Asclepias syriaca, commonly known as common milkweed, is a flowering perennial herbaceous plant native to North America. It contains milky latex which can be toxic to sheep, cows, and other larger mammals (including humans) if it is consumed in large amounts. Many insect species are attracted to common milkweed, most notably monarch butterflies.
I. Appearance and Characteristics
Asclepias syriaca, commonly called common milkweed, butterfly flower, silkweed, silky swallow-wort, and Virginia silkweed, is a species of flowering plant. It is native to southern Canada and much of the United States east of the Rocky Mountains, excluding the drier parts of the prairies. It is in the genus Asclepias, the milkweeds. It grows in sandy soils as well as other kinds of soils in sunny areas.
A. syriaca is a clonal perennial for growing up to 1.8 m (5 ft 11 in) tall. Individual plants grow from rhizomes. All parts of common milkweed plants produce a white latex when cut. The simple leaves are opposite, sometimes whorled; broadly ovate-lanceolate. They grow to 10–28 cm (4–11 in) long and 4–12 cm (1+1⁄2–4+3⁄4 in) broad, usually with entire, undulate margins and reddish main veins. They have very short petioles and velvety undersides.
The highly fragrant, nectariferous flowers vary from white (rarely) through pinkish and purplish and occur in umbellate cymes. Individual flowers are about 1 cm (0.4 in) in diameter, each with five horn-like hoods and five pollinia. The seeds, each with long, white flossy hairs, occur in large follicles. Fruit production from self-fertilization is rare. In three study plots, outcrossed flowers had an average of about 11% fruit set.
A. syriaca can become aggressive. It spreads aggressively from rhizomes and may not be suited to small gardens and formalized plantings. The plant is winter hardy in USDA zones 3–9; it has a preference for moist but well drained soils, but is tolerant of dry conditions and clay soils. It is ideal in semi-dry places where it can spread without presenting problems for other ornamental species.
II. How to Grow and Care
Sunlight
Common milkweed prefers full sunlight. It grows best in an open area where there are six to eight hours of sunlight per day.
Temperature and Humidity
Common milkweed tolerates a wide range of temperatures and humidity. But because it is native to eastern parts of the United States, it will not do well in extreme and extended heat or humidity. The plant goes dormant in the winter.
Watering
Common milkweed does not need watering except in the driest conditions. Water deeply, giving the plants between one to two inches of water, then wait until the top inch of soil is dry before watering again. Overwatering common milkweed can result in a lethal fungus.
Soil
Common milkweed adapts to various soil environments. The most suitable soil is slightly acidic and well-drained. If the planting soil is sticky, add some leaf mold, organic fertilizer, river sand, or a similar compound and stir evenly to improve soil texture. You can also add a little soil at the base of the plant to encourage growth. This will prevent plant lodging and provide more nutrients to the roots.
Fertilizing
Common milkweed does not require much fertilizer, so doesn’t need to be fertilized often. After flower buds appear, a little phosphate fertilizer ensures growth during the flowering period. Apply organic fertilizer once before winter. This will increase the plant’s ability to withstand the harsh environment and survive the season. During dormancy, the physiological activities of common milkweed will slow down or even stop. Overfertilization causes excessive accumulation of nutrients, which leads to root rot. Hence, stop fertilization after the plant enters winter dormancy.
Pruning
Regularly remove infected plants and wilting leaves to keep common milkweed healthy. If you are located in a tropical or subtropical climate, you can cut off branches in early spring to promote germination. Given the high toxicity of common milkweed, wear gloves when handling plants to prevent poisoning. Keep common milkweed away from children, avoid direct contact, and do not eat the plant.
Propagation
Propagating common milkweed by taking cuttings can be easier than dividing rhizomes because milkweed tends to grow deep taproots, which can be tricky to dig up. With cuttings, you can create new plants in a short period of time which is ideal if you want to get a quick start to creating a butterfly garden. Here’s how to propagate milkweed plants via cuttings:
- Take cuttings of common milkweed in the middle of the summer when stems are green. With a sharp, sterilized garden cutting tool, cut stems that are four inches long. Choose green stems with three to five leaf nodes.
- Remove the lower leaves of each stem and keep the top two pairs intact. Coat the bottom of the stems with a rooting hormone.
- Place stems standing up in a potting medium that is an 80/20 mix of perlite to peat moss.
- Place pots in a shady cool spot out of any direct sunlight while the stem is forming roots. Keep the soil evenly and continuously moist.
- When roots have formed, transplant the cuttings into the ground within six to ten weeks.
How to Grow from Seed
Common milkweed seeds scatter on their own when the seed pods burst open in the fall. The flowers produce warty seed pods two to four inches long that split when ripe to cast many fine seeds to the wind. If you let the plant go to seed, they will sprout in distant corners of your yard (and beyond), thanks to the silky appendages that allow the seeds to waft on the slightest breeze. They are rather like the seeds of dandelions in this regard. This occurs right before the onset of cold weather so the seeds can naturally go through cold stratification. You can mimic nature by simply scattering milkweed seeds in the late fall, on a weed-free site, and covering them with no more than ¼ inch of soil.
The alternative is to start common milkweed seeds indoors. Take into consideration that the cold stratification process to increase the germination rate takes 30 days so start the seeds sometime in March. Here’s how it’s done:
- Wrap the seeds in a damp paper towel and place them in a zippered plastic bag in the refrigerator. Leave it undisturbed for about 30 days.
- Fill peat pots 3/4 of the way with seed-starting potting soil. Moisten the soil until it is just damp.
- Place one or two seeds in each pot. Cover the seed with a 1/4 inch of soil.
- Water the seed from the bottom up. Put the peat pots on a flat pan and add 1/2 inch of water to the tray. The pots will absorb the water.
- Place the pots on a sunny windowsill, under grow lights, or in a greenhouse. Keep the seeds evenly moist. You can expect sprouts within two weeks.
- Transplant the peat pots directly in the ground in the spring. The pots break down over time without disrupting the roots of the milkweed.
Pests and Diseases
The usual suspects are attracted to common milkweed, including milkweed bugs and milkweed beetles (both don’t do too much harm, in fact), aphids, whiteflies, scale insects, spider mites, thrips, and leaf miners. The oleander aphids (Aphid nerii), also called milkweed aphids, look like tiny orange eggs. They suck the sap out of the stems and leaves, cause flowers and pods to die off, and they can even kill the plants. If you see flies swarming around the milkweed plants, they are most likely attracted by the honeydew that the aphids produce when feeding on the plants.
However, it is important not to use pesticides on milkweed or any other plants in a butterfly or pollinator garden, as the same chemicals that kill destructive insects will also kill butterflies and their larva.4 Instead, use a hose, spray from a bottle, or fingernails to scrape off the offenders.
Snails and slugs also love young tender milkweed plants. Snail bait works well and won’t harm monarchs, but as the plants grow, the snail problem minimizes.
Watch for fungus problems such as leaf spot, verticillium wilt, and root rot. Trim infected leaves and branches affected by leaf spot, but the other two fungus issues may be difficult to overcome.
III. Uses and Benefits
- Ornamental uses
The beautiful common milkweed is an ornamental wildflower ideal for butterfly gardens, cottage gardens, prairie landscapes, and more. It provides a beautiful canopy of fragrant flowers that support the butterfly population. This low-maintenance plant is resistant to deer and pests, making it a great addition to any garden.
- Other uses
The plant’s latex contains large quantities of cardiac glycosides, making the leaves and stems of old tall plants toxic to humans and large animals. The young shoots, young leaves, flower buds and immature fruits are all edible raw.
Euell Gibbons, the author of Stalking the Wild Asparagus (1962), wrote that milkweed is bitter and toxic. However, he may have inadvertently prepared common dogbane (Apocynum cannabinum), a poisonous somewhat similar-looking plant instead. Gibbons devised a method to remove the bitterness and toxicity by plunging the young shoots into boiling water and cooking for one minute, repeating the procedure at least three times to make the plant safe to eat. Some modern foragers consider the bitterness and toxicity issue a myth. The plants have no bitterness when tasted raw, and can be cooked like asparagus, with no special processing.
Failed attempts have been made to exploit rubber (from the latex) and fiber (from seeds’ “floss”) production from the plant industrially. The fluffy seed hairs have been used as the traditional background for mounted butterflies and other insects. The compressed floss has a silk-like sheen. The plant has also been explored for commercial use of its bast (inner bark) fiber, which is both strong and soft.
U. S. Department of Agriculture studies in the 1890s and 1940s found that common milkweed has more potential for commercial processing than any other indigenous bast fiber plant, with estimated yields as high as hemp and quality as good as flax. Both the bast fiber and the floss were used historically by Native Americans for cordage and textiles. Milkweed has also been cultivated commercially to be used as insulation in winter coats.
Traditionally, in both North America and Europe, the plant was used to treat respiratory infections such as pleurisy.
Find Where to Buy the Best Common Milkweed (Asclepias syriaca)
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