Asclepias syriaca | Common milkweed
Regulated invasive species of Union concern under the European Regulation on the prevention and management of the introduction and spread of invasive alien species [1143/2014].
Invasive species - risk of Medium Impact
Introduction pathways - 1
Escape from Confinement
Introduction pathways subclass - 1
Introduction pathways - 2
Introduction pathways subclass - 2
Transport of habitat material
NAPRA Ireland risk assessed
Perennial deep-rooted rhizomatous weed. Grows to 2m tall on a stout downy stem (CABI, 2017). Leaves are oblong, opposite and smooth margined and can be 20cm long by 11cm wide (EPPO, 2015). Leaf undersides are covered in short white hairs (CABI, 2017). Domed, slightly drooping clusters of white, pinkish or purple flowers, which appear from June to August. Seeds are flat, ovoid and hairy with a silky tail enabling wind dispersal over long distances (CABI, 2017; Bhowmik, 1994). Roots are both horizontal and vertical and can extend 4m deep (Sarkany et al., 2008). Roots have adventitious root buds. All plant parts exude latex (thick milky substance) when broken (European Commission, 2017).
Forms dense stands that outshade and outcompete native plants. Tolerates drought (Bhowmik, 1994) and most soil conditions with the exception of very wet ground (CABI, 2017). Contains allelopathic chemicals that inhibit the germination of other species (Sarkany et al., 2008) thus increasing their competitive advantage. Can be a weed of arable land, vineyards and young forestry plantations reducing crop yields significantly (Bhowmik, 1994). This plant contains highly toxic cardenolides (glucosidic substances), which are toxic to herbivores and humans (European Commission, 2017). Symptoms of toxicity include dullness, bloating, high temperature, fats and weak pulse, spasms, coma (Stevens, 2000).
Sand dunes, roadside, wastelands, forests (Q-bank, undated).
Mechanism of impact
There is currently an EU wide ban on the sale, growing and keeping of this plant (European Commission, 2017).
Young plants can be removed by digging with a spade but care must be taken to remove the entire root.
Cutting exacerbates the problem by stimulating re-sprouting and should be avoided (Q-bank, undated). It results in the spread of root fragments and each root bud can grow a new plant (CABI, 2017).
Application of glyphosate in June (Bhowmik, 1994) gives control of this plant but consideration must be given to bees collecting nectar from this plant.
In susceptible areas, to limit establishment crop rotation can be practiced, including a winter crop (EPPO, 2015). Ploughing will kill seedlings if carried out within 3 weeks of emergence (EPPO, 2015).
Thrives in a wide variety of habitats including woodland, wetlands, agricultural land, river valleys, wasteland, ditches, road and rail sides. It tolerates most soil conditions but doesn’t like very wet ground (CABI, 2017). Flourishes in fertile moist ground (Bhowmik, 1994).
Asclepias speciosa (Showy milkweed) – this species has rounded and heart shaped leaves, very hairy stems and longer flowers than A. syriaca (CABI, 2017). Will hybridize with A. syriaca (Stevens, 2000).
A. tuberosa (Butterfly weed) – this species has orange flowers (CABI, 2017).
Native to North America and Canada. Its native range extends from Southern Canada south to NE Oklahoma, NW Georgia and Texas and east from N Carolina to Maine (USDA-NRCS, 2017).
Records submitted to Data Centre in 2023
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How can you help
Report any sightings to the National Biodiversity Data Centre. Dispose of unwanted plants responsibly. Do not purchase or plant in your garden.
Bhowmik, PC. (1994). Biology and control of common milkweed (Asclepias syriaca). Review of Weed Science 6, 227e250 : https://www.researchgate.net/publication/256486001 Site accessed 13 September 2017.
CABI. (2017). Invasive Species Compendium. Datasheet report for Asclepias syriaca (Common milkweed). Wallingford, UK: CAB International. http://www.cabi.org/isc/datasheetreport?dsid=7249 Site accessed 13 September 2017.
European and Mediterranean Plant Protection Organisation (EPPO). (2015). Risk Assessment of Asclepias syriaca. EPPO. Paris.
European Commission. (2017). Invasive Alien Species of Union Concern. European Union, Luxembourg.
Q-bank. (undated). Asclepias syriaca L. Common milkweed. Factsheet. http://www.q-bank.eu/Plants/Factsheets/Asclepias_syriaca_EN.pdf Site accessed 13 September 2017.
Sarkany, E.S., Lehotczky, E., Tamas, J., Nagy, P., 2008. Spreading, ecology and damages by the Common milkweed (Asclepias syriaca L.) in Hungary. Cereal Res. Commun. 36(3), 1571–1574. https://www.researchgate.net/publication/295714962 Site accessed 13 September 2017.
Stevens, M. (2000). Plant Fact Sheet. Common milkweed Asclepias syriaca L. United States Department of Agriculture - Natural Resources Conservation Services (USDA-NRCS). https://plants.usda.gov/plantguide/pdf/cs_assy.pdf Site accessed 13 September 2017.
USDA-NRCS. (2017). The PLANTS Database. The PLANTS Database. Baton Rouge, USA: National Plant Data Center. https://plants.usda.gov/core/profile?symbol=ASSY Site accessed 13 September 2017.