Sunday, October 11, 2015

Milkweeds, Monarch Butterflies and Colorado

"Plant milkweeds!" they say. But which ones?
common milkweed, A. syriaca
common milkweed, Asclepias syriaca
The number of monarch butterflies (Danaus plexipus) seen in the eastern United States has declined dramatically over the last couple decades. Monarch experts, e.g. Drs. Lincoln Brower and Orley Taylor, make a strong case that the decline has been caused by several factors: First, 1), changes in agriculture in the U.S. midwest that have reduced the number of milkweeds for monarch butterflies to feed upon. In particular, use of herbicide-resistant crops eliminated the common milkweed, Asclepias syriaca (dogbane family, Apocynaceae) which had been growing as a weed in the corn and soybeans. Simultaneously, high prices for corn and soybeans, now used for biofuels (ethanol) as well as human and animal food, favored expansion of weed-free cropland into areas that previously supported wild plants including milkweeds. Another cause of monarch butterfly decline is, 2), loss of forests in their overwintering grounds in Mexico, due to illegal logging and poor management. In addition, 3), periods of unfavorable weather reduced growth of monarchs and allowed their natural enemies, from birds and ants to fungal diseases, to find and kill them. (See the Taylor lecture with all the data; link). To counter the decline in monarch butterflies, many changes may be required, but much can be done by having more milkweeds available for the butterflies. Monarch butterfly caterpillars eat milkweeds and accept no substitutes. Consequently, concerned experts and citizens urge planting milkweeds so that there is ample food for monarch caterpillars. Adult butterflies get norishment for the nectar from flowers of many plant species so adult food is rarely an issue.

milkweeds in Colorado
wild milkweeds in Colorado, showy milkweed
If a major part of the problem is reduction in milkweed numbers, then planting milkweeds is a very effective response. To aid that, a variety of organizations are advising people all across North America to plant milkweeds, providing links and advice, and in some cases giving away milkweed seeds. (Monarch Watch, Save our Monarchs, Xerces Society )

However, not everyone should just write away for free seeds and toss them into their yard. My concerns are from the point of view of growing well-adapted native plants. If the plant dies, you've wasted your time and energy and any monarch caterpillars relying on it die too. Conversely, introducing troublesome new weeds would be an undesirable outcome of this campaign. Plant the right milkweed for your area.

North America is a big place and no milkweed is native across the entire continent. Thus, the right milkweed--native, adapted to local conditions--varies across the continent.

Choosing the right milkweed is more difficult in some places than in others. The most famous monarch butterflies, those from the eastern United States, overwinter in the mountains of Mexico and come north in the spring, first to breed in Texas and a month or so later, monarchs that hatched in Texas fly into the central midwest to breed themselves. For all those areas, the common milkweed, Asclepias syriaca, is native, fast growing, hardy, and the preferred food of monarch butterfly caterpillars. Planting the common milkweed from central Texas to the Canadian border and east to the Atlantic will hugely improve conditions for monarch butterflies.
common milkweed, A. syriaca
common milkweed, Asclepias syriaca
However, the monarch butterflies found in California and surrounding states neither migrate to forests in Mexico nor eat common milkweed. Their numbers are down (link see information on life cycle of west coast monarchs) but the causes are not likely to be mainly planting herbicide-resistant soybeans or Mexican logging. Conservationists recommend planting milkweeds, but milkweed species native in the West, not the common milkweed. See Xerces Society brochures for California and the Great Basin.

Colorado is between the two regions and rarely mentioned in monarch butterfly/milkweed discussions.

The central U.S. gets drier and drier going west to the base of the Rocky Mountains. The common milkweed drops out in eastern Kansas and the native roadside milkweed of Colorado is the showy milkweed, Asclepias speciosa. Historically, monarch butterflies generally stayed on the eastern side of the Great Plains, where common milkweed grows. Monarch butterfly populations may have dropped in Colorado and Wyoming, but they were never considered important parts of the North American monarch butterfly populations, but rather a place that monarchs migrated through or wandered into accidentally.

showy milkweed, Asclepias speciosa
showy milkweed, Asclepias speciosa
Apparently, in Colorado, showy milkweed is the milkweed to plant to help monarch butterflies. That is what Monarch Watch (link) recommended in response to my inquiry. Likewise, the Xerces Society recommends planting showy milkweed for monarch butterflies all across the West.

showy milkweed, Asclepias speciosa
showy milkweed, Asclepias speciosa
The showy milkweed is the native, hardy milkweed that people on the Front Range see growing on roadsides. It needs less water than the common milkweed so is a better choice for Colorado's low rainfall. (Common milkweed, if grown here, should be treated like Kentucky bluegrass and given supplemental water). As you can see from my pictures, showy milkweed looks a lot like the common milkweed. 

Both common milkweed and showy milkweed will spread from rhizomes (belowground stems) forming clumps from a single plant and often coming up some distance away.  Gardeners need to be prepared to cut back unwanted stems. 

Also native to Colorado, and being given away as free seeds by Minnesota's Save Our Monarchs (link), is Asclepias incarnata, the swamp milkweed. This is a handsome plant with reddish pink flowers. However, as its name indicates, it is a plant that requires a lot of water. It will not do well in dry Colorado gardens. The Flora of Colorado says it is found "along ditches and streams, in marshes and in other wet areas of the plains and foothills." (Ackerfield 2015 p. 93). For gardens that run down into wetlands or with a swampy component, this is terrifically beautiful plant and a fine monarch butterfly host.

swamp milkweed, Asclepias incarnata
swamp milkweed, Asclepias incarnata
Another very attractive prairie milkweed loved by monarchs, butterfly milkweed (also known as butterfly weed and pleurisy root, Asclepias tuberosa), is native here on "sandy, gravelly or calcareous (chalk, limestone) soils in open woodlands or pinyon-juniper communities, 5000-7000 ft." (Ackerman, p. 94). That makes it sound abundant but it is not common. In western Colorado it is found "near springs" (Weber and Wittmann, 2012 p. 59). Butterfly milkweed is also found across most of eastern North America, areas all generally moister than the plains of Colorado. The University of Missouri describes it as drought-tolerant, but eastern Colorado's normal is St. Louis's drought. In a watered garden or with some shade it should survive, and, in the right spot, thrive.

butterfly milkweed, Asclepias tuberosa
butterfly milkweed, Asclepias tuberosa
Kansas prairie

butterfly milkweed, Asclepias tuberosa
butterfly milkweed, Asclepias tuberosa
eastern Nebraska
The discussion above focuses on the Front Range of Colorado and applies to the plains of eastern Colorado. In the Rocky Mountains and for areas of the Western Slope, the milkweed to plant is still showy milkweed (see Xerces Society on the Great Basin) though butterfly milkweed is a lovely choice if you have wetlands or have water to spare. Note that western Colorado is the far eastern edge of the range of the western monarch butterflies.

One might think since Colorado is not on the mainline of monarchs that it is irrelevant for Coloradans  to grow milkweeds. I'd take a different approach. What if we make this a welcoming place for monarch butterflies? We live in an era of change. Scientists in the Arctic or working on mountain tops are recording the arrival and survival of animals that could not previously live in that area. Animals (and plants, but they're slower) move as conditions change. If monarch butterflies encounter a rich supply of milkweeds, they are likely to leave eggs. If those eggs survive, we can gradually develop breeding populations of monarch butterflies for whom the Colorado is home, aiding nationwide efforts by helping expand monarch breeding grounds. 

Comments and corrections welcome. 

monarch butterfly in Maryland
monarch butterfly seen in Maryland, Sept. 2015

Afterthought: A weed is defined as a plant in the wrong place. If we grow milkWEEDS in our gardens, will we have created a contradiction, a wanted weed?

Ackerfield, J. 2015. Flora of Colorado. Botanical Research Institute of Texas, Fort Worth, TX.
Finley, B. 2014. Rescue strategy hatching for dwindling monarch butterflies. The Denver Post Dec. 27, 2014 link
Monarch Watch link is giving away swamp milkweed seeds  link Their pollinator seed mix will get you started gardening for pollinators, but it is made of plants that are native in many places, not local to any particular area. Choosing local natives works better for any particular spot, but gardener has to learn what those are.
Taylor, O. R. (Chip). Monarch butterfly conservation: the challenges ahead. Aug. 2013.YouTube Video
Weber, W.A. and R. C. Wittmann. 2012. Colorado Flora, Western Slope. 4th edition. University of Colorado Press, Boulder.
Xerces Society (Invertebrate conservation) link

Kathy Keeler


  1. Great Article with a beautiful vision...having just relocated from New Jersey to Denver and a lover of the Monarch Butterfly of my many questions is "will I see the Monarch Butterfly?" I continue to run a program back East for urban youth and one of my favorite projects is Teaching with Monarchs - Journey and Transformation though a wonderful organization Monarch Teachers Network.

  2. Monarch butterflies come through Colorado. I remember seeing one last year. Four days ago I saw a monarch flying through my backyard. That's the only one I've seen this year--but if you aren't outside you won't see them and I haven't been able to spend many hours in the garden recently--so I was (and am) delighted.