Common Milkweed or Asclepias syriaca is a superb Monarch support and a mainstay of Monarch waystations. But it is so much more. This plant feeds more than 450 insects at different life stages. Diverse insects consume nectar, sap, leaves, flowers and seeds5. Admittedly weedy, its lovely blooms are fragrant and enhance any scent garden1,5.
Common Milkweed thrives in full sun and well-drained soils. Like other “weeds”, it’s not picky about growing conditions. A. syriaca grows in clayey, sandy or rocky calcareous soils, loamy soils and high clay or sand soils.
Asclepias syriaca springs up in disrupted areas. Human activity disrupts natural areas. But natural disruptions occur where water rises and falls, for example, along streams and lakes.
In nature, Common Milkweed is found in moist and dry black soil prairies, sand prairies, sand dunes along lake shores, fields, pastures, abandoned fields, vacant lots and along railroads, fence rows and roadsides.
Common Milkweed absolutely lives up to its weedy name. It spreads by seed and by long creeping rhizomes that pop up in unexpected places. Once established, this plant is extremely difficult to remove.
These habits are offset by Common Milkweed’s value to insects and pollinators. It’s best to let it grow where you don’t mind its vigorous and abundant nature—a wild area, a meadow or a prairie garden.
Asclepias syriaca offers a strong erect form in the garden. The pale, cylindrical central stem supports opposite jade-green leaves. These leaves range from pale to dark green on top and are pale green with short, dense hair underneath. Each leaf has a distinctive central vein with small veins spreading to the edges.
The milky sap, which gives Milkweed its name, oozes whenever part of the plant is broken. The sap is called a latex and contains 2% latex. The latex is thick and sticky.
Most ingeniously, caterpillars will reduce the sap’s flow by taking tiny bites closer to the stem. The sap leaks out there. When the caterpillar begins to eat a leaf, there’s less latex at the feeding site. Thus, less chance the sticky stuff will foul the larva’s mouth2,3.
For humans and other mammals, there are other concerns. The milky sap contains cardenolides, specifically cardiac glycosides. Contact with the skin or eyes causes irritation. If eaten or exposed to mucous membranes, cardiac glycosides can disrupt the nervous system, the kidneys, the muscles (which includes the heart) and the human’s/animal’s acid/base balance3,4,5.
The monarch and other insects that consume milkweed have turned this to their advantage. As the caterpillars/insects consume milkweed their bodies store the cardiac glycosides. If a bird or animal eats them, they taste bitter and can make the predator feel sick.
Predators soon learn to leave them alone. The orange/black or red/black coloring of milkweed insects signals, “Stay away! Not good for you!” The technical term for this warning coloration is ‘aposematic’5.
Milkweed blooms don’t dazzle but flower in 1930’s vintage shades. The drooping balls of florets are soft and dusky lasting one to one and a half months. Colors range from greenish white to greenish pink to rosy-pink to purplish-pink to reddish purple.
Each cluster averages 30 individual florets but can have up to 100. Their sweet, vanilla fragrance drifts from the three to five umbels on each plant.
Up close, each floret is a fascinating feat of engineering all directed toward an exceptional pollination system. Each a quarter of an inch across, the florets have five reflexed (bent back) petals and five raised hoods with curved horns. The hoods have lighter colors than the petals.
In the center of each floret, is a cylindrical structure formed by two fused stigmas. It’s called the stigmatic column.
Between the hoods, are the stigmatic slits. These slits hold the pollinaria. Unique structures, pollinaria hold waxy sacs of pollen. These are transferred instead of the loose, powdery pollen used by most flowers. Milkweeds and orchids are the only known plants to have them6,7.
The milkweed pollinium (or pollinarium) consist of a blackish-brown oval gland (corpusculum) with a slit, two translator arms hanging from the gland and two pollinial sacs. Before each sac, a knee bend of approximately 900 occurs in the arm. This bend enables the rotation of each sac during pollination.
In the milkweeds, the pollinial sac sit inside the stigmatic column and only the corpusculum is visible between the hood structures. When an insect lands on the flower, one of its legs may slip into the stigmatic slit between two hoods. As it tries to free itself, the leg moves upward toward the slit in the corpusculum. Bristles in the chamber keep it from going back down.
Insects must be strong to free themselves. Large butterflies, predatory wasps and long tongue bees are most likely to remove pollinaria. Lost legs and dead smaller insects both occur from failed escape attempts.
When a pollinaria is removed, it begins to dry. The pollinial sacs rotate 90o during drying. The rotation moves them into the correct position for pollination.
When the insect lands on another milkweed, the knee bend (not the corpusculum oval) slides into the space between the hood petals. The translator arm follows then the rotated pollinial sac. The pollinial sac slides into a space in the stigmatic column and pollination is completed.
When the insect continues pulling upward, the translator arm breaks. The insect keeps the remaining part of the pollinaria.
It’s also possible to start a chain of pollinaria during this process. As the broken translator arm slides between the hoods, it can hook the corpusculum slit of this floret’s pollinaria adding a fresh pollinaria to the partial remaining one. Clumps and chains of all sorts develop this way. These groups of pollinaria may increase chances of pollination6.7.
Part I, ends here, with the exciting conclusion of a fertilized milkweed. Part II will continue with photos and information about the insects that use Common Milkweed. What a crowd it is—including flies, wasps, bees, butterflies, moths, skippers, plant bugs and beneficial insects!
See you next time to discover what Wild Things are in the Garden!
Mary
References
- Hilty, J., n.d., Common Milkweed, Asclepias syriaca, Milkweed family, (Asclepiadaceae), https://www.illinoiswildflowers.info/weeds/plants/cm_milkweed.html
- Berkov, A., 28.October.2010, Plant Talk: Inside the New York Botanical Garden; Plant Profile: The Extraordinary Common Milkweed, How Do Insects Feed on this Plant with Sticky White Latex? https://www.nybg.org/blogs/plant-talk/2010/10/science/plant-profile-the-extraordinary-common-milkweed/#:~:text=Plants%20in%20the%20genus%20Asclepias,Asklepios%2C%20the%20ancient%20Greek%20physician.
- Bowe, Scott, 11.September.2018, Milkweed is More Than Just a Common Weed, WXPR, https://www.wxpr.org/natural-resources/2018-09-11/milkweed-is-more-than-just-a-common-weed
- Nelson, M. and Alfuth, D., 24.February.2021, Milkweed (Ornamental Plants Toxic to Animals), X-number: XHT1276, https://hort.extension.wisc.edu/articles/milkweed-ornamental-plants-toxic-to-animals/
- Taylor, David, n.d., Common Milkweed, (Asclepias syriaca L.), Plant of the Week, https://www.fs.usda.gov/wildflowers/plant-of-the-week/asclepias_syriaca.shtml
- Eldredge, E.P., 2015,11,00, Milkweed Pollination Biology, Plant Materials Technical Note NV-58, Natural Resources Conservation Service, https://www.nrcs.usda.gov/plantmaterials/nvpmctn12764.pdf
- Betz, R.F., Struven, R.D., Wall, J.E. & Heitler, F.B., 1994, Insect Pollinators 12 Milkweed (Asclepias) Species, T.B. Bragg and J. Stubbendieck (eds.) Proc. Of the Thirteenth North American Prairie Conference. Univ. of Nebraska, Lincoln, NE.
Wild Things in the Garden 