Asclepias is a genus of herbaceous, perennial, flowering plants known as milkweeds, named for their latex, a milky substan ce containing cardiac glycosides termed cardenolides, exuded where cells are damaged. The genus contains over 200 species distributed broadly across Africa, North America, and South America. It previously belonged to the family Asclepiadaceae, which is now classified as the subfamily Asclepiadoideae of the dogbane family, Apocynaceae. Many milkweed butterflies, including monarch butterflies, rely exclusively on milkweed plants as a food source for their larvae. These plants contain acrid milky juices that probably make the larvae and their subsequent stages distasteful to predators.
Most milkweeds have milky juice, flowers with five united petals, podlike fruits, and, usually, tufted seeds. Male and female parts of each flower are united in a single structure, and the flowers are typically borne in clusters. The pollen is characteristically massed in bundles called pollinia, pairs of which are linked by a yokelike bar of tissue contributed by the stigma of the pistil. Parts of the pollinia stick to visiting insect pollinators, which then carry them to other flowers to facilitate cross-pollination. This method of pollination is complex, but, when successful, the great numbers of pollen grains transferred result in the production of many seeds. The silky-haired seeds are drawn out of their pods by the wind and are carried off.
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Types of Milkweed
A. asperula is also known as “antelopehorn milkweed,” “spider antelopehorns,” “green-flowered milkweed,” and “spider milkweed.” This plant can have a sprawling or upright growth habit, with stems reaching one to two feet in length. This perennial is clump-forming with stems that are densely covered with minute hairs. As the green seed pods grow, they curve to resemble antelope horns. It has pale, greenish-yellow flowers, tinged maroon that bloom March to October. This species is native to the southwestern and south-central US, which includes Texas, Oklahoma, Kansas, Nebraska, New Mexico, Colorado, Arizona, Utah, Idaho, California, and Nevada.
A. cordifolia is commonly known as“heartleaf milkweed” – like its common name, its species name “cordifolia” also refers to its heart-shaped leaves. This species is native to the West Coast of the US, including California, Nevada, and Oregon. A. cordifolia is sometimes also called “purple milkweed” but is not to be confused with A. purpurascens, which is also referred to by that same common name. Blooming in spring and summer, A. cordifolia produces clusters of reddish-purple flowers that are held in a loose panicle. Upright plants reach one to two feet tall, and their heart-shaped leaves are blue-green in color, and tinged with purple.
A. eriocarpa is known for its wooly seed pods, which give it its common name, “woolypod milkweed,” as well as its species name, “eriocarpa.” This species is native to California, Nevada, and Baja Mexico, this species is also known as “kotolo” or “Indian milkweed.” Increasing the tactile interest of this plant, the wavy foliage of A. eriocarpa is often coated in white hairs, giving it a silvery appearance. Leaves are oval or lance-shaped, and arranged in pairs or whorls. Blooming from May through October, this plant holds pink and white or cream flowers aloft on upright stalks that reach one and a half to three feet tall.
Another western native, A. erosa also goes by the common name “desert milkweed.” Its native habitat is the desert southwest, specifically southern California, Arizona, Nevada, Utah, and northern Baja California. Desert milkweed has white to yellow flowers and a green to yellow stem and blooms April to October. Identification is somewhat difficult because its leaves vary from mostly smooth to covered with fine cream-colored hair. This plant has an upright growth habit and grows to be one to three feet in height.
Also known as “tall milkweed,” A. exaltata is native to eastern North America. In the US, it ranges from Maine south to Georgia and Alabama, and west to Tennessee, Missouri, Iowa, and Minnesota. In Canada, it is native to Ontario and Quebec.It has an upright growth habit and can grow to be up to five feet tall. Its green leaves are elliptical and pointed, with a smooth top side and a hairy underside. With its leaves and its tall stature, it bears some resemblance to pokeweed, giving this species another common name, “poke milkweed.” Rather than holding its flowers in dense, upright spheres like some Asclepias species, A. exaltata has open clusters of white and lavender flowers that hang delicately from the inflorescence, blooming from May through August.
Also known as narrowleaf milkweed or Mexican whorled milkweed, is native to parts of the western US, California, Idaho, Nevada, Oregon, and Washington in particular. The plant reaches 20 to 40 inches tall with an upright growth habit, and a wispy appearance due to its narrow leaves. The long, narrow leaves are pointed, and green to grayish-green in color. The flower heads are soft pink, with individual flowers that are pink and white, blooming from June to September. The seed pods are smooth, narrow, and tapered. These can be removed before they split to prevent the growth of unwanted volunteers.
Sandhill milkweed, or A. humistrata, is native to the southeastern United States, in Florida, Georgia, South Carolina, North Carolina, Alabama, Mississippi, and Louisiana in particular. Also called “pinewoods milkweed,” this plant can have either an upright or a sprawling growth habit. Its broad leaves are green with a purple tinge and lavender veins. Stems grow to a length of one to three feet. The flower clusters are a soft shade of creamy pink or purple, blooming from March through June.
Also known as pink milkweed, swamp milkweed, rose milkweed and swamp butterflyweed, this perennial has large blossoms composed of small, rose-purple flowers. A. incarnata can reach up to five feet tall. The deep pink flowers are clustered at the top of a tall, branching stem and bloom June to October. Swamp milkweed has smooth, narrow, tapered seed pods. This species is very widespread, native to most of eastern Canada as well as most of the US, with the exception of Oregon, Washington, California, and Mississippi.
A. perennis, commonly called “aquatic milkweed.” This short-lived perennial is native to coastal areas of the southeastern US and into the Ohio valley, including Florida, Georgia, South Carolina, Alabama, Arkansas, Louisiana, Mississippi, Texas, Tennessee, Kentucky, Illinois, Indiana, and Missouri. This species is also sometimes called “white swamp milkweed,” but it is not to be confused with the white variety of A. incarnata discussed above. The stems of aquatic milkweed are purplish green with medium green leaves, and the plant grows from one to three feet tall. Blooming from May through September, buds have a pink tint, opening to white. Flowers are held aloft in delicate umbels. Unlike other species native to the US, aquatic milkweed seed pods lack the hairy “silk” that helps with seed dispersal, since its seeds are transported by water rather than wind.
Commonly called “purple milkweed,” in addition to Wisconsin, A. purpurascens is native to much of central and eastern North America. These plants reach two to three feet tall. Their leaves are broadly oval shaped and pointed, and green with central leaf veins that are purple in color. Blooming from May through July, this species has intensely colored rose pink to reddish-purple flowers held in a dense, rounded umbel.
A. speciosa also known as showy milkweed, is a widespread species that is native to western and central North America. Plants are upright with stout stems, and typically grow two to four feet tall, occasionally reaching six feet in height. Foliage is green to silvery green, and the leaves are oval shaped. Blooming from June through July, showy milkweed has large pink and white, ball-shaped flower heads. Seed pods are large and often have a spiny or warty texture. A. speciosa will spread via rhizomes or seeds, but it doesn’t spread as aggressively as its relative, common milkweed.
Common milkweed, or A. syriaca, is native to all parts of the eastern and central US with the exception of Florida. Upright plants can reach up to six feet tall and have stout stems, with leaves that are light to dark green on top with lighter green undersides. Leaf shapes are variable for this species and can be lance-shaped, oval, oblong, or elliptical. Blooming from June to August, fragrant clusters of pink or purple flowers are ball-shaped, and droop slightly on their stems. The seed pods of this species are large and can be warty or spiny. Don’t plant this in your flowerbed or it will take over. It has a wide-spreading root system and needs an area all its own, where it can really stretch out. The flower is very fragrant and attract many pollinators in addition to Monarch butterflies.
Commonly called “butterflyweed” (or “butterfly weed” written as two words), A. tuberosa is native to most of the eastern two-thirds of the United States, with a range that stretches from the East Coast westward to Minnesota, the Dakotas, Utah, and Arizona. This Asclepias is also known by many other common names, including “butterfly milkweed,” “orange milkweed,” “pleurisy root,” and “chigger flower.” Blooming from May through September, this species’ showy, flat-topped clusters of flowers are a brilliant reddish orange. Naturally occurring yellow varieties "Hello Yellow" also exist. Butterflyweed grows to be one to three feet tall, and has dark green leaves that are narrow and tapered. These plants can produce multiple stems from the same root crown.
Commonly called “whorled milkweed,” “horsetail milkweed,” or “Eastern whorled milkweed,” A. verticillata is native to most of the Eastern two-thirds of the US, from Vermont south to Florida, and west to Arizona, Kansas, Wyoming, and Montana. In Canada, whorled milkweed is native to the provinces of Manitoba, Ontario, and Saskatchewan. This plant is very wispy, with medium-green, needle-like leaves that are whorled around the stems, giving this species its common name. Plants grow to be one to two and a half feet tall. From April through September, white blooms appear in small, flat-topped clusters. This woodland species grows well in soil that has a low to medium moisture content, and it can thrive in full sun or part shade.
Like antelope horns mentioned above, A. viridis is sometimes called “green-flowered milkweed.” And like that plant, its blooms have a greenish tint, giving it its most common name, “green milkweed.” Also like antelope horns, A. viridis is sometimes called “spider milkweed,” since crab spiders (species in the Thomisidae family) like to hunt on its flowers. It is native to the central and eastern central US, including Alabama, Arkansas, Florida, Georgia, Illinois, Indiana, Kansas, Kentucky, Louisiana, Mississippi, Missouri, Nebraska, Ohio, Oklahoma, South Carolina, Tennessee, Texas, and West Virginia. Plants reach one and a half to two and a half feet tall and have a spreading, open growth habit, bearing medium-green, oval- to lance-shaped leaves. The margins of these leaves are sometimes wavy. Blooming from May to July, flowers are pale green with rose or purple centers. The flowers are held in irregular clusters, and there is usually just one cluster per plant.
A. californica is also know as California Milkweed. This perennial is a white-woolly plant with milky sap and deep purple flowers. The plant can reach maximum height of 3 feet. A distinguishing feature is the white fuzz that covers its stems. It blooms May to July. This species is native to Central and southern California.
A. variegata commonly called the Redring milkweed or White milkweed. This perennial has small white flowers with purplish centers crowded into round, terminal clusters that resemble snowballs and blooms May to September. Not to be confused with swamp milkweed (Asclepias incarnata), white swamp milkweed flowers are white and restricted to floodplain forest and along streams. It is native to eastern North America, where it is found in Canada and the United States. It is most common in the Southeastern United States, and becomes rare in the northern edge of its range.
A. texana commonly called as Texas milkweed. This grows in the higher elevations of western and southwestern Texas, such as around the Trans-Pecos region and the Edwards Plateau. This is the only white milkweed that grows in the state. It likes moister areas like river banks despite not needing a lot of water, so if your yard is near a stream, this might be a great variety for you. It does need full sun and well-draining soil that’s slightly acidic.
A. angustifolia commonly called as Arizona milkweed. Another white-flowered variety of milkweed, with an occasional light-pink shading. This grows in the mountains and woodlands with more water in Arizona. It grows to about 3 feet and does need protection from harsh summer sunlight.
Tropical milkweed (A. curassavica) is a non-native species of milkweed from the Caribbean that is frequently available at garden centers. The plant grows 3 to 4 feet tall, and has orange and red flowers. Tropical milkweed grows best in full sun and with regular water. In tropical areas, it will bloom all year. Because this evergreen plant can delay the migration of monarchs and carry pests year-round, experts recommend against planting tropical milkweed in areas where it will overwinter.
When to Plant Milkweed
Milkweed can be grown from seed or transplants.
Start seeds indoors about 4 to 8 weeks before your last frost date in the spring.
Alternatively, sow seeds directly into the garden soil in the fall or in early spring.
Choosing and Preparing a Planting Site
Milkweed plants require full sun and a lot of space.
Milkweed does best in well-draining soil, although some species, like swamp milkweed, prefer to grow in soil with higher moisture levels.
Plant in the back of flower beds or create a bed for just milkweed.
How to Plant Milkweed
Scatter seeds on top of the soil and cover with about ¼ inch additional soil.
Seeds will germinate in 7-10 days.
Thin seedlings to 2 inches apart.
Transplant seedlings when 3-6 inches tall.
Plant transplants in blocks rather than long rows. Plant milkweed 18-24 inches apart.
Water after planting and keep soil moist until plant are established.
Add mulch around the plants to keep the soil moist and discourage weeds.
How to Grow Milkweed
Water plants if soil is dry, but avoid overwatering.
Plants generally do not need supplemental fertilization.
Avoid using insecticides/herbicides in areas around milkweed.
Plants may not bloom the first year, but leaves will still provide a food source for butterfly caterpillars.
Like most wildflowers, milkweed is easy to grow and requires very little pampering. Most species are not seriously bothered by heat, drought, deer or other pests. And because milkweed is a native plant that tolerates poor soils, fertilization isn’t necessary.
The best soil type for milkweed often depends on its native habitat. Most varieties are extremely forgiving and will grow well in average garden soil. Swamp milkweed is an exception and requires moist, humus-rich soil.
You can mulch milkweed if you want to control weeds or retain moisture, but not all varieties will benefit. Swamp milkweed will appreciate your water-retention efforts, but milkweeds that prefer dry soil, such as common milkweed and butterfly weed, are usually better off with no mulch.
As with many flowering perennials, removing withered flowers can result in new buds—prolonging the availability of nectar for monarchs and other pollinators. Simply remove the flower cluster just above the first set of leaves.
Handling precautions and toxicity:
Be aware that the toxic alkaloids in the sap of milkweed that help protect the monarchs from predators can cause eye and skin irritation and are poisonous to pets and other animals when ingested. Take the appropriate precautions and wear gloves, long sleeves, and long pants when working with these plants.
Propagating Milkweed from Cuttings
Cut fresh green stems (1/3 inch diameter) from young milkweed plants.
Recut the stems underwater and coat the bottom of the stems with rooting hormone.
Place the stems in moist sand, vermiculite, or potting soil.
The stem cuttings will root in 6-10 weeks and will be ready to be transplanted outdoors.
Pests and Plant Diseases
Aphids: Aphids which are often found on milkweed plants are yellow, soft, oval, and huddle together on new shoots, stems, buds, and leaves. They damage the plant by sucking liquid from the plant, eventually stressing the plant and killing it when infestation is high.
Whiteflies: Whiteflies are annoying little winged insects that destroy plants when the nymphs suck plant juices and exude a sticky substance. Whiteflies are on the list of the top 10 pest in every part of the west. They are not considered a great pest for milkweed plants but when they are it’s a problem. Their sticky substance on the leaves hinders a monarch caterpillar’s digestive system and appetite. If this begins to happen, the larvae need to be moved to clean plants.
Scale Insects: Scale insects are closely related to mealybugs and aphids, however they are different because they have a shell-like covering that camouflages them and protects them from natural enemies. Adult scales suck plant juices through tiny filamentous mouth parts and eventually kill the plant. Scale eggs hatch beneath the shell and seek their feeding sites in the spring and summer.
Spider Mites: Often red, sometimes yellow or green, spider mites are tiny arachnids (eight legs). They damage a leaf by feeding from its liquid. This results with yellow-stippled leaves. You will see fine webbing across the leaves, especially on the underside and around the stems. Spider mites thrive in dry dusty areas. They rank among the top ten garden pests throughout the west.
Thrips: These are near microscopic pests that rasp the leaf tissue and drink the juices the plant secretes. Infestation causes leaves to twist, stick together, and discolor. Plants are more vulnerable in a greenhouse than outdoors. Milkweed plants with thrips are not a good diet for monarch larvae because of nutrients taken out of the leaves.
Leaf Miners: Most leaf miners that use milkweed are very small wasp larvae. They feed between the two layers of a leaf material making a swirling design. Their consumption of the leaf parts eventually kills the leaf. They are extremely difficult to control when large numbers of milkweed plants are cultivated.
Snails and Slugs: Snails and slugs are great fans of milkweed plants, especially young tender plants. They are rated as the overall worst garden pests. They hide by day and feed at night. It can be devastating to check on your milkweed plants in the morning and they are only sticks. These pests are difficult to get rid of because they come from all surrounding areas.
Leaf Spot (fungus): Leaf spots on milkweed plants are usually red, brown, or black. The spots often enlarge and coalesce infecting the entire leaf which drops. Severe infections can defoliate the plant. The fungus spores that cause leaf spot are airborne or waterborne. The disease is far less serious in dry climates with low rainfall.
Root Rot(fungus): Excess water can suffocate plant roots if there are not enough spaces in the soil. Most plants need well drained soil and good aeration. Overwatering causes water-mold fungi if the water stands too long around roots, especially with warm soil.
Verticillium Wilt (fungus): Verticillium fungus invades and plugs the water-conducting tissues in the roots and stems. It is one of the most widespread and destructive plant diseases in California. A common symptom is a wilting of one side of the plant or the entire plant. The leaves begin to turn yellow and then brown, and the plant dies. The fungus can survive in the soil for years.
Benefits of Milkweed
Because the relationship between caterpillars, butterflies, monarch migration and milkweed is such a complex issue, it sometimes feels like it’s out of our control. Nothing could be further from the truth!
Milkweed works to build stronger biodiversity in your neighborhood and beyond, and it has several other benefits, too.
Provides Pest Control, Including Stink Bugs
Milkweed actually has the power to make your life easier in the garden. A Washington State University study investigating the pest-control aspects of the plant turned up some really interesting findings:
It is a cheap and simple way to support pollinator health and to get pests under control.
Native milkweed plants attract beneficial insects like parasitic wasps, carnivorous flies and predatory bugs that suppress common pests like aphids, leafhoppers, thrips and even stink bugs
Another recent study highlighted a Georgia peanut farm that successfully used milkweed plantings to increase Tachinid fly numbers. Why would you want these insects? They act as parasites to pesky stink bugs, offering inexpensive, chemical-free pest control.
Helps Clean Contaminants
The “silk” found in milkweed pods is often utilized to help absorb contaminants during oil spills.
Interestingly, the seed pod fibers absorb more than four times the amount of oil compared to the plastic-based materials currently used during oil spill cleanup projects. Encore3, a Canadian company, created milkweed fiber-based kits that absorb 53 gallons of oil at a rate of .06 gallons per minute.
How does that cleanup rate compare to the polypropylene products on the market? It sponges up spilled oil twice as fast.