Milkweed is a common plant in our rural landscapes. Most folks think of the common milkweed and showy milkweed when they picture this plant. These are broadleaf representatives of the milkweed family. Of the five most common milkweed species found in the western states, the western whorled milkweed, Asclepias subverticillata, is often overlooked.
I have frequently observed western whorled milkweed along the edge of roads leading out of Cedaredge where I ride my bike. It is also growing in my yard. It is a native of the west and Mexico, growing at the lower elevations. This milkweed sometimes goes by other names such as horsetail and poison milkweed.
This species grows 1 to 3 feet tall, has smooth un-branched narrow leaves (3/8" wide) that are 2-4 inches long growing in whorls. Broken stems and leaves exude a sticky milky latex substance. White flowers appear in umbrella-like clusters at the ends of its branches. Flowering takes place in mid to late summer. Resulting seed pods that are narrow and pointed, ranging from 2 to 4 inches long. They contain many flat brown seeds with silky tufts used for wind dispersal.
This milkweed can grow as single plants but more often in groups. It is a perennial, reproducing by seed and deep horizontal roots. Because of these extensive roots, established western whorled milkweed is more difficult to control. It should be mowed, pulled or hand cultivated early in its development. Once firmly established, herbicides will need to be used. The two most commonly used are glysophate products and 2,4-D + Dicamba formulations. Be careful when using herbicides containing dicamba as it does have soil activity and could adversely affect nearby desirable plants and trees.
This milkweed can be found in pastures, hay fields, orchards, gardens, ditch banks, roadsides and fencerows. It can be quite toxic to livestock. Plants contain a neurotoxin rather than the cardenolides contained in broadleaf milkweeds. While highly toxic to livestock, it is generally unpalatable to most animals and is usually ingested when other more desirable forage is limited or when in contaminated hay. You should eliminate it from livestock holding areas or hayfields if possible. They say that as little as 1-4 ounces can kill a sheep. Other animals that could be sickened include cattle, horses, goats, alpacas, poultry and rabbits.
An interesting side note to all this is that insects feeding on milkweeds gain protection from predators because of the toxins they ingest. These insects are not affected by these toxins but their predators would be poisoned if they ingested these milkweed-feeding insects. These insects include the late appearing yellow milkweed aphids, red milkweed longhorn beetles and of course the monarch and queen butterflies that everyone so enjoys.
Jim Leser retired to Cedaredge in 2007 after a career with Texas A&M University Extension in entomology. He is a member of the Cedaredge Tree Board and a Colorado Master Gardener.