What's the deal with Milkweed?

Monarch caterpillar on Antelope Horn milkweed 

What's the deal with milkweed? 

by Leah Churner

In an outtake of Episode 6, I ask park ranger LaJuan Tucker about native versus non-native milkweeds. Listen to the clip and read on for a primer on milkweeds. What are they? Why is tropical milkweed controversial? And if you live in Texas, what other perennials will attract Monarchs to your garden?

Why are milkweeds important? 

Monarch butterflies (Danaus plexippusmigrate thousands of miles each year, and their round-trip annual journey takes four or five generations to complete. Milkweeds (genus Asclepias) are the sole group of host plants for this butterfly. In other words, the milkweed plant is an essential nursery for Monarchs: it is the only plant on which the butterfly will lay eggs, and the only food source for Monarch caterpillars. Monarchs cannot complete metamorphosis without milkweeds!

Planting milkweeds is especially important in Texas because Eastern North American Monarchs fly through the state twice a year on their way to and from Mexico. Here is a really cool Google Earth “tour” of the Monarch migration path that explains this process visually. 

There are many native milkweed species in North America. These perennial plants, which have evolved in tandem with the Monarchs, display bloom and dormancy patterns that match the migration path of the butterflies. For instance, I have noticed that the local native Antelope Horn milkweed (A. asperula), grows quickly and blooms in the spring and fall, when the Monarchs are passing though the Hill Country, but disappears entirely in the summer and winter, when the butterflies have gone north or south. 

Unfortunately, most milkweed species have notoriously low rates of germination, so growing native milkweed from seed at home is quite difficult. It is possible, however. Here is the Wildflower Center’s guide to germinating milkweeds

What is tropical milkweed, and why is it controversial? 

Tropical milkweed (Asclepias curassavica) is the most widely commercially available species of milkweed. It is native to South America and grows upright with orange, red, and yellow flowers. This species' popularity in the nursery trade is due to its growth habit. Most milkweeds form a taproot, a single center root that grows straight down like a carrot, rather than a fibrous system that fans out close to the surface. Plants with taproots are difficult to grow in pots. Tropical milkweeds don’t grow a taproot, so they are therefore more amenable to greenhouse production. 

Monarchs will lay eggs and enjoy nectar from tropical milkweed. But the plant can pose some dangers to the butterflies. Because tropical milkweed is native to the Southern Hemisphere, it's not synced to the seasons in North America. This species has a much longer flowering period than native species, and will tend to keep blooming until frost. This can confuse the butterflies into sticking around late into the fall. Late-blooming tropical milkweed delays the Monarchs' natural migration pattern, causing them to die from cold before they can reach Mexico. Fortunately, there is a VERY SIMPLE solution to this, as LaJuan points out in the episode. Cut your tropical milkweeds to the ground in October. Or mow it, if you have a lot. 

A better, native alternative to tropical milkweed is butterfly weed, Asclepias tuberosa. Due to popular demand and growing concern about Monarchs, Asclepias tuberosa has become more available at nurseries in recent years. Because it looks similar to tropical milkweed, look for the plant by its Latin name, A. tuberosa, to make sure you are buying the correct species. Tuberosa is native to most of the continental US and will not interfere with Monarchs' migration.

Side note: I also saw Antelope Horn milkweed for sale at the Natural Gardener for the first time this year! So far my only experience with that plant is in the wild. If you bought some of that, please drop me a line and tell me about it! info@hothousepodcast.com

What should I plant to support Monarchs and other butterflies? 

Buy Asclepias tuberosa, if you can find it at a nursery or local botanical garden sale. If you can’t find a native, tropical milkweed is better than nothing, but do remember to cut it back in the fall if you live in Texas. (If you are further north, find out the window of migration through your area and cut it back after that. This map can help you figure that out.)  

But wait! There's more. Once safely in the butterfly stage, Monarchs expand their diet. They eat nectar from a variety of plants enjoyed by other butterflies, including lantanas, mistflowers, and verbenas. All of these plants bloom in the spring and fall, but they may be a little tired at the end of the summer; as LaJuan mentions in the episode, wildlife gardeners should also consider planting fall-blooming perennials to bolster the buffet of nectar-rich options for butterflies. These landscape plants put on their biggest bloom show in September and October, just as the Monarchs are making their way south through our area. Here is a list of easy-to-grow fall bloomers for Texas gardens listed in order of my personal preference: 

  • Mexican bush sage (Salvia leucantha) 
  • Autumn sage (Salvia greggii) 
  • Copper canyon daisy (Tagetes lemmonii)
  • Mexican mint marigold (Tagetes lucida) 
  • Fall aster (Symphyotrichuym oblongifolium)
  • Coral vine (Antigonon leptopus) 
  • Cigar plant (Cuphea micropetala) 

Final fun fact: It used to be believed that Monarchs are the only butterfly species that migrates both north and south each year. My 1981 copy of the Audubon Society Field Guide to North American Butterflies says so. But that assertion has since been challenged by studies of other butterflies, including the Painted Lady butterfly and the Admiral butterfly, both members of the genus Vanessa. Researchers in the UK discovered in 2012 that the Painted Lady butterfly (Vanessa cardudi), which lives on every continent except Australia and Antarctica, does make an annual migration north and south, and in fact its migration is even farther than the Monarch’s, taking six generations to complete. The reason nobody ever noticed? The butterflies are flying south at such high altitudes — climbing over a half-mile in the sky — that they are invisible to the human eye! Painted Ladies and Admirals are common in Central Texas, but their populations vary greatly year to year. What causes their fluctuating numbers remains an unsolved mystery. 



leah churner