Common milkweed

Encyclopedia Article

Common milkweed              Asclepias syriaca L.

Family:   Apocynaceae (Dogbane family)  Formerly in Asclepiaceae
Life cycle:  Perennial reproducing by seed and vegetative rootstalks.
Native status:  Native to North America
Habitat:  Pastures, roadsides, crop fields

General description:   Unbranched stem up to five ft tall; opposite leaves oblong up to 8 in long; stems covered with dense, soft pubescence; large clusters of pink flowers;  seed pod is 3 to 4 in long, 1 to 2 in wide and covered with soft prickles.

Key ID traits:  Erect stem with large oppositely arranged, oblong leaves; all plant parts emit milky sap when damaged.

Similar species:  Hemp dogbane resembles common milkweed in early spring, but mature dogbane has much smaller leaves and the stem is branched. Hemp dogbane stems are glabrous. Sullivant’s milkweed (AKA Sullivan’s, prairie milkweed) is similar to common milkweed in appearance, but it does not tolerate disturbance as well as common.  Leaf midveins of Sullivant’s milkweed are pinkish, and leaves are more erect on the stem and have a rubbery feel.

Miscellaneous:   Common milkweed is native to Iowa, but was much less prevalent prior to conversion of prairie to cropland.  Common milkweed is the Asclepias species best adated to disturbance, so it has increased whereas other milkweed species declined. Common milkweed is the primary food source for larvae of the monarch butterfly in Iowa and the Cornbelt.  Adoption of glyphosate resistant crops has greatly reduced the amount of common milkweed found in Iowa crop fields.  Monarch populations have declined along with the common milkweed, although other factors are involved in the butterflies decline.


Opposite ovate leaves on unbranched stems.  Milkweed is often found in clumps due to regeneration from underground rootstocks.

 


The majority of common milkweed plants emerge from overwintering roots rather than seed.


Milkweed flowers are some of the most complex in the plant kingdom. Insects must pull a sack of pollen from the stigmatic slit (without getting stuck), and then transfer the pollen to another flower.  Monarchs are poor pollinators of milkweed flowers.


Common milkweed pods are covered with soft spines.

 


Seed are dispersed by wind.  The pappus (floss) was used to fill life preservers during WWII and is used to stuff pillows.

 


Vegetative rootstocks are responsible for patches of common milkweed.

 


Common milkweed produces the majority of monarchs in Iowa and surrounding states.  Eggs are layed on milkweed species and larvae only feed on their foliage.

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