Common milkweed

Asclepias syriaca L.

Common milkweed follicles
Common milkweed dehiscent follicles
Common milkweed stand in flower

Images above: Upper left: Common milkweed follicles (Antonio DiTommasso, Cornell University). Upper right: Common milkweed dehiscent follicles (Scott Morris, Cornell University). Bottom: Common milkweed stand in flower (Randall Prostak, University of Massachusetts).

Identification

Other common names:  silkweed, milkweed, cotton-weed, Virginia silk, wild cotton, silky milkweed, common silkweed, showy milkweed, swallow-wort

Family:  milkweed family, Asclepiadaceae

Habit:  Erect, unbranched, perennial herb spreading by deep thickened storage roots.

Description:  Seedlings have light green, smooth stems.  Cotyledons are 0.25-0.5” (0.6-1.3 cm) long, largely untapered, dull green with prominent veins, long-stalks, and round tips.  True leaves are opposite, waxy, pointy tipped, and dark green with a prominent, white mid-vein.  Vegetative sprouts arising from underground roots are far more robust and common than seedlings; sprout stems are capped with a folded clump of leaves that unfolds as the stem elongates.  Mature plants reach 3-5 ft (0.9-1.5 m) tall on a single hollow, hairy, unbranched stem; the stem is green, turning red with maturity.  Leaves are opposite and borne upon 0.4” (1 cm) long stalks.  Leaves are green and hairless on the top, lighter green and hairy on their undersides.  Leaf midveins are prominent and white.  Secondary veins do not reach the edge of the gently tapered, 3-8” (7.5-20 cm) long, ellipse-shaped leaves.  The mature plant has a large underground system of thick, white horizontal storage roots.  In deep, well-drained soil, roots can penetrate to 8 ft (2.4 m) (Robbins et al. 1942).  The entire plant exudes a white, milky sap when cut.  Globes of fragrant, stalked, purple-pink to white flowers grow from upper leaf axils and stem tips.  Seeds develop in 3-5” (7.5- 13 cm) long, teardrop shaped, bumpy, hairy, spiny, gray green seed pods; pods tend to grow in pairs.  Upon seed maturity, pods split open, shedding up to 200 seeds each, and turn grayish brown.  Pod interiors are glossy yellow.  Seeds are large, 0.25-0.5” (0.6-1.3 cm), brown, oval with one flat end; the center of the seed is raised and surrounded by a thin, papery margin.  Long, silky hairs are attached to the flattened end.

Similar species:  Hemp dogbane (Apocynum cannabinum L.) differs from common milkweed by its green-white flowers, branching habit in its upper third, smaller leaves measuring 2-4.5” (5-11 cm) long, and long, narrow, curved seedpods.

Management

Common milkweed does well in grain and early planted corn because the shoots emerge after tillage and planting.  In contrast, soil preparation for summer planted crops eliminates the first flush of shoots and forces the plant to use additional root reserves to regenerate shoots.  Shoots can emerge from roots well below the plow layer, so a single deep moldboard plowing will not control this weed (Evetts and Burnside 1974).  Consequently, repeated shallow cultivations that cut shoots before they can replenish the roots are required.  For heavy infestations, a cultivated fallow period from late spring through summer will help get the weed under control.  Carbohydrate storage in the roots reaches a minimum in July to September (Bhowmik 1994) so mid-summer fallow may be most effective at exhausting the roots.  Buds on the roots become dormant in late summer (Bhowmik 1994), so fallowing at this time is not effective.

Rotation with alfalfa helps reduce vigor of common milkweed populations due to frequent mowing (Bhowmik 1994, Cramer and Burnside 1982).  Hay crops that are only mowed once per year do not help control this weed.  Seedling establishment of common milkweed was highest in spring wheat, intermediate in soybean, and lowest in corn, reflecting the duration of competition throughout the season (Yenish et al. 1997).  Generally, any crop that establishes a leaf canopy before common milkweed emergence and maintains a competitive canopy through the season, will greatly suppress this weed (Bhowmik 1994).

Common milkweed also serves several beneficial functions within agroecosystems (DiTommaso et al. 2016).  It hosts aphids that provide a food source for parasitic wasps which, in turn, attack and control the European corn borer (Ostrinia nubilalis).  It also contains compounds that support reproduction and survival of the monarch butterfly (Danaus plexippus) during their migration to overwintering sites in Mexico (Seiber et al. 1986).  Consequently, maintenance of low milkweed densities in cropping fields may contribute to a balanced landscape management strategy that realizes the ecological benefits of common milkweed while avoiding its agricultural liabilities (DiTommaso et al. 2016).

Ecology

Origin and distribution:  Common milkweed is native to eastern North America and occurs from Georgia to Oklahoma, and northward to southern Canada (Bhowmik 1994).  It has been introduced into Europe and Japan (ISC), and the Willamette Valley of Oregon (USDA Plants).

Seed weight:  4.2 mg (Stevens 1932), 2.5 mg from unfertilized plants to 3.8 mg from fertilized plants (Willson and Price 1980), 3.5 to 6.1 mg for viable seeds (Evetts and Burnside 1973), 4.0 to 7.4 mg (Farmer et al. 1986), 4.4 to 6.5 mg (Morse and Schmitt 1985).

Dormancy and germination:  Seeds are dormant when shed from the mother plant (Bhowmik 1994) and require 1 to 9 weeks of cold (41-48 °F = 5-9 °C), wet conditions (Baskin and Baskin 1977, Evetts and Burnside 1972, Jeffery and Robinson 1971) or about one year of dry storage (Bhowmik 1978) to break dormancy.  The longer the exposure to cold conditions, the lower the temperature required for optimum germination (Baskin and Baskin 1977).  This ensures that seeds will not germinate when shed in the fall, but will be ready for germination the spring following dispersal (Bhowmik 1994).  Seeds germinate at constant temperatures from 59-95 °F (15-35 °C), but germination is best at fluctuating day/night temperatures from 68/50 to 95/68 °F (20/10 to 35/20 °C) (Baskin and Baskin 1977, Evetts and Burnside 1972, Farmer et al. 1986).  Seed germination increases in the presence of nitrate (Evetts and Burnside 1972).  Light has little influence on germination of cold stratified seeds (Baskin and Baskin 1977).  Optimum pH for germination is 4 to 8 (Evetts and Burnside 1972).

Seed longevity:  Seeds survive in the soil for up to 9 year (Bhowmik 1994) but most seeds will probably germinate or die within the first 3 years following production (Bhowmik and Bandeen 1976, Anderson 1999).

Season of emergence:  Seedlings emerge primarily in the spring (Baskin and Baskin 1977, Sauer and Feir 1974).  Optimum temperature for emergence is 81 °F (27 °C) with minimal emergence at temperatures below 59 °F (15 °C) (Bhowmik 1978).  Shoots from overwintering rootstocks emerge from mid-spring throughout the summer, but peak emergence occurs in late spring and early summer (Bhowmik 1994).

Emergence depth:  Seeds germinate poorly on the soil surface but seedlings emerge well from 0.2” (0.5 cm) to 1.6” (4 cm) (Bhowmik 1978, Evetts and Burnside 1972).  A few seedlings can emerge from 2.4 to 2.8” (6 to 7 cm) (Evetts and Burnside 1972, Jeffery and Robinson 1971, Yenish et al. 1996).  New shoots can emerge from rootstocks buried as deep as 3.5 feet (107 cm) in the soil, but most emerge from roots in the top 12” (30 cm) (Evetts and Burnside 1974).

Photosynthetic pathway:  C3

Sensitivity to frost:  Common milkweed seedlings and shoots normally do not emerge until after the last frost, and the shoots usually begin to die back prior to the first frost in the fall, so frost has little consequence for this species.  When present at the first frost in the fall, this species was entirely killed (Stevens 1924).

Drought tolerance:  Full grown plants are drought tolerant because of their deep taproot, but seedlings are more drought sensitive than other weed species (Bhowmik 1994, Evetts and Burnside 1972).

Mycorrhiza:  Common milkweed had a low mychorrizal status in prairie settings (Dhillion and Friese 1994).   Low levels of infection by one mycorrhizal species increased milkweed biomass but infection by another species decreased biomass (Vannette and Hunter 2011).

Response to fertility:  Common milkweed grows well on soils with relatively low nitrogen, but plant size responds to balanced fertilization with N, P and K (Hochwender et al. 2000).  More biomass is allocated to roots than to shoots under low than high fertility conditions (Hochwender et al. 2000).  Reproductive pod and seed output is also increased by a balanced fertilizer (Willson and Price 1980).  It tolerates pH from 4 to 10 (Cramer and Burnside 1982) but does best on slightly alkaline soils. 

Soil physical requirements:  The species does not tolerate wet soils.  It thrives on soils with a wide range of textures provided drainage is adequate (Bhowmik 1994).  Given its common occurrence on headlands and roadsides (Bhowmik 1994, Sosnoski et al. 2007), it appears to tolerate compaction.  It tolerates salt (Bhowmik 1994) and can grow at substantially higher salt concentrations than sorghum (Cramer and Burnside 1982).  It is absent from boron deficient soils (Bhowmik 1994).

Response to shade:  Common milkweed grows best in 30-100% of full sunlight (Bhowmik 1994).  Mid-summer shading of 75% had little effect on pod and seed production (Willson and Price 1980).  Shade from a dense annual weed leaf canopy in the absence of soil competition substantially reduced reproduction of common milkweed roots (Evetts and Burnside 1975).

Sensitivity to disturbance:  The roots of mature plants can penetrate 7-12 feet (210-373 cm) into the soil and root fragments are capable of resprouting from as deep as 4-5 feet (120-150 cm) (Evetts and Burnside 1974).  This regeneration capacity protects them from damage during tillage and cultivation.  Seedling roots develop buds about three weeks after emergence; these buds are capable of regenerating a new shoot if the original is destroyed (Evetts and Burnside 1972, Stamm-Katovich et al. 1988).  Plants in the 3-leaf stage are capable of 65% resprouting when clipped at ground level and plants in the 4-leaf stage or greater are capable of 100% resprouting with multiple shoots (Jeffery and Robinson 1971).  Thus, occasional mowing or tillage will not prevent the development of a large colony (Bhowmik 1994).

Time from emergence to reproduction:  Seedlings do not flower the first year (Bhowmik 1994).  Shoots from older plants flower roughly 6-8 weeks after emergence (Bhowmik 1994, Doll 2002, Sauer and Feir 1974).  Seeds mature about 6 weeks after flowering (Anderson 1999, Bhowmik 1994, Evetts and Burnside 1973).  In Missouri, plants reached peak pod production in late July/early August and pods dried and opened by the end of September (Sauer and Feir 1974).

Pollination:  Common milkweed is self-sterile and cross pollinated by insects, mostly bees and wasps.  Bumble bees are primarily responsible for pollination by day, but moths pollinate at night (Morse and Fritz 1983).  Pollen generally is dispersed short distances to the same plant or to neighboring plants (Pleasants 1991).  Nectar is abundant in common milkweed and represents the primary attraction for insect transport and germination of pollen (Kevan et al. 1989).  Although any given flower can function as either a pollen donor or a seed producer, on average, 58% of flowers in a cluster become pollen donors and the others receive pollen and can potentially produce pods with seeds (Willson and Rathcke 1974).  The majority of pollinated flowers abort, with very few producing mature seed pods (Bhowmik 1994).

Reproduction:  Stalks typically produce 4-6 pods (Sauer and Feir 1974, Willson and Rathke 1974), each containing 100-425 seeds (Bhowmik 1994, Morse and Fritz 1983, Stevens 1932, Willson and Price 1980, Willson and Rathcke 1974).  The number of pods depends on the level of pollination (Morse and Fritz 1983).  Normally, vegetative reproduction does not begin until the second or third year of life (Bhowmik 1994).  An undisturbed plant in southern Ontario produced a colony of 56 shoots during its fourth growing season (Bhowmik 1994).

Dispersal:  Seeds are dispersed primarily by wind (Bhowmik 1994).  Each seed has a tuft of fine, silky hairs that provide buoyancy in the air (Morse and Schmitt 1985).  Seeds often cling to the pods for several months after the pods open, which prolongs the period of dispersal into late winter.  Root growth can spread the plant up to 10 ft (3 m) per year (Anderson 1999).  Root fragments are dispersed within fields and occasionally between fields on tillage implements.  Root segments as small as 1” (2.5 cm) can produce shoots (Anderson 1999).

Common natural enemies:  Common milkweed flowers can be destroyed by the mid-summer herbivors Tetraopes tetrophthalmus and Diabrotica cristata, while pods are destroyed in late summer by T. tetrophthalmus, Rhysseratus lineaticollis and Oncopeltus fasciatus (Willson and Rathcke 1974).  It hosts several specialized insects including caterpillars of the monarch butterfly (Danaus plexippus), the milkweed longhorned beetle (Tetraopes teraophtalmus) and the small milkweed bug (Lygaeus kalmii), but they usually do little to control the weed.  Virus diseases cause yellowing or mottling, clumping of stems, and deformed stems and leaves (Bhowmik 1994).  

Palatability:  Milky sap contains a poison that causes nausea and potential heart damage.  Nevertheless, young shoots, buds and flowers can be used as a pot herb provided the water is changed repeatedly during cooking to remove the bitter sap (Gaertner 1979).  Common milkweed is poisonous to livestock when 1-2% of body weight is consumed, but toxicity decreases as plants mature.  Dried milkweed in hay remains toxic.  (Burrows and Tyrl 2006) 

Notes:  Common milkweed appears to contain allelopathic compounds that inhibit the growth of selected crops and weeds (Bhowmik 1994).

References:

  • Anderson, W. P.  1999.  Perennial Weeds: Characteristics and Identification of Selected Herbaceous Species.  Iowa State University Press, Ames.
  • Baskin, J. M., and C. C. Baskin.  1977.  Germination of milkweed (Asclepias syriaca L.) seeds.  Bulletin of the Torrey Botanical Club 104:167-170.
  • Bhowmik, P. C.  1978.  Germination, growth and development of common milkweed.  Canadian Journal of Plant Science 58:493-498.
  • Bhowmik, P. D.  1994.  Biology and control of common milkweed (Asclepias syriaca).  Reviews of Weed Science 6:227-250.
  • Bhowmik, P. C., and J. D. Bandeen.  1976.  The biology of Canadian weeds. 19. Asclepias syriaca L.  Canadian Journal of Plant Science 56: 579-589.
  • Burrows, G. E., and D. J. Tyrl.  2006.  Handbook of Toxic Plants of North America. Blackwell: Ames, IA.
  • Cramer, G. L., and O. C. Burnside.  1982.  Distribution and interference of common milkweed (Asclepias syriaca) in Nebraska.  Weed Science 30:385-388.
  • Dhillion, S. S., and C. F. Friese.  1994.  The occurrence of mycorrhizas in prairies: Applications to ecological restoration.  Thirteenth North American Prairie Conference 13:103-114.
  • DiTommaso, A., K. M. Averill, M. P. Hoffman, J. R. Fuchsberg, and J. E. Losey.  2016.  Integrating insect, resistance, and floral resource management in weed control decision-making.  Weed Science 64:743-756.
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  • Seiber, J. N., L. P. Brower, S. M. Lee, M. M. McChesney, H. T. A. Cheung, C. J. Nelson, and T. R. Watson.  1986.  Cardenolide connection between overwintering monarch butterflies from Mexico and their larval food plant, Asclepias syriaca.  Journal of Chemical Ecology 12:1157-1170.
  • Sosnoskie, L. M., E. C. Luschei, and M. A. Fanning.  2007.  Field margin weed-species diversity in relation to landscape attributes and adjacent land use.  Weed Science 55:129-136.
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