The Importance of Milkweed to Monarchs

Question: Why is milkweed important to the monarch butterflies? What is the best milkweed to plant and when and how should it be planted?

Thank you for asking about monarchs and ways to encourage them to visit your garden. Plant choice matters. We hope this will inspire you to plant milkweed this fall or next spring!

So, why milkweed is important to monarchs? Many butterflies need a single kind of plant for their caterpillars (larvae) to feed on. For the monarch butterfly, that plant is milkweed, of the genus Asclepias (uh-sklee-pee-us). Milkweed leaves are the only food source that supports their growing caterpillars.

Monarch butterflies spend the winter in central Mexico and fly north to the U.S. and southern Canada in the summer. Here they breed and lay their eggs on the many species of milkweed (Asclepias spp.). Over the last decade, the Eastern migratory population of monarch butterflies has declined around 80%. The cause of the monarch’s decline is attributed to the loss of their breeding habitat across North America. The severe decline in milkweed has been correlated with insecticide use, herbicide-tolerant corn and soybeans, suburban development, and wide swaths of invasive species. Without milkweed, there would be no monarch butterflies.

Plant the best milkweed for your location:
Gardeners who are concerned about the decline of the monarch butterfly are planting milkweed to increase their breeding habitat. More milkweed means more leaves to support growing monarch caterpillars. Although there are many species of milkweeds, deciding which milkweed species to plant depends on a few variables. The best milkweed is the one that is native to your area, is easiest to grow, and that feeds healthy monarch caterpillars. The comprehensive tome, Flora of Virginia, lists 15 milkweed species that are native to Virginia. Five of these species are considered good support for monarchs. Although female monarchs will lay eggs on any of these species, according to the USDA Agricultural Research Service team, they lay more eggs on swamp milkweed (Asclepias incarnata) and common milkweed (Asclepias syriaca). A warning about common milkweed: it is an aggressive spreader that grows to 6 feet tall and needs plenty of room because it spreads quickly over a large area. While it flourishes with ease, it may not be a good choice for a cultivated garden. Other recommended milkweeds for Virginia gardens: butterfly weed (A. tuberosa), whorled milkweed (A. verticillata) and poke milkweed (A. exultata).

Starting with plants or seeds? Plants can be started in the garden in spring or in fall without much trouble. It’s easy to find some milkweed plugs (like butterfly weed and swamp milkweed) at local nurseries and sometimes at big box stores. For other milkweed species, growing from seed may be the best option since plants may be scarce.

For best results with spring planting, sow seeds directly and cover with ¼-inch of soil and water. To improve germination, seeds sown in spring should be cold stratified (subjecting seeds to cold and moisture before germinating). The easiest way to do this is to place the seeds in moistened soil covered in plastic, or place between moist paper towels in a plastic bag, and refrigerate for 3 to 6 weeks. Remove seeds and soak in warm water for 24 hours before planting. For fall planting, just scratch the soil surface lightly, scatter the seeds and water. No need for cold stratification – nature will do the rest!

Plant milkweed in your yard and the beautiful yellow, black, and white striped caterpillars will come!

A tip for gardeners in Central Virginia:
In September, you can see many monarch caterpillars as they munch on an abundant buffet of milkweed at Big Meadows on the Skyline Drive in the Shenandoah National Park.

References

“Flora of Virginia”, Weakley, A. S., Ludwig, J. C., Townsend, J. F., Crowder, B., Botanical Research Institute of Texas Press, 2012.

“Asclepias — Or How I Learned to Love Milkweed”, Pat Chadwick, Piedmont Master Gardeners Association, The Garden Shed, 2016.

“Quasi-extinction risk and population targets for the Eastern, migratory population of monarch butterflies (Danaus plexippus)”, Semmens, B., Semmens, D., Thogmartin, W. et al.

“Which Milkweeds Do Monarch Butterflies Prefer?”, Jan Suszkiw, Agriculture Research Service Office of Communications, USDA.

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