at least 9 yrs in glass bottles (Minhall 1977)
b. common milkweed was unable to survive storage in water for 1, 2, or 12 months at 40°, 30°, and 5°C, respectively (Evetts and Burnside 1972)
c. death of seeds under dry seed storage occurred at 40°C stored for 5 months (Evetts and Burnside 1971)

VI. Vegetative Reproduction of Common Milkweed

1. Common milkweed can reproduce vegetatively by adventitious root buds (root stock) on underground root system
2. A. syriaca seedlings become perennial within 21 days after germination (Cramer and Burnside 1981)
3. Sprouting does not occur on seedlings until they have been clipped; this suggest apical dominance
4. Root buds are dormant in winter; in the spring, buds become active and produce a new aerial shoot (Bhowmik and Bandeen 1970)
a. dormancy of root bud can be broken with high temperature
5. In a 4 year study, 1 seedling produced 56 stalks and 95 seedling in a 9 m2 area (Bhowmik and Bandeen 1970)
6. Roots of common milkweed typically develop to a depth of 100-120 cm (Bhowmik and Bandeen 1970), but have been observed to penetrate as far down as 373 cm into a silty clay loam soil (Evetts and Burnside 1974)
7. The root system can spread 10 ft per year

VII. Population Dynamics of Common Milkweed

A. Population shift
1. common milkweeds began moving from roadsides and waster areas into cultivated land in later 60's and early 70's (Bhowmik and Bandeen 1976)
a. possibly, the shift to cultivated fields was due to removal of competing annual weeds with herbicides that do not control A. syriaca, reduced tillage practices, and decreased crop rotation
2. seedlings establish in patches in cultivated fields as shoots emerge, spread to adjacent areas
B. Competition with other plant species
1. common milkweed is not a good competitor
2. light competition did not reduce the height of common milkweed as much as soil
competition (Evetts and Burnside 1975)
3. height of A. syriaca was not significantly reduced when competing with green foxtail, but not with redroot pigweed
4. common milkweed was shown not be a good competitor with 6 annual weed species
(Bhowmik and Bandeen 1973b)
5. A. syriaca can compete with sorghum and causes yield loss (Evetts and Burnside 1975)

VIII. Response of Common Milkweed to Herbicides and Other Control Tactics

A. Cultural control (Martin and Burnside 1980)
1. a single tillage operation when common milkweed seedlings are less than 3 weeks old will kill the majority of seedlings (included under mechanical control by some authors)
2. alfalfa cut 3 times a year will depleted the root system and eliminate stands of common milkweed
3. growing winter wheat also reduces milkweed stands
B. Chemical control
1. control of A. syriaca is often variable with herbicides depending on the growth rate, growth stage and time of treatment (Cramer and Burnside 1981)
2. commonly used herbicides for common milkweed seedlings include (Burnside 1977):
a. metribuzin (Sencor)
c. atrazine
d. combinations of the above herbicides
3. commonly used herbicide for established stands of common milkweed (Burnside 1977):
a. amitrol (Cytrol)
b. glyphosate (Roundup)
1. 70% control or better was obtained using glyphosate (2.2 kg/ha) at early bud-stage (Cramer and Burnside 1981)
2. 70% control or better was obtained using glyphosate and amitrol at late-bud stage (Cramer and Burnside 1981)
3. glyphosate is the most effective for controlling established stands of
common milkweed in most cases

IX. Economic Importance of Milkweeds

A. As a weed caused a:
1. reduction in yield of sorghum of 720 kg/ha (Evetts 1970) and 2 to 10% (Cramer and Burnside 1982)
2. reduction in yield of oats of up to 26 % (Knowles 1950)
3. reduction in yield of soybeans of 12 to 19% (Cramer and Burnside 1982)
4. estimated to cause $3.2 million in yield loss (all crops combined) (Evetts 1977)
B. As a beneficial:
1. food
a. Chippewa Indians used young flowers in stewed form (Berkman 1949)
b. substitute for asparagus (Millspaugh 1892)
2. fiber
a. use bast fiber of stem for textile

3. latex
a. as commercial rubber (Berkman 1949)
4. floss (coma)
a. as a substitute for kapok (used as a stuffing material) (Whiting 1943)
1. used instead of kapok to stuff life jackets in WW II
5. flowers
a. source of honey for butterflies, bees and other insects
6. leaves
a. serve as food for insects (Danaeidae)
7. potential for liquid fuels (Campbell 1983, 1985)
a. polyphenol, oil, polymeric hydrocarbons

X. Success of Milkweeds

A. Characteristics of the common milkweed which make it successful:
1. Kephart (1981) suggests that the autogamous nature of common milkweed and floral mechanisms requiring insect pollination insure advantages of genetic variability promoted by outcrossing in addition to the assurance of vegetative (uniparental) reproduction (or simply, two forms of reproduction, one that promotes genetic diversity and the other maintains the present genotype)
2. has an efficient means of seed dispersal; plumed seeds can travel relatively long distances to colonize new areas
3. seeds can germinate in most soils of varying pHs or altitudes, as long as the soil is moist and there is adequate drainage and thus can colonize a wide range of habitats
a. because they can colonize many different habitats, common milkweed can recolonize a site more efficiently than weeds that are adopted to a particular habitat (if milkweeds are controlled in the field, they may be able recolonize with seed from neighboring plants in the pasture or fence row that was ignored by the farmer.
4. the common milkweed, as well as other milkweeds, produce alkaloid compound (cardiac gylcosides) which deter feeding by most herbivorous insects and are distasteful or poisonous to vertebrates
5. few preemergence have much of effect on controlling common milkweeds; metribuzin may control seedlings in some cases; glyphosate is one of the few herbicides that will control milkweed in most situations
6. it can a new plant from
B. Characteristics of the common milkweed which make it a more successful weed than other milkweed species:
1. common milkweed can germinate and grow in most habitats; other milkweed species have narrow microhabitat tolerances
a. for example, butterfly milkweed tends to colonize relatively undisturbed sites such as roadsides, fence rows, and waste lands, whereas common milkweed colonizes crop land and pasture land in addition to these other sites
2. A. syriaca also produces 4 times as many seeds per pod and almost twice as many seeds per pod as A. incarnata, A. tuberosa, and A. verticillata
C. Characteristics of milkweed which make it less successful than other weeds:
1. common milkweed is not a good competitor for light and particularly for soil resources (water, nutrients, etc.) (Evetts and Burnside 1975)
2. common milkweed is a perennial and not an annual weed
a. if an A. syriaca seed colonizes a new area it takes at least two years before it produces seeds, whereas has a good chance of producing seed the year it colonizes
3. common milkweed is an outcrossing species that is highly self-sterile and each flower only produces 5 pairs of pollinia (which restrict the chances of pollination)

Berkman, B. 1949. Milkweed-A war strategic material and a potential industrial crop for sub- marginal lands in the United States. Econ. Bot. 3:223-239.

Bhowmik, P. C. 1978. Germination, growth and development of common milkweed Can. J. Plant Sci. 58:493-498.

Bhowmik, P. C. and J. D. Bandeen.1970. Life history of the common milkweed. Weed Sci. Soc. Amer. Abstr. No. 12.

Bhowmik, P. C. and J. D. Bandeen. 1973. Reproductive nature of common milkweed (Asclepias syriaca L.) Can. Bot. Assoc. 8th Annu. Meeting 30 pp.

Bhowmik, P. C. and J. D. Bandeen. 1976. The biology of Canadian weeds. Can. J. Plant Sci. 56: 579-589.

Bookman, S. S. 1984. Evidence for selective fruit production in Asclepias [speciosa]. Evolution 38:72-86.

Burnside, O. 1977. Cultural, mechanical, and chemical control of common milkweed. Proc. Annu. Meet. North Cent. Weed. Control. Conf. 32:107-110.

Campbell, T. A. 1983. Chemical and agronomic evaluation of common milkweed, Asclepias syriaca. Econ. Bot. 37:174-180.

Campbell, T. A. 1985. Growth analysis in common milkweed (Asclepias syriaca). Can. J. Bot. 63:2345-2349.

Cramer, G. L. 1977. Life history of common milkweed. Proc. Annu. Meet. North. Cent. Weed Contr. Conf. 32:99-100.

Cramer, G. L. and O. C. Burnside. 1981. Control of common milkweed (Asclepias syriaca) with postemergence herbicides. Weed. Sci. 29:636-640.

Cramer, G. L. and O. C. Burnside. 1982. Distribution and interference of common milkweed (Asclepias syriaca) in Nebraska. Weed. Sci. 30:385-388.

Doyon, D. 1958. Study of the geographical distribution of the common milkweed (Asclepias syriaca L. ) in North America. Quebec Soc. Prot. Plants Rep. 40:41-113.

Eisikowitch, D., P. G. Kevan, S. Fowle, and K. Thomas. 1987. The significance of pollen longevity in Asclepias syriaca L. under natural conditions. Pollen Spores 29:121-128.

Evetts, L. L. 1970. Ecological studies with common milkweed. M. S. Thesis, Univ. of Nebraska,
Lincoln, Neb. 70 pp.

Evetts, L. 1977. Common milkweed--the problem. Proc. Annu. Meet. North. Cent. Weed Contr. Conf. 32:96-99.

Evetts, L. L. and O. C. Burnside. 1971. Ecological studies with common milkweed. Proc. North Cent. Weed Conf. 26:62

Evetts, L. L. and O. C. Burnside. 1972. Germination and seedling development of common milkweed and other species. Weed Sci. 20:371-378.

Evetts, L. L. and O. C. Burnside. 1973a. Common milkweed seed maturation. Weed Sci.

Evetts, L. L. and O. C. Burnside. 1973b. Early root and shoot development of nine plant species.
Weed Sci. 21: 289-291.

Evetts, L. L. and O. C. Burnside. 1973c. Milkweed--a persistent perennial that reduces yields. Neb. Farm, Ranch, and Home Quart. 20:12-15.

Evetts, L. L. and O. C. Burnside. 1974. Root distribution and vegetative propagation of Asclepias syriaca L. Weed Contr. Weed Res. 14:283-288.

Evetts, L. L. and O. C. Burnside. 1975. Effect of early competition on growth of common milkweed. Weed Sci. 23: 1-3.

Gaertner, E. E. 1979. The history and use of milkweed (Asclepias syriaca L.). Econ. Bot. 33:119-123.
Groh, H. 1943. Notes on common milkweed. Sci. Agric. 23: 625-632.

Jeffery, L. S. and L. R. Robison. 1971. Growth characteristics of common milkweed.
Weed Sci. 19: 193-196.

Kephart, S. R. 1981. Breeding systems in Asclepias incarnata L., Asclepias syriaca L., and Asclepias verticillarta L. Am. J. Bot. 68:226-232.

Knowles, G. 1950. Weed control experiments. Can. Dep. of Agr. Exp. Farm Ser. Div., Field
Husb., Soils, and Agr. Exp. Progr. Dep. pp. 52-60

Martin, A. R. and O. C. Burnside. 1980. Common milkweed - weed on the increase. Weeds Today. 11: 19-20.

Millspaugh, C. F. 1892. American medicinal plants. Dover Publishers Inc., New York.
1974 reprint.

Minshall, W. 1977. The biology of common milkweed. Proc. Annu. Meet. North Cent. Weed Control. Conf. 32:101-104.

Moore, R. J. 1946a. Investigations on rubber-bearing plants. III. Development of normal and aborting seeds in Asclepias syriaca L. Can. J. Res. 24: 56-65.

Moore, R. J. 1947. Investigations on rubber-bearing plants. V. Notes on the flower biology and pod yield of Asclepias syriaca L. Can. Field Natur. 61: 40-66.

Oegema, T. and R. Fletcher. 1972. Factors that influence dormancy in milkweed seeds. Can. J. Bot. 50: 713-718.

Stevens, O. A., 1932. The number and weight of seeds produced by weeds. J. Bot. 19:784-794.

Whiting, A. G. 1943. A summary of the literature on milkweeds (Asclepias spp.) and their utilization. U.S.D.A. Bibl. Bull. No. 2. 41pp.
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