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OSU Master Gardener volunteers utilize objective, research-based information to diagnose plant problems and offer sustainable solutions. This blog will highlight scientific studies that may be of interest to OSU Master Gardeners (and others) who would like to know more about the art and science of home horticulture. Any opinions expressed in this blog are the author's and not necessarily those of Oregon State University.
Updated: 1 hour 2 min ago

Milkweed for Monarchs: does it make sense in Oregon?

Mon, 06/29/2015 - 4:57pm
(Originally published in the March 2015 issue of The Gardener's Pen newsletter)

Larvae (left) and adult (right) of the Monarch butterfly, Danaus plexipuss.  Photo Credit:  Dr. Jeffrey Miller, Oregon State University Professor of Entomology.

Ecology of the Monarch Butterfly, Danaus plexippus,  in the Pacific Northwest:  Monarchs are common on the east side of the Cascade Mountains, but very uncommon on the west side.  Monarch caterpillars feed on milkweed during the early summer.  Adults fly from spring to fall when they migrate south.  Found in open habitats, particularly along roadsides and fencerows. (Adapted from: Miller and Hammond, 2003. Lepidoptera of the Pacific Northwest. USDA FHTET-03-11)

Monarch butterflies are specialist insects, with specialized digestive systems and feeding behaviors that are adapted for feeding on plants in the genus Asclepias (milkweed plants).

Asclepias plants have toxic chemicals (cardiac glycosides) and a latex sap that deters most insect feeding.  Monarchs store the cardiac glycosides in their exoskeleton, which circumvents the need to metabolize these nasty chemicals and also makes them poisonous to vertebrate predators.

The latex sap of milkweeds gums up the mouthparts of many insects ~ causing them to starve.  Monarchs deal with the latex sap by clipping the veins on milkweed leaves, allowing the latex to ‘bleed out’ of the plant before they feed.

Monarch adults are migratory.  East of the Rocky Mountains, monarch butterflies fly south to overwintering sites in Mexico each the fall, and return north in the spring.  Scientists have have noted that overwintering populations of Monarchs in Mexico have significantly declined over the last two decades (Brower et al. 2012).  Three factors have been implicated in the decline of eastern monarchs:  (1) loss of forest habitat in Mexico, where the butterfly overwinters; (2) loss of breeding habitat/milkweed plants in the United States due to land development and increased use of herbicides in Roundup-Ready crop fields; (3) occasional extreme weather conditions that decrease the length of the breeding season.

Known migration routes, breeding territories and overwintering areas for Monarch butterflies.  Map reproduced courtesy of MonarchWatch.org.
Although most North American monarchs overwinter in Mexico, those that live west of the Rocky Mountains generally overwinter at one more than 300 sites along the California coast.  These monarch ‘groves’ tend to be within a few km of the ocean, which is thought to moderate temperature, and are usually protected from the elements in some fashion.  Unlike eastern monarchs, who may fly thousands of miles from Canada to Mexico, western monarchs usually migrate no more than 100 miles.  Their breeding sites are thought to range as far north as western Canada, and as far south as southern Arizona, in the mountains and foothills of California, the Pacific Northwest and the Great Basin States.  General dogma has been that monarchs may wander into southern Oregon, during their spring migration to breeding sites or their fall migration to overwintering sites in California.  But the truth is, we really don’t know that much about where western monarchs breed.

Still, many groups have advocated that Oregon gardeners plant native milkweed plants to support western monarchs ~ particularly because there has been about an 80% decline in western monarch numbers recorded from California overwintering sites since 1997.  The factors implicated in western monarch decline include:  (1) milkweed loss following prolonged drought, (2) land development that reduces overwintering habitat and/or breeding habitat, and (3) pesticide use.

Does it make sense for Oregon gardeners ~ particularly those in Western Oregon ~ to plant milkweed to support western monarchs, given that conventional dogma suggests that monarchs don’t migrate through or utilize breeding sites in Western Oregon?

I suggest that it can’t hurt for Oregon gardeners to plant milkweed in an effort to support the Western Monarch.  Although monarchs may not be common outside of southern Oregon, what little data there is suggests that monarchs may at least be migrating through ~ and in some cases may be breeding in ~ broad areas of Oregon.  What data do I have to support this assertion?


  1. A map of the known and potential monarch breeding areas in the western U.S. includes (as best as I can read) monarch breeding records in Jackson, Josephine, Klamath, Lake, Harney, Lane, Benton, Washington, Multnomah, Wasco and Deschutes counties.  I do not have access to the data that was used to construct the map, but it appears as if the researchers are relying on museum records.  So, the identified breeding sites probably represent a record of a monarch specimen from a museum, which was collecting during the summer at a particular locale.  If this is true, there are records of summer monarchs in both eastern and western Oregon locales.  These may be ‘vagrants’ that wander off of their migration path, but they may also be breeding adults.
  2. The Butterflies and Moths of North America site has user-verified records (with photos) of monarchs reported for nearly every Oregon county.  I was able to access details for the three most recent sitings.  I’ve paraphrased the details of the sitings, below, so that you can see that there is evidence of monarch breeding in southern Oregon (caterpillars in Josephine County) and adult migration through the Willamette Valley (strong adult flights ~ rather than tattered-winged vagrants).
    • June 10, 2014, Benton County, OR, one adult monarch sipping nectar from showy milkweed (Asclepias speciosa).
    • June 21, 2014, Black Bear Bar along a wild and scenic section of the Rogue River, OR.  Monarch caterpillars munching on showy milkweed.
    • June 20, 2014, Lane County, OR, one adult monarch flying around and sipping nectar from Buddleia.  Flight was strong and direct.  Perhaps a migrant. Over the past few years, several organizations have been promoting the planting of milkweed plants, in order to provide host plants for monarch butterflies.
  3. The adults I've seen in Oregon (Douglas County, Lane County, Linn County) have had intact wings and scales - not what I expect from strays far from their host plants/flight path.

What type of milkweed should you plant?  Opt for native milkweeds, and avoid tropical milkweed (Asclepias curassavica).  The Xerces Society has a wonderful publication that details the milkweed plants native to Oregon.  These include:

  • Asclepias cordifolia (purple milkweed):  scattered in south and southwest Oregon
  • Asclepias cryptoceras ssp. Davisii (Davis’ milkweed): scattered in central and eastern Oregon
  • Asclepias fascicularis (narrow-leaved milkweed):  scattered across Oregon
  • Asclepias speciosa (showy milkweed):  widespread across Oregon

You will most easily be able to find seed of Asclepias speciosa from local nurseries, who may also have Asclepias tuberosa (butterfly weed, native to the Eastern US) and Asclepias syriaca (common milkweed, native to Europe).  Asclepias tuberosa and Asclepias syriaca, like many milkweeds, can have weedy tendencies.  They are called milkWEEDS, after all.  But, with this weediness comes the potential to invade areas outside of your garden.

Thus, when selecting milkweed, try to stick to native species that are appropriate for your area ~ such as Asclepias speciosa ~ in order to limit the introduction of non-native plants in natural areas.  

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