Milkweed in the Garden

Gardeners often lament the “weed” part of the common name of Asclepias – milkweed. This descriptor seems to diminish not just the beauty, but the acceptance of this plant as a worthy selection for home gardens. In 2017, butterfly weed (Asclepias tuberosa) was selected as the Perennial Plant Association’s Perennial of the Year. This designation raised the profile of milkweed as a native plant that is not just ecologically important, but beautiful as well.

Around this same time, in 2016, The Garden Shed published its first article on Asclepias. In the last six years, there has been a surge of interest in native plants, as well as an increased appreciation of the relationship between native plants and the insects that have evolved with them. The most well-known example of an evolutionary relationship between a larval host plant and an insect is the relationship between milkweed and the monarch butterfly (Danaus plexippus). On July 21, 2022, the International Union for the Conservation of Nature (IUCN), a Switzerland-based conservation organization that monitors the status of wildlife, added the migrating monarch butterfly to its “red list” of threatened species and categorized it as “endangered” — two steps from extinct. Gardeners are eager to join the effort to create monarch habitats by planting milkweed and other native pollinator plants, in both public/community spaces and in home ornamental gardens.

This article will focus on milkweed species native not just to North America, but to areas as specific as zip code, county, state, or regions, with a reminder on how to use several native plant databases. I will also present a first-hand example of starting milkweed from seed, and how to assess the characteristics of different milkweeds and their suitability for particular site conditions. We will also briefly look at some of the current research on tropical milkweed (A. curassavica).

A REVIEW OF NATIVE PLANT DATABASES

I used three native plant databases to identify milkweed species native to my area, which is zip code 22901, Albemarle County, Virginia. These databases are included in the Source list at the end of the article. You can also broaden your search to include plants native to states, or to regions. After comparing the lists, you can then choose which “native” definition provides the plants best suited to your taste and to your site conditions. Site conditions might include characteristics such as full-sun, acidic soil, wet in spring, and heavy deer and rabbit pressure.

For this article, I’ll concentrate on five milkweeds: the first four are native to zip code 22901, and whorled milkweed is native to Albemarle County:

• Common ( syriaca)
• Swamp ( incarnata)
• Purple ( purpurascens)
• Butterfly weed ( tuberosa)
• Whorled ( verticillata)

MILKWEED OVERVIEW

There are 72 species of milkweed native to the U. S. and Canada, with native species in all the lower 48 states. Common milkweed (A. syriaca) is the most well-known species of native milkweed. Milkweeds are named for the milky sap in their stems, leaves, and pods. This sap contains latex and complex chemicals called cardenolides. The cardenolides make the plants unpalatable to most creatures other than milkweed specialists.

Monarch butterflies lay their eggs only on milkweed plants; larvae or caterpillars eat the milkweed leaves for food. Cardenolide analysis has shown that 85% to 92% of monarchs overwintering in Mexico had eaten common milkweed (Asclepias syriaca) as caterpillars. Once they mature to the butterfly stage, monarchs draw the nectar from milkweed flowers and from other nectar-producing flowers.

 

 

 

WHICH MILKWEED IS BEST?

Over the past two decades, monarch numbers east of the Rockies have fallen by 80-90%. The decline is partly attributed to shrinking milkweed habitat, especially in the Midwest. A team of Agricultural Research Service (ARS) and university scientists monitored the egg-laying preferences of female monarch butterflies on 9 different milkweed species. Although female monarchs will lay eggs on all nine species, swamp milkweed (A. incarnata) and common milkweed (A. syriaca) averaged the highest number of eggs. The findings indicate that while female monarchs do make choices, they don’t specialize in reproducing on a single milkweed species. What’s more, their egg-laying preference can change according to the time of season, the prevalence and habitat of the milkweed species they encounter, and the plants’ robustness and maturity. For these reasons, the researchers caution against focusing restoration efforts on a single preferred species.

COMMON MILKWEED – Asclepias syriaca

This species thrives in full sun in average to well-drained soils, and is commonly found in pastures, field edges, and along roadsides. It grows 4-6’ tall. It has a long bloom period, from June to August, when it bears large clusters of fragrant, dusty pink flowers in spherical umbels atop the plant. The ovate leaves are approximately 6” long and are borne on a single stalk. The undersides of the leaves are covered with short woolly hairs.

Growing Milkweed from Seed

In fall of 2020, another master gardener gave me a few dried pods of common milkweed (A. syriaca). I researched how to grow milkweed from seed and looked at an online video about how to separate the seeds from the silk. The instructions were to place the dried pods (the floss had already started to burst) with a few coins in a paper bag and shake. I had to add quite a few coins before I began to see some results. I then cut a diagonal hole in the bottom corner of the bag as instructed and after some shaking, the separated seeds came out. Asclepias seeds need cold stratification to germinate, which means that a period of 30-60 cold days is needed to wake the seeds from dormancy. It’s easy to find instructions online about how to do this indoors (see one such site from the University of Florida). The cold stratification process is naturally achieved by planting the seeds outdoors in the fall and exposing them to the natural freeze/thaw cycles of a Virginia winter. This is what I did, and I think I was lucky in a couple of ways: first, the seeds were already brown and hardened and 2) my planting date of November turned out to be optimal. (See this link for a video on how to remove seeds from a dried pod that has not yet burst.)

The planting spot I prepared has acidic clay soil and is in full sun. I raked the soil smooth, added some compost because the clay soil is compacted, raked it smooth again, added the seeds, covered them very lightly with soil, and watered. Mark the spot so that you remember where to look the following spring. Be aware that common milkweed seedlings may not appear until late April in Zone 7. Unfortunately, I underestimated the resiliency of milkweed seedlings and did not thin them sufficiently. Spacing between plants provides better air circulation which helps mitigate problems from fungus such as downy mildew.

An Overabundance of Milkweed

Common milkweed spreads through seed dispersal and through underground rhizomes. Of the five milkweeds being discussed, common milkweed is the most aggressive spreader. This means that you need to consider whether the spot you’ve chosen will give the milkweed enough room to spread. I am growing a large patch of common milkweed within a pollinator bed that includes other native plants. Containment is an issue because I’ve planted a spreading plant within a bordered area. Common milkweed is usually recommended for open areas without defined borders. I am digging up mature milkweed at both ends of the milkweed patch, as well as any small plants that appear outside the patch. When the rhizomes are severed during hoeing or pulling, a new plant can form from each rhizome segment. If possible, dig up the entire plant, including the roots, and remove as much of the rhizome as possible. This is easier to do when the plants are young, and the soil is moist. I also cut back milkweed to thin the patch and allow air to circulate. This cutting-back must be done before the monarchs start laying eggs on the undersides of the leaves, or after the egg-laying season. Usually, one egg is laid per plant, and it can be very difficult to spot. There are multiple generations of monarchs, and the first monarch eggs in Virginia, zone 7, are generally observed in April. Many monarch caterpillars were visible last year on our milkweed patch in September. See this link to the “Annual Life Cycle” of the monarch.

 

If you want to save the pods but not allow the seeds to disperse, tie a rubber band or a string around the pod as it starts to harden and dry in the fall. Or you can just cut off the pods to prevent self-seeding.

 

 

 

Flowering

It takes two growing seasons to get blooms from A. syriaca. I planted seeds in November 2020; seedlings appeared in April 2021; those plants flowered in June 2022.

Is Milkweed Enough?

The best habitat for monarchs contains a variety of native milkweeds as well as native nectar plants. This combination provides adult monarchs with a food source throughout the season. The Xerces Society’s “Monarch Nectar Plant Guides” offers a list of suggested nectar-producing plants by state. You might then check those recommendations against more local native plant databases, and against your own site conditions.

OTHER MILKWEED SPECIES

Swamp Milkweed (A. incarnata)

Swamp milkweed grows best in wet conditions with full sun or partial shade in soils with neutral to acidic pH. It is native to wet sites along streams, ponds, or bogs but adapts to drier sites in moist clay or loam soils. It can reach a height of 4-5’ and should be spaced 2-3’ apart. Lance-shaped, smooth leaves are 3-6” long. The flowers are much more ornamental than the flowers of common milkweed. Five-petaled pink to rose-purple flowers mature in mid-spring and last into early fall. This species spreads through rhizomes and wind-blown seeds but it is not an aggressive spreader. Its clump-forming habit is a nice addition to a perennial garden.

I planted swamp milkweed plants in a site that is very wet in spring and then dries out during the summer months. Although the plants bloomed nicely the first year, they didn’t survive the winter. I assume they may have been weakened by the dry conditions. The plants were also heavily attacked by oleander aphids which devoured all the blooms in two days. These alien aphids commonly attack other species of milkweed as well. A strong stream of water will help dislodge the aphids; insecticidal soaps are also recommended for heavy infestations. Be mindful that both these treatments will damage monarch eggs along with the aphids. Ladybugs and other beneficial insects may control smaller infestations. Swamp milkweed seems to be finding a place in general nurseries in addition to native plant nurseries.

Purple Milkweed (A. purpurascens)

This species is similar in appearance to common milkweed, but its flowers are a more vibrant hue of purple to deep pink, and they bloom from late spring to early fall. The flowers are larger than those of swamp milkweed. There were conflicting reports by well-known sources on whether purple milkweed is rhizomatic. This is an important point when trying to identify milkweed that would be easier to contain in a garden setting. According to the U.S. Forest Service, Eastern Region:

“Asclepias purpurascens is a perennial herb emanating from a stout non-rhizomatous rootstock, ranging to one meter in height.”

I asked Ian Caton of Wood Thrush Native Nurseries if he had experience with this plant. Ian could attest that purple milkweed is not rhizomatic; it spreads by seed. As a less-aggressive spreader, purple milkweed is well suited to a perennials garden. In the wild, its populations are described as having a wide range throughout the eastern and midwestern U.S., but the populations are usually small and scattered. A. purpurascens is most commonly found along woodland edges and roadsides in mesic to well-drained soils that are not high in organic material. It prefers a site with morning sun and afternoon shade.

Unfortunately, purple milkweed seeds and plants are not readily available in the trade. Native nurseries are probably the best source for plants, and there are some seed sources online. See this list of native plant nurseries provided by the Virginia Native Plant Society.

Butterfly Weed (A. tuberosa)

This milkweed has been embraced for its bright orange flowers, its manageable size for perennial gardens, and its less aggressive spreading habit. It is naturally found in poor, coarse soils of dry fields, rock outcrops, and other steep slopes. The plant is easily grown in average, dry to medium, well-drained soil in full sun. Crown rot can be a problem in wet, poorly drained soils. It usually grows 1.5-3’ tall with multiple stems, and has a long bloom season, from June to early September. Deadheading may stimulate a second flush of flowers about a month after deadheading. This species is one of the best nectar plants for a wide variety of pollinators. Mature plants may freely self-seed in the landscape if seed pods are not removed prior to splitting open. Seeds need a three-month cold stratification; in November, plant the seed outside and it will easily germinate the following spring. The plant is difficult to transplant because of its long taproot. It does not spread by rhizomes. Unlike many of the other milkweeds, butterfly weed has a water translucent sap rather than a thick milky sap. Although deer resistant, it may be eaten by rabbits. You may want to mark this plant’s location at the end of the season because it can completely disappear in winter and then reappear in late spring. Like other milkweeds, it is susceptible to attack by the invasive alien oleander aphids. This plant is widely available commercially.

 

 

Whorled (A. verticillata)

Whorled milkweed naturally occurs in sandhills, thin woodlands, and barren outcroppings. It is drought resistant, preferring dry, nutrient-poor conditions with low competition from other plants. Although it flowers best in full sun, it can tolerate some shade. It spreads by rhizomes and seed but is not considered to be an aggressive spreader. Its flowers are delicately shaded from white to pale green. A long bloom time, from June through September, and a clump forming habit make this an attractive plant for rock gardens or dry sunny hillsides. Its unbranched, single stems bear 2-3” long, narrow leaves arranged in whorls along the stem. The plant grows 1-3’ tall. In addition to being a host plant for the monarch butterfly, whorled milkweed attracts a variety of nectar-seeking insects, including bees, wasps, flies, butterflies, and skippers. Deer avoid this plant because it is one of the more toxic milkweeds. It is highly toxic to livestock and is considered a weed in range areas. I haven’t found this plant at general nurseries; native plant nurseries would be good options. This species will easily grow from seeds planted outside in the fall.

Tropical Milkweed (A. curassavica)

I am going to spend some time discussing tropical milkweed because it is easy to find at general nurseries and it is sometimes commercially labeled “butterfly weed” or simply “milkweed.” Native to central and south America, it is also commonly called bloodflower and scarlet milkweed. It has become a popular selection because it is a very attractive plant with a long bloom period of brightly colored red and orange flowers, and it doesn’t have the spreading characteristic of some of the native milkweeds. It is very attractive to butterflies, bees, and other insects, as well as hummingbirds. Monarch butterflies use this species as a host plant. However, monarch advocates such as Monarch Joint Venture and Xerces Society are vehement in their warnings NOT to plant nonnative milkweeds.

Monarch advocates are concerned that tropical milkweed may interfere with monarch migration and reproduction. In northern areas, A. curassavica grows later in the season than native milkweed species. More research is needed to determine whether the presence of tropical milkweed may encourage monarchs to breed at a time when they should be migrating.

A protozoan parasite of monarch butterflies, Ophryocystis elektroscirrha (OE) can be carried over on tropical milkweed when it does not die back in winter. Native milkweeds die back after blooming, and the OE parasite dies along with them. Much of the controversy around tropical milkweed centers on whether the effects of tropical milkweed on monarchs extend beyond USDA Hardiness Zones (9-10) where tropical milkweed survives the winter. This is a complicated question and depends on the abundance of non-migratory monarchs relative to the entire monarch population, and the degree of contact between migratory and winter-breeding monarchs.

According to a study from the NIH in 2019:

Conclusion: Our study provides evidence that exposure to tropical milkweed can increase monarch reproductive activity, which could promote continued residency at year-round breeding sites and decrease monarch migratory propensity.

The study also notes that year-round breeding activity is associated with high local densities and greater transmission of the debilitating protozoan parasite OE.

Climate Change Effects

Scientists are also looking at the higher concentrations of cardenolides that occur in tropical milkweed. Cardenolides are the toxins that monarchs ingest from milkweed. Studies jointly conducted in 2018 by researchers at Louisiana State University and the University of Michigan looked at the effects of climate change on monarchs and milkweed. They found that warmer climate increases the potency of cardenolides to levels that may become detrimental to the monarchs themselves.

Faldyn and Elderd found that the local warming associated with climate change can produce chemical changes in milkweed that in turn affect monarch butterflies when they and their larvae feed on certain species of this plant, particularly the nonnative milkweed, Asclepias curassavica.

On the other hand, some monarch advocates are in favor of tropical milkweed. They point out that the most pressing threat to monarchs is extreme habitat loss, and tropical milkweed could  help increase habitat because the species is available and easy to grow. More research is needed to address the warnings some scientists have raised about this species.

SUMMARY

This article concentrated on identifying Albemarle County native milkweeds that would be good candidates for inclusion in home gardens. All these species act as larval hosts to the monarch butterfly and offer nectar-rich flowers attractive to many types of insects. The milkweed species were evaluated in terms of their spreading characteristics, as well as their requirements for moisture, light, and soil type. Some of the more interesting milkweed candidates are not easily found in the trade at this point, but all the species may be seed-grown outside in the fall to achieve the required cold period prior to germination. Nonnative tropical milkweed was also discussed. While more research is needed on tropical milkweed’s effect on monarch reproduction and migration, encouraging the selection of a nonnative species seems to be going against the learning curve on how to promote an ecosystem based on the evolutionary relationship among native plants, insects, and wildlife.

FEATURE PHOTO: Common Milkweed (A. syriaca) in Garden. Photo: Susan Martin

SOURCES

“Asclepias – Or How I Learned to Love Milkweed,” The Garden Shed, June 2016.

Native Plant Finder by Zip Code, National Wildlife Federation (ranked by the number of butterfly and moth species that use them as host plants for their caterpillars)

Piedmont Virginia Native Plant Database, Albemarle.org (by plant categories in VA by county or state)

Native Plant Finder, Virginia Department of Conservation and Recreation (plants native to Virginia according to 3 regions: Mountains, Piedmont, and Coastal)

“Interactions with Milkweed,” Monarch Joint Venture

“Which Milkweeds Do Monarchs Prefer?” USDA

“Harvesting Milkweed Seed: A Pod and a Plan,” Xerces Society

“How to Collect Milkweed Seeds (Without the Fluff),” SaveOurMonarchs Foundation

“If you plant it, They will come,” University of Florida

“Milkweed is taking over my perennial garden. How can I save some for the monarch butterflies but keep it from spreading?” University of New Hampshire Extension

“Annual Life Cycle,” Monarch Joint Venture

“Monarch Nectar Plant Guides,” Xerces Society

“Asclepias incarnata,” NC State Extension

“Featured Creatures: Oleander Aphid,” University of Florida

“Asclepias purpurascens,” U.S. Forest Service

“Conservation Assessment for Purple Milkweed (Asclepias Purpurascens),” USDA Forest Service, Eastern Region, 2003

“Native Plant Nurseries,” Virginia Native Plant Society

Ian Caton, Wood Thrush Native Nursery

“Butterfly Weed, Asclepias tuberosa,” University of Wisconsin Extension

“Butterfly Weed (Asclepias tuberosa L.),” U.S. Forest Service

“Asclepias verticillata,” NC State Extension

“Asclepias verticillata,” Lady Bird Johnson Wildflower Center Plant Database

“Tropical Milkweed – A No-Grow,” Xerces Society

“Is Tropical Milkweed Killing Monarch Butterflies?” Monarch Butterfly Garden

“Exposure to Non-Native Tropical Milkweed Promotes Reproductive Development in Migratory Monarch Butterflies,” NIH, National Library of Medicine, National Center for Biotechnology Information

“Global Warming Can Turn Monarch Butterflies’ Favorite Food into Poison,” Science Daily, 4/8/2018

Q&A Related to Research About Tropical Milkweed and Monarch Parasites,” Monarch Joint Venture

“UGA research on milkweed breeding featured on the cover of HortScience,” University of Georgia CAES Newswire 7/29/2021

“A Closer Look at Monarchs and Milkweed. Latest Information from Xerces,” Kim Eierman, Ecobeneficial, 2014.