Review of Xerces Society’s Milkweed Guide

Milkweeds: A Conservation Practitioners Guide — Plant Ecology, Seed Production Methods and Habitat Restoration Opportunities review by Claudia Larsen. To view or download the full publication for free, click here.

In response to outcries from conservationists and researchers, the US Fish and Wildlife Service is currently conducting a status review of one of our favorite butterflies, the Monarch, to be included on the endangered species list. Many of us are aware of the Monarch’s population decline that has been well documented by researchers. Weather, habitat destruction of overwintering grounds in California and Mexico, and loss of food source on migration routes have caused great concern in the last few years.

The Xerces Society’s insight into factors that influence Monarch butterfly populations has pointed to many things we cannot control. However, the increased production and planting of the Monarch food plants, milkweeds, is certainly an environmental movement that can be achieved on a large scale in the United States.

In 2010, in collaboration with the USDA Natural Resources Conservation Service, Monarch Joint Venture, and other community partners, the Xerces Society launched Project Milkweed, a program designed to produce new sources of milkweed seed in areas of the Monarch’s breeding range where seed has not been reliably available (California, the Southwest, Texas and Florida), to raise public awareness about milkweeds’ value to Monarchs and native pollinators, and to promote the inclusion of milkweeds in habitat restoration efforts.

I urge anyone interested in this subject to read the full publication, which can be viewed or downloaded for free by clicking here. If you want to skip to the end, there is a nice synopsis of how to establish milkweeds from seeds that would be helpful to the amateur grower (p106-109), as well as a complete list of milkweeds native to the U.S. and Canada. If you are a researcher, the acknowledgements contain much, if not most, of the scientific literature available on milkweeds and related science.

Milkweed Biology and Ecology 

There are 72 species of milkweed (Asclepias) in Canada and the United States (except Alaska and Hawaii) that are adapted to varied climate conditions. Texas contains 37 species, New England has 10 species, and Arizona, California, Florida, Texas and Utah all have endemic species. Florida hosts 21 native species of Asclepias, two of which are endemic.

Milkweed has ethnobotanical history dating back to native American populations. Cloth and rope were woven from fibers and stems, plant sap was used to cure ringworm and bee stings, and roots were made into a drink to treat venereal disease or coughs. Butterfly milkweed is sometimes called “pleurisy root” and sold as herbal medicine today.

In the 1940s, milkweed seed fibers were used to fill life preservers. Today, they continue to be used as hypoallergenic filling in blankets and pillows. During the 1970s and 80s, milkweed was tested as a biofuel resource, which was later destined to be uneconomical, just like their possibility of being a substitute for crude petroleum and natural rubber.

Some cool facts about milkweeds:

• They are named for the milky sap that is found in their stems, leaves and pods and makes them unpalatable to animals. (The oddball is Asclepias tuberosa, which does not have sap.) The plants contain chemicals called cardenolides in all their plant parts, and Monarch butterflies have adapted to sequester the chemicals to use in their own defense. Milkweed species with higher levels of cardenolides are believed to be less valuable to Monarch survival since high levels are toxic to the caterpillars. Unfortunately, these chemicals are also toxic to animals which feed on them, especially sheep and goats who are browsers. Milkweed chemicals have been evaluated as a breast cancer treatment.
• Milkweed growth habit varies from 5-6 feet tall to low-growing and sprawling; leaves vary widely and may be opposite, alternate or whorled. Except for aquatic varieties, milkweeds have deep roots. Those growing in Midwest prairies have been documented growing 6-12 feet below ground.
• Milkweed flowers may be erect or drooping and are arranged in clusters called umbels. Nectar is stored in the upper part (corolla), which may also have variable hood shapes. Flower color may be white, yellow, green, purple, pink, orange or red.
• Seeds are contained in a pod (follicle) and are attached to white fluffy pappus, which disperses them in the wind. Reproduction also occurs on some species by vegetatively produced shoots on the roots, so plants appear to be in a colony.

Pollination facts:

• Milkweeds produce high quality nectar and depend on a variety of insects (bees, wasps, butterflies, moths, flies, beetles and true bugs) to transfer pollen for seed production.
• Although Monarchs visit milkweeds for nectar, the swallowtail butterflies and Nymphalis species are more frequent visitors.
• Their unusual pollen is similar to that of orchids, which is carried in special sacs called pollinia that are located in vertical grooves in the flowers.
• Milkweeds are self incompatible and rarely form hybrids between species.

The Value of Milkweeds to Wildlife 

Besides Monarchs, research has shown that milkweeds are of value to native bees, honey bees and beneficial insects like true bugs, wasps and flies that are predators of crop pests, making them an important factor in natural biological control. Milkweeds provide nectar for various bumblebees, carpenter bees, digger bees, sweat bees, plasterer bees and leafcutter bees. Seed bug, longhorn beetles and leaf beetles use milkweed for food and shelter. Other butterflies and moths using milkweeds as important nectar sources include the queen butterfly, dogbane tiger moth, unexpected cycnia, milkweed tussock moth, and Pygoctenucha terminalis (which has no common name). Hummingbirds (red-throated and black-chinned) may nectar on some species, while birds (flycatchers, chickadees and orioles) use seed fluff as nest liner.

A special note on non-native honey bees: Milkweeds are valued for high quality honey production, receiving a class 1 or 2, which means one-acre of milkweed can produce up to 45 pounds of honey. Some of the species that honey bees frequent are clasping milkweed, common milkweed, purple milkweed, prairie milkweed, swamp milkweed and whorled milkweed.

Milkweed Propagation and Seed Production 

Propagating and planting more milkweeds is an obvious solution to helping Monarchs, but regionally grown seed is mostly scarce. So the focus on increasing production of milkweeds is vital. This manual details information and processes for collecting wild seed with landowners’ written permission or permits from public land. Xerces suggests harvesting only 20% of the population in order to maintain the wild population. After the first season, seed can be collected from foundation plantings. Seed is mature when pods split open and seed is brown, but there are multiple challenges with seed maturing at different times and efficient separation of seed and white pappus fluff. Seeds can be hand harvested on multiple occasions with one person for a field up to one acre using a mesh seed capture bag.

Seeds should be dried several days before processing and all seed-feeding bugs removed. Screening seed through fabric hardware cloth works but results in fiber floating everywhere. A 5.5 hp shop vac with 16-gallon capacity is successful but may have variable results. Igniting seed with fire in small batches is discouraged. Apparently, there is need to develop further methods for cleaning large volumes of seed pods, and pages 83-93 can show you how in “Build Your Own Small-scale Cleaning Equipment.” There is no data on how long milkweed seed can be stored.

Seed germination poses a whole set of other problems for growers because seeds wait to be after-ripened and meet environmental temperature requirements. Most published information is based on research for Common milkweed (A. syriaca) and Butterfly milkweed (A. tuberosa). Stratification by exposing seed to cold, moist conditions is recommended. This can be achieved by sewing seed outside in fall or providing artificial stratification at 40-41F (refrigerator) and sewing in spring with temperatures between 65-80F.

From the manual: “Combine the seeds with an equal or greater volume (up to three times) of dampened sand, perlite, vermiculite, or other sterile media. The media should be moist but not wet; it should not drip when squeezed. Place the seed and media in a sealed plastic bag and store the bag in a refrigerator for 4–6 weeks. The standard temperature range of a household refrigerator will be perfectly suitable for this seed treatment. The seeds should be chilled but not frozen, because the formation of ice crystals can damage seed. Store the seeds in the fridge until you are ready to plant. Sow the contents of the bag immediately upon removing it from cold storage. Whether seeds are sown indoors or outdoors, the soil in the planted area must be kept moist for the entire germination period.”

Note: There is still variability of success reported between seedlots, regions and researchers using these techniques. The need for light or length of dry storage to aid seed germination is apparently still debatable.

To establish fields of Asclepias for seed production, this manual recommends direct sowing in rows using a specialty seed drill that is modified for milkweed seed; transplanting small 2- to 5-month old seedlings by hand; and hand-sowing seed by incorporating into the upper inch of soil with a rake, followed by tamping the soil with a roller or by walking on it.

All methods require adequate site preparation to reduce weed cover, and irrigation as needed the first season by applying 1 inch of water per week from either irrigation or natural rain. Ideal seed planting depth is ½ to ¾ inch and the seed rate is 2-20 seeds per linear foot.

When growing milkweeds in containers for later transplanting, use those that are 2½ to 4½ inches deep. Careful handling and methods to reduce transplant shock when moving to field conditions are needed. Mulch or a fabric or plastic weed barrier can be used for field plantings to reduce weeds. Growers may employ a water wheel or vegetable transplanter for planting, then tamp air pockets and water afterward. Some growers use alternate rows of grasses or compatible forbs or legumes to reduce weeds, since herbicides are never recommended for milkweed fields.

Plants may produce seed the first year, but years 2-5 will produce the most seed. These methods are based on research with common (A. syriaca) and butterfly milkweeds (A. tuberosa). (Extensive details and production pictures are in the manual.) Management needs for the crop include pest and disease control and monitoring soil fertility. It is not known if supplemental fertilization is beneficial for milkweeds, except to correct a deficiency in N-P-K. Organic amendments may enhance growing conditions.

Parasites, herbivores and disease pathogens may affect seed production crops as well. The major insect problem is the yellow-orange oleander aphid, but other insects have evolved with milkweeds and do not warrant chemical control. The manual has an extended description and pictures of aphid species that feed on milkweeds as well as other pests mentioned below. Sooty mold and phytoplasma diseases may result from aphid infestations, and again the authors offer the most comprehensive list of pathogens causing disease on milkweed. Besides fungal pathogens, milkweeds are susceptible to bacterial infections, viruses and other biological organisms. Natural predators of aphids to encourage are lady beetle larvae and adults, hoverfly larvae, and brown and green lacewing larvae.

Large and small milkweed bugs are a problem because they suck on milkweed pods, making the seeds inviable and various beetles eat milkweed leaves. The “Chemical Control – Pesticide Selection” section in this chapter details why we should not use systemic or contact herbicides and explains that even horticultural oils and soaps are toxic. Organic-approved pesticides can be as harmful as conventional products.

So what happens to your crop when Monarchs lay eggs on your crop and hatch voracious caterpillars? Answer: Accept reduced seed yield. Collect caterpillars and transfer to a “designated” crop area where they are allowed to eat. Advertise for “foster homes” in a nature park or school garden. Last resort: Use insecticide and realize that killing a particular group of caterpillars is less of a loss than losing a whole crop of seeds that will be utilized by many, many more Monarchs and their offspring.

Using Milkweeds in Habitat Restoration Plantings

This chapter promotes the planting of milkweeds in residential and institutional landscaping, farmland conservation easements, roadside rights-of-way and designated pollinator gardens. It stresses the need to time maintenance activities that promote Monarch migration calendars; the need to emphasize the increased value milkweeds and bees will provide to crop pollination; and of course, the need to preserve natural stands of milkweeds on public and private lands. This chapter has examples of various case studies in restoration plantings and describes incentive programs set up by the USDA for farmers and conservationists.

Operations this large always uproot deeper questions and concerns. There are now 20 species of milkweed seed and plants commercially available in the US. The Xerxes Society is examining geographic guidelines for moving plants out of adapted regions that might result in potential changes in plant gene pools. Also of concern is the effect of planting milkweeds where they have not been previously found. How would current Monarch migrations adapt or change to new habitat areas, latitude or elevation change? Can we certify that milkweed growers are pesticide-free for systemics that remain in the plant and threaten safety of all insects that use milkweed for nectar and hosts?

The Xerces guide also discourages the use of non-native tropical milkweed and cultivars in plantings. They do not provide the same resources as native milkweeds and promote sedentary (non-migrating) Monarch populations that become susceptible to disease.

To view or download the full publication for free, click here.

Click to learn more about Florida’s Monarch butterflies and the milkweed they depend on.