by Tony
Knight
Colorado State University Cooperative Extension
Master Gardener
Larimer County
Are they weeds or wild flowers? It’s a common question
whether milkweeds are found growing in people’s gardens or pastures, and
it depends on one’s perspective. A weed can be a native or non-native
plant that is unwanted in a particular area at a particular time. The milkweeds
(Asclepias sp.) are represented as 108 different species in North America,
many native to Colorado.
A very common species in western states is the showy milkweed (Asclepias speciosa),
while milkweed (A. syriaca) is common in the eastern states. The generic name
Asclepias is derived from the Greek God of healing, Asklepios, because milkweeds
have been used medicinally since ancient times.
Showy milkweed is a stout perennial, up to four feet tall, with an underground
root system and milky sap. The light-green, large leaves are oval, pointed at
the tip, opposite and hairy underneath. Ten to twenty star-like flowers with
white-pink petals are produced in clusters from wooly stalks radiating from
a central point. The seed pods typically ripen in late summer and release numerous
brown seeds with attached silky hairs that aid
in wind dispersal.
Some species of milkweed contain compounds that are toxic to the heart and the
nervous system of horses, livestock, poultry and people. These compounds, called
cardenolides, affect the heart in the same way as the drug digitalis derived
from foxglove (Digitalis purpurea). Milkweeds with narrow, grass-like leaves,
such as the western whorled milkweed (A. subverticillata), have other toxins
that affect the nervous system, causing seizures and even death.
Most milkweed poisoning is attributed to the narrow-leafed grass-like species
that becomes invasive because of its spreading root system. These plants can
spread into pastures and hay meadows where hungry animals graze. Milkweed’s
toxicity is retained even after plants have dried, making them dangerous in
baled hay. The showy milkweed is rarely eaten by animals because of its tough
fibrous stems and milky sap that animals find distasteful.
The fact milkweeds contain toxic chemicals has been discovered by Monarch butterflies
to protect themselves from predation. Monarch caterpillars feed on the showy
milkweed and absorb the toxins from the plant, storing them in their skin as
a protective mechanism. These toxins remain as the larva pupates into a butterfly.
What is even more remarkable is that the Viceroy butterfly has evolved to look
like the Monarch butterfly, deceiving birds.
Before assuming that all milkweeds should be removed from our environment, consider
the monarch butterfly and its dependence on the milkweed for existence. The
showy milkweed is a wild flower, not a problem to livestock, and critical to
the monarch butterfly’s existence. On the other hand, if narrow-leafed
whorled milkweed is present in your pasture or hay meadow, it should be controlled
because it’s invasive and highly poisonous.
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Figure 1 (taken by Tony Knight)
Showy milkweed (Asclepias speciosa) showing characteristic flowers and large
leaves.
Figure 2 (taken by Tony Knight)
Narrow leafed or whorled milkweed (Asclepias subverticillataIa) showing the
much narrower grass-like leaves, and the fluffy seeds.
Figure 3 (taken by David Cappaert, Michigan State University)
The caterpillar of the Monarch butterfly (Danaus plexippus) feeding on milkweeds.
The authors have received training through Colorado State University Cooperative Extension's Master Gardener program and is a Master Gardener volunteer for Larimer County.
Gardening and Insect Fact Sheets are available on-line by clicking HERE.
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