A. syriaca can be an aggressive and persistent weed and contains several poisonous glucosidic substances (cardenolides) known to be poisonous to sheep, cattle, and occasionally horses (Anderson, 1999).
Louis Hebert sent seeds of Asclepias syriaca from Quebec City, Canada to Paris and Cornut’s treatise published in 1635: ‘Canadensium plantarum aliarumque nondum editarum Historia’, describes both A. syriaca L. and A. incarnata L. A. syriaca was first identified as Apocynum majus syriacum rectum; Linnaeus then classified it as A. syriaca L. in 1753 (Gaertner, 1979).
Asclepias is a widespread genus with 76 species and 91 accepted taxa overall. There are 14 species of milkweed native in Canada (White, 1996). A. syriaca is the most widespread and locally abundant species of the genus (Hartzler and Buhler, 2000); it occurs in 38 states of North America.
According to Endress and Bruyns (2000) Asclepiadaceae is now treated as sub-family Asclepiadoideae in the family Apocynaceae.
A. syriaca (common milkweed) is a perennial herb with long-spreading rhizomes. Stems stout, erect, to 2 m tall, with short downy hairs and milky juice; leaves opposite, smooth margined, oblong, 10-20 cm long and 5-11 cm wide, with prominent veins; upper surface smooth, lower covered with short white hairs. Flowers sweet-smelling, pink to white, in large, many-flowered (the number per inflorescence varies greatly, from less than 10 to more than 120) axillary and apical bell-like clusters. The long-lived flowers produce copious amounts of nectar (Wyatt and Broyles, 1994), flowering from June to August, depending on initial growth, climate, and location (Anderson, 1999). Seed brown, flat, oval, 6 mm long, 5 mm wide, with a tuft of silky white hairs apically. All plant parts contain latex; shoots from established plants arise from adventitious root buds, emerging in April and May. The root system is composed of horizontal and vertical roots. In established stands, vertical roots may penetrate the soil to depths of 3.8 m (Anderson, 1999).
Contrary to its specific name, A. syriaca is native to northeastern, north central and southeastern United States and adjacent areas of Canada, and has spread to cultivated areas (Hartzler Buhler, 2000). This plant grows throughout the Great Plains ecoregion from southern Canada south to northeastern Oklahoma, northwestern Georgia, and Texas; and east from North Carolina to Maine (USDA-NRCS, 2010). It is naturalized in cultivated ground and dry grassland in various parts of central and southern Europe (IENICA, 2003).
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.
A. syriaca colonizes a variety of communities from woodlands to cleared grasslands and marshlands. It grows in clumps or patches in meadows, fencerows, roadsides, railways, waste places, reduced-tillage fields, and other open habitats.
A. syriaca is genetically self-incompatible, although low-levels of self-compatibility have been reported (Wyatt and Broyles, 1994), and this species has an unusual late-acting form of ovarian rejection. Flowers are self-sterile and insect-pollinated (Anderson, 1999), and pollen dispersal via a pollinium results with a fruit sharing the same outcrossed paternal parent (Hochwender, 2000). Only around 2% of the flowers produce mature pods, with an average plant producing 4-6 pods, containing approx. 150-425 seeds.
A. syriaca has a pollen delivery system whereby packets of pollen (pollinia) are transferred as a single unit during pollination; this method of reproduction is only seen elsewhere in some members of the Orchidaceae (Pleasants, 1991).The vegetative habit and weedy reproductive strategy A. syriaca makes it one of the most distinct milkweeds (Wilbur, 1976).
In a study using isozyme polymorphisms to study multiple paternity in A. syriaca, Gold and Shore (1995) found no statistical evidence for multiple paternity within fruits but extensive multiple paternity between fruits on the same ramet, suggesting potential choice of mates through selective fruit abortion.
Physiology and Phenology
A. syriaca grows as a colony of plants and each colony develops from a single plant arising from a seed or root segment, it has underground rootstocks that develop adventitious buds which give rise to new individuals; a large clonal group can comprise several thousand stems (Wilbur, 1976).
Vertebrate herbivores browse A. syriaca and results from a study by Hochwender (2000), showed that in low nutrient conditions, this species allocates resources to the storage organ, providing greater biomass when damaged; therefore an ability to compensate for damage might be important for this species.
A. syriaca is adapted to wide range of climatic and edaphic conditions and will tolerate soil conditions in any textural group, but is most commonly found on well-drained soils of loamy texture and grows best in full sunlight or light shade. It does not tolerate excessive moisture. A. syriaca has been found to be the most abundant invasive species in open sand grasslands (Botta-Dukat, 2008).
Approximately 12 species of herbivore consume A. syriaca in eastern North America and nearly all are specialists of Asclepias spp. because of particular mandibular structures, feeding modes, and the way they avoid the defensive latex production of this species (Van Zandt and Agrawal, 2004).
Only 3% of A. syriaca flowers produce seed pods, as most fall from the inflorescence approximately 10 days after opening (Anderson, 1999). A. syriaca spreads mainly by seeds which are wind pollinated and are dispersed on tufts of floss(Bhowmik, 1982). A. syriaca can produce large numbers of seeds, especially compared to other milkweed species, which facilitates dispersal over a long distance (White, 1996). Contrary to this, however, Pleasants (1991) found that much pollen dispersal in this species was short-distance.
Losses due to this species have been recorded in Sorghum production as A. syriaca populations increased (NAPPO, 2003). The floss attached to seeds also tends to clog air intake devices on combines during harvest (Anderson, 1999).
A. syriaca can be an aggressive and persistent weed and contains several poisonous glucosidic substances (cardenolides) known to be poisonous to sheep, cattle, and occasionally horses (Anderson, 1999). All parts of the plant contain the potentially toxic substances and A. syriaca is toxic to poultry; some milkweeds have been declared ‘noxious weeds’ in some North America states (White, 1996).
The specialist herbivores Tetraopes tetraophthalmus and Danaus plexippus have developed adaptations that allow them to sequester plant-generated cardenolides to defend themselves against predators (Hochwender et al., 2000).
Populations of the plant are thought to be beneficial as a source of nectar for butterflies, bees and other insects and A. syriaca plays an important part in the life cycle of the monarch butterfly (Danaus plexippus). Since D. plexippus requires A. syriaca as a larval food (Urquhart and Urquhart, 1979), D. plexippus might be at risk in Canada if noxious weed legislation were to be enforced and resulted in the elimination of milkweed (Hartzler and Buhler, 2000).
A. syriaca is considered medicinal in the USA and foliage or latex is used as a remedy for cancer, tumours and warts. It is also thought to be beneficial in treating asthma, catarrh and cough. Good quality fibre is obtained from the inner bark of the stem and can be used to make twine.
A. speciosa (showy milkweed) is similar to A. syriaca but it has broad, oval and rounded or heart-shaped leaves, densely woolly stalks, and fewer and longer flowers. Apocynum cannabinum (hemp dogbane) could be confused with A. syriaca but can be distinguished by its narrow, elliptical leaves 5-12 cm long, which are not hairy (Anderson, 1999). A. tuberosa (butterfly-weed) can also be mistaken for A. syriaca; it looks similar but it has orange flowers and the sap is not as milky.
Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.
Removal of stalks by cutting can aggravate the problem as this stimulates growth from underground buds, and when cultivated, the resulting small root fragments will grow into a new plant.
Seedlings can be controlled with selected herbicides as pre-emergence treatments Bhowmik (1982).
Aboveground plant parts may be destroyed by herbicides including 2, 4-D, mecoprop, dicamba and MCPA, but growth of adventitious roots is stimulated by this action. Amitrole-T and glyphosate will kill top growth and result in restricted regrowth during the following season. Glyphosate seems to be most effective overall and may reduce A. syriaca occurrence in crop fields (NAPPO, 2003).