How to Grow Milkweed (Asclepias) Plants

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Monarch butterflies will only lay their eggs on milkweed plants.
Monarch butterflies will only lay their eggs on milkweed plants. (Image: herreid/iStock/Getty Images)

Milkweeds (Asceplias spp.) are native perennial plants that are found throughout most of North America. They get their name from the milky sap that oozes from the main stem and leaves. The common milkweed (Asclepias syriaca) is hardy in U.S. Department of Agriculture plant hardiness zones 3 to 9 and grows to be 2- to 6-feet tall. Blooming from early to mid-summer, fragrant flowers are born in umbels, clusters of small flowers on short stalks. Planting milkweeds is recommended to help preserve dwindling Monarch butterfly (Danaus plexippus) populations. There are several convenient techniques for growing and propagating milkweed plants.

Growing Conditions

Milkweeds grow in a variety of conditions including clayey, sandy or rocky high alkaline soils. They tolerate partial sun but prefer full sun. Planting milkweeds in full sun in a loamy, well-drained soil will help them grow vigorously. They usually bloom in their second year and increase their numbers through underground shoots, or rhizomes.

Because milkweeds are tenacious and adaptable, they generally do not require fertilization, though young seedlings can benefit from a granular slow-release fertilizer application in the spring when new growth starts. Nitrogen is an important nutrient for vigorous vegetative growth and will benefit young milkweeds the most. Use a fertilizer that has nitrogen only, such as 10-0-0, applying at the rate of about 0.1 pound per 100 square feet, though follow label directions. Sprinkle the fertilizer over the area and then water it in to work the nutrients into the soil and to prevent fertilizer burn to plant tissues.

Growing From Seed

As the concern for the survival of Monarch butterflies grows, more stores and garden centers are selling milkweed seeds in packets. You can also collect seeds from the pods of the milkweed plants. When the pods are mature, they start to split. Do not collect seeds before this splitting occurs, or they won't be ripe. Seeds should appear brown and not pale or white. Store the seeds in a paper bag and keep them dry. The seeds need a period of cold to germinate. A simple technique is to sprinkle the collected seeds around in the fall after a killing frost and wait for them to germinate in the spring.

Another technique is to germinate the seeds indoors after they have been stratified. Stratification involves giving them a treatment of cool, moist storage to simulate winter conditions for germination. To stratify milkweed seeds, place them in a cold, moist potting soil in a dark place for several weeks or place the seeds between moist paper towels in a plastic bag. After a period of three to six weeks, the seeds can be planted in warm, moist soil. The advantages to startling your milkweed plants indoors are a higher germination rate and easier establishment outdoors with well-rooted transplanted seedlings.

Planting in Beds

Stratified seeds can be sown in flats filled with potting soil. Water the flats thoroughly so that the water drips through the bottom before planting the seeds. Scatter the seeds on the soil surface 1/4 to 1/2 inch apart, then cover the seeds with an additional 1/4 inch of soil. Mist the flats to dampen the additional soil. Covering each flat with clear plastic will help them stay moist. Place each flat in a warm, sunny location. Seeds will germinate in seven to 10 days and will grow to the appropriate height -- about 3 to 6 inches high -- in four to eight weeks.

Prepare a bed for the seedlings by removing weeds and mixing the soil with a rich compost material. Seedlings can be transplanted in spring after danger of frost has passed. Plant seedlings 6 to 24 inches apart depending on species.

Keep the soil moist but not over saturated. In the absence of rain, water seedlings lightly each day but do not allow standing water puddles. Mulching will help retain moisture.

Growing From Cuttings

Milkweeds can be harvested and divided when the plants are dormant in the fall to obtain cuttings of the rhizomes. The buds of common milkweeds are about 2 inches apart. Each section of the replanted rhizome should have at least one growing point or bud. You can also simply pull up the rhizomes of milkweeds and cut off a section and replant at the same depth. Replant the sections in the fall before the ground freezes at 32 degrees Fahrenheit to give them time to develop their root systems through winter. Watering the cuttings the first year will help their survival. Wear gloves when cutting milkweed rhizomes, because the milky sap can be irritating.

Selecting Milkweeds

There are over 110 species of milkweed in North America. Some have more attractive features for garden settings than the common milkweed. Butterfly weed (Asceplias tuberosa, USDA zones 4 to 10) is commonly sold in nurseries. Butterfly weed has attractive leaves and flowers and does not take over flowerbeds like common milkweed. Swamp milkweed (Asclepius incarnata, USDA zones 3 to 9) has attractive long-lasting pink flowers, attracts monarchs and is suitable for the home garden. Since many milkweeds species are tropical, ensure you select one that will grow well within your hardiness zone.

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