Help Save The Monarchs With These 4 Great Milkweeds

Monarch butterfly on swamp milkweed

Last week, I was manning the booth at the Master Gardener Demo Garden when someone plopped a tall, spindly plant down onto the table. It looked pitiful; the flowers were gone and the leaves had tiny holes in them. But upon closer inspection, I spotted a few lantern-shaped chrysalises and some colorful caterpillars working their way up the stems. The plant was none other than milkweed. And the ‘lanterns’ contained baby monarchs in the process of forming.

We placed the milkweed next to some Joe-Pye weed and sat back to see what would happen.


With a wing-span of over 4 inches, the monarch butterfly is considered by many to be the ‘king’ of all butterflies. According to the National Wildlife Federation, however, monarch populations in North America are in sharp decline. Over the past twenty years, their numbers have plummeted by an astonishing 90 percent, prompting scientists to question what’s happening. They suspect the answer may lie with a plant called milkweed.

Close-up of swamp milkweed, Asclepias incarnata

Why milkweed you might ask? Because milkweed is the only host plant for monarchs. And monarch caterpillars feed exclusively on its leaves. Yet milkweed has been disappearing, too. Over the last few decades, large-scale clearing of land for development coupled with heavy pesticide use has resulted in the plant becoming increasingly scarce. This has placed the tiny species that depends on it in jeopardy.

Indeed, the problem is so big that according to one National Public Radio report, monarch populations overwintering in Mexico that once averaged around nine hectares (1995 to 2002), in 2014 took up less than one.*


Every spring, millions of Eastern monarchs begin their annual migration from Mexico to Canada, a journey of roughly 3,000 miles. And every fall, they do the same trip in reverse. The insects stop three times to breed along the way, each time going through four stages of development: egg, larvae (caterpillar), chrysalis and butterfly.

Once they emerge from their chrysalises, the butterflies live for just four to eight weeks, long enough to fly hundreds of miles further north to lay their eggs. Then the stages of development are repeated and the next generation continues the journey.

Millions of monarch butterflies flying in Mexico

In all, it takes three generations of butterflies to reach Canada.

Interestingly, the fourth generation butterfly (born in Canada) lives a lot longer. This is the monarch that travels all the way back to Mexico to begin the whole process again in the spring. It lives for six to eight months. (To read more about the monarch butterfly cycle, click here.


The milkweed family (Asclepias) includes around 120 species, most of which are native to the United States and Canada. There are two major groups: narrow-leaved and broad-leaved. Among these, the plant that people most often associate with milkweed is the broad-leaved Asclepias syriaca. Known as common milkweed, it produces large balls of mauve-pink flowers and blooms in early to mid summer.

Monarchs are attracted to all milkweeds, however, and draw no distinction between species.

Milkweed plants at the Denver Botanic Gardens


Monarchs and milkweed have an intimate relationship. In March, after emerging from hibernation, monarchs find a mate, then begin their annual journey north. On the way, they search for a place to lay their eggs and produce the next generation. This can only happen on milkweed.

Once laid, the eggs take about four days to hatch. The emerging caterpillars then spend another two weeks feeding on milkweed leaves until fully grown.

But the story doesn’t end there. Once fully grown, the caterpillars attach themselves to a milkweed stem and begin the process of metamorphosis. It generally takes an additional two weeks for the adult butterfly to emerge.

Monarch caterpillar chrysalis

And once the adult emerges, the process begins all over again.


Aside from providing food and a place to live, milkweed has another key benefit for the monarch butterfly. As the caterpillars undergo transformation in chrysalis, they ingest toxic chemicals secreted in the plants’ sticky sap. These chemicals, called cardenolides, provide them with a defense against would-be predators such as birds and mice.

Monarch caterpillar munching on milkweed

Specifically, cardenolides are so toxic and bitter to other vertebrate species that many have learned to associate the bad taste with the butterflies’ orange and black markings. Moreover, even as adults (when monarchs no longer eat milkweed), they are still able to sequester the toxins that they ingested as larvae. As a result, this provides them with protection against many predators during the remainder of their lifespan.

Monarchs’ distinctive colors help ward of predators

Monarch butterflies only lay one egg per leaf, but they lay eggs on lots of leaves and always on more than one plant. They are mini eating machines. Three days after the milkweed arrived to the fair, the caterpillars had devoured the plant they rode in on. And in their place hung rows of tiny chrysalises.


Nowadays there are a number of milkweed and butterfly weed varieties that have been developed specifically for the garden. Available in different colors and flower forms, they add a natural beauty to the garden. Most importantly, planting a few of these plants will help save the monarchs, too.

Common milkweed, Asclepias syriaca

Ready to save the monarchs with milkweed? Here are some great new varieties to try, available at many local nurseries.

BUTTERFLY WEED, Asclepias tuberosa

This bright orange beauty blooms from May to September. It won Perennial of the Year 2017 for its brilliant flowers, hardy, upright stems and good drought-tolerance. The blooms look great combined with yellow daylilies or white daisies. And, they especially shine against purple-leaved plants like smoke bush.

Butterfly weed, Asclepias tuberosa

Orange not your thing? Then try ‘Hello Yellow’, available through White Flower Farm.

Asclepias tuberosa ‘Hello Yellow’

Or try a hot-color mix of yellow, gold, orange and scarlet, Asclepias tuberosa ‘Gay Butterflies’ also available at White Flower Farm.

Asclepias Gay Butterflies Mixture

SWAMP MILKWEED, Asclepias incarnata

Similar in shape to roadside milkweed, Swamp milkweed ‘Cinderella’s’ deep pink flowers with white centers make it a standout in the garden. And its vanilla-scented flowers are a treat for the senses. Growing 3′ to 4′ tall, it blooms all summer with regular deadheading.

Swamp milkweed ‘Cinderella’ available at

Milkweed generally blooms from June to August. All varieties need full sun and well-drained soil. If you observe caterpillars or chrysalises on your plant, wait until the butterflies have flown away, then simply cut the plant to the ground.

* Monarch populations are measured in hectares, which roughly equal 2 1/2 acres.


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