How to Grow and Care for Common Milkweed

Common milkweed (Asclepias syriaca) is a native herbaceous perennial that appeals to butterflies—especially the monarch butterfly. Asclepias is the only plant family that serves as the host plant for monarch butterfly egg laying. The monarch larvae, the hatchling caterpillars, feed exclusively on milkweed leaves. Without milkweed, there can be no monarch butterflies.

Common milkweed grows quickly to two to four feet in height. It has a narrow vertical growth habit and thick, long, oblong green leaves that grow to about eight inches.

Plant seedlings in the early spring after the danger of frost has passed and direct-sow seeds in the ground in the late fall. Once established, milkweed spreads rapidly by self-seeding if seed pods are not removed. In late spring to midsummer, fragrant clusters of pink-purple flowers appear. The leaves of milkweed and the milk-like substance within are toxic, except for monarch butterflies. Milkweed is not only toxic to humans and pets but also to cows and other livestock, which is why farmers dread having milkweed growing in their pastures.

Common Milkweed Care

• Choose a site with well-draining soil in full sun. There is no need to fertilize or enrich the soil.
• Plant common milkweed about 18 inches apart; their rhizomatous roots will quickly fill in the space between plants.
• Common milkweed might not be the best choice for formal perennial borders because of its tendency to get weedy and spread aggressively. It's better suited for naturalized areas like open fields and meadows and butterfly gardens.
• Remove the seed pods before they open to reduce spreading.

Light

Common milkweed prefers full sunlight. It grows best in an open area where there are six to eight hours of sunlight per day.

Soil

This plant prefers dry to medium average, well-drained soil. It tolerates dry conditions, infertile soil, and rocky conditions. It thrives in soil with a pH range of 4.8 to 7.2.

Water

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.

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.

Fertilizer

There's no need to fertilize common milkweed plants. Common milkweed tolerates poor soils.

Types of Related Milkweed Plants

Plants in the milkweed family are the single most important source of food for the threatened monarch butterfly, and planting a patch or two in your landscape is an important contribution to the continued existence of the species. Over 100 native species of milkweed plants are found in the United States. In addition to common milkweed, here are some of the most popular types of milkweed within the Asclepias genus, which grow to different heights and bloom colors:

• Butterfly weed (Asclepias tuberosa): 1 to 2 feet tall; light orange blooms; native to the northeast United States.
• Swamp milkweed (Asclepias incarnata): 4 to 5 feet tall; mauve, pink, or white blooms; native to the northeast and southeast United States.
• Showy milkweed (Asclepias speciosa): 1 to 3 feet tall; rose-purple and pink blooms; native to the western United States.
• Purple milkweed (Asclepias purpurascens): 3 to 4 feet tall; red-purple blooms; eastern North America.
• Whorled milkweed (Asclepias verticillata): 1 to 2 feet tall; white blooms; native to eastern North America.
• Sandhill milkweed (Asclepias humistrata): 1 to 3 feet tall; pink, lavender, or white blooms; native to the southeast United States.

Propagating Common Milkweed Plants

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:

1. 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.
2. Remove the lower leaves of each stem and keep the top two pairs intact. Coat the bottom of the stems with a rooting hormone.
3. Place stems standing up in potting medium that is an 80/20 mix of perlite to peat moss.
4. 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.
5. When roots have formed, transplant the cuttings into the ground within six to ten weeks.

How to Grow Common Milkweed 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 cover 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:

1. 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.
2. Fill peat pots 3/4 of the way with seed-starting potting soil. Moisten the soil until it is just damp.
3. Place one or two seeds in each pot. Cover the seed with a 1/4 inch of soil.
4. 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.
5. 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.
6. Transplant the peat pots directly in the ground in the spring. The pots break down over time without disrupting the roots of the milkweed.
   

Common Pests & Plant 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. 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.