Milkweed a must for attracting monarch butterflies

May 26, 2010

The larval stage, or caterpillar, of the monarch butterfly feeds exclusively on the milkweed plant. Photo courtesy UF/IFAS Santa Rosa County

Now is the time to plant your butterfly magnets

Whether you call it butterflyweed or milkweed, this is one group of “weeds” you want in your landscape. They are butterfly magnets. The leaves are the preferred food source for the caterpillar of several species of butterflies, including monarchs. The flowers provide nectar for both butterflies and hummingbirds.

Plants in the genus Asclepias are collectively known as the milkweeds.  They get their common name from the milky color of its sap.  The sap contains toxins, called cardenolides, which the monarch has adapted to be able to ingest. These toxins serve to protect both the plants and the monarchs from predators.

This group of herbaceous plants contains over 100 species across North America. In the past, they have grown abundantly in disturbed habitats such as agricultural landscapes and along roadsides.  However, milkweed plants are in serious decline. Urban and suburban development is eliminating monarch habitat by displacing agricultural landscapes where an estimated 90% of milkweeds occur.

Butterflyweed or Asclepias tuberosa. After the early-summer flower clusters, green blimp-shaped pods form. These open to reveal silky parachuted seeds. Photo courtesy UF/IFAS Santa Rosa County

Though most members of the genus are tropical, there are 21 native milkweed species in Florida.  Due to their ornamental value, tropical milkweeds are the ones you are most likely to find in local garden centers.
Asclepias curassavica, commonly called Mexican butterflyweed, blood-flower or scarlet milkweed is native to South America.  It is considered a nonnative, tropical milkweed.

Scarlet milkweed contains much higher amounts of toxins than native milkweeds. Monarchs do not discriminate between Asclepias that have high or low toxicity.  Higher toxicity milkweed may hinder larval performance, but it may also provide additional protection from predation.

The increase in tropical milkweeds in mild climates has one potentially harmful effect. It may result in the establishment of year-round monarch populations in new locations. This development might seem beneficial, but research demonstrated that migrating monarchs have a much lower occurrence of parasites than those that do not migrate. Therefore, any new resident populations resulting from an increase in tropical milkweeds are likely to be heavily infected with parasites.
However, according to University of Florida professor Dr. Jaret Daniels, planting tropical milkweed is better than no milkweed at all.
The preferred choice is native milkweed.  One example is butterflyweed or Asclepias tuberosa. It lives as far north as Ontario to New Hampshire, west to South Dakota, south to Arizona into Mexico and east to Georgia and Florida. It does best in hardiness zones 3 to 9.

While some of native species may not be quite as showy as scarlet milkweed, they work perfectly well as monarch attractants. Gardeners should rest assured that monarchs are well adapted to consuming these native species. For example, while butterflyweed (A. tuberosa) has tougher leaves than scarlet milkweed (and in fact monarchs need to eat more of this plant than other species to get the nutrients they need), caterpillars that eat only A. tuberosa grow just as well as those that eat other species including A. curassavica.

So the message is clear—no milkweed, no monarch butterflies.  When you’re adding plants to your landscape this year, think about adding native milkweeds.  It’s a good thing.

Theresa Friday is the Residential Horticulture Extension Agent for Santa Rosa County.  The use of trade names, if used in this article, is solely for the purpose of providing specific information. It is not a guarantee, warranty, or endorsement of the product name(s) and does not signify that they are approved to the exclusion of others.

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1 Comment
    1. I enjoyed your comments on the Monarch & milkweed. I would like to know if the milkweed attracts the Monarch when the flowers are gone.

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