Monarchs and Milkweed

April 18, 2020

Milkweed is a North American native plant that is a food source for hundreds of small creatures but critical for monarch butterflies. Monarchs lay their eggs on the underside of milkweed leaves and nowhere else.


The plant’s name comes from its milky sap which contains toxins called cardiac glycosides. Monarch caterpillars feed exclusively on milkweed foliage. Ingesting plenty of the toxins renders them toxic to predators.


Most adult monarchs live roughly one month. But about every fourth generation lives six to eight months. It is this generation that migrates to warmer areas for the winter and returns in spring to reproduce.


The loss of milkweed in their habitat has led to a 90 percent decline in monarch butterflies over the past two decades.


Native Americans used the plant to treat warts and lung ailments such as pleurisy. Because of this, the great taxonomist Linnaeus named the milkweed family Asclepias after the Greek god of healing. There are more than 100 species in the tribe. If a gardener would like to plant milkweed to help out butterflies, it is important to choose a species native to that particular habitat.


Three readily-available species are native to our area. Common milkweed ( A. syriaca), butterfly weed (A. tuberosa), and swamp milkweed (A. incarnata). All three of these require a period of cold stratification to germinate. The easy way to achieve this would be to sow the seeds in autumn, allowing Mother Nature to do her thing. If you don’t want to wait to plant milkweed, you can place the seeds onto a damp paper towel and put that into a Ziploc bag. Place the bag in the refrigerator (not the freezer) for 30 days.


Butterfly weed has a tap root. It does not like to be transplanted. After the cold treatment, start it in peat pots. When the seedlings are ready to go into the garden, space them 18 inches apart burying the pot completely so that you do not disturb the root. I love the  crayon-bright orange flowers, but it also comes in taxi cab bright yellow.


Common milkweed reproduces by seed and by rhizomes. It can form substantial clumps but does not spread aggressively. All Asclepias produce lots of seeds each of which is covered in silky white threads that are borne on the wind which is why it is sometimes called “silkweed”.


Swamp milkweed, as the name implies, wants a moist environment. It did not do well in the bog. It wants constant moisture but not wet feet. Unlike butterfly weed, it tolerates partial shade as well as full sun. Also called Red Swallowwort, it is taller than the other two species. The flowers are pink or white with a vanilla fragrance.


When we first moved here in 1980 I would see drifts of butterfly weed along the back roads. I think that the last time I saw even one roadside plant was in 2016 when some fellow master gardeners and I were tending an area pollinator garden. 


Next Wednesday is Earth Day. What a great time to do something to support an endangered species by planting some milkweed!


Master gardener Ann Bartlett loves to see butterflies fluttering through the ornamental beds that surround her home. 

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