Milkweeds (Asclepias) get their name from the sticky white sap that oozes from the leaves when they are damaged. More than 100 species of this herbaceous perennial are native to the U.S. and Canada. Many have adapted to different regions of the country and a wide range of climates and terrains, from deserts and rocky areas to marshes and open prairies. Some species grow exclusively in specific regions while others will thrive in just about any habitat.
Three species of milkweed are good all-around choices for gardens in most regions of the country: common milkweed (A. syriaca), swamp milkweed (A. incarnata), and butterfly weed (A. tuberosa). The last two are highly ornamental and available in a variety of cultivars. To help sustain monarchs and other butterflies, you should plant at least a few milkweed species that are native to your area. Download this milkweed information sheet from Monarch Joint Venture for regional recommendations.
The petite, star-shaped flowers of milkweed are exquisitely designed for pollination. They grow in clusters of five nectar cups, each with incurved horns above the petals. When an insect lands on a flower, its feet slip between the cups and the pollen sacs attach themselves to the legs. When the insect moves to the next flower, the horns collect the pollen. Equally well designed are the large, fluff-filled seed pods that develop from the fertilized flowers. In the fall, these proficient self-sowers split open to release hundreds of seeds borne on silken parachutes.
2 to 5 feet, depending on the species
Why it’s a must for monarchs:
Milkweed is both a food source and a host plant on which the monarch lays its eggs, depositing them on the underside of the leaves. The larvae then feed on the leaves after hatching, but cause no permanent damage to the plant. In turn, the toxic chemicals contained in the sap of milkweed plants make both the caterpillars and adult butterflies unappetizing to predators. “[During monarch migration] flight is fueled by nectaring on the flowers and is punctuated by laying eggs on milkweeds. To grow and sustain each generation, milkweed is the only food needed,” says Agrawal.
Milkweed’s highly fragrant and nectar-rich flowers are an enticement for other pollinators as well. Frequent visitors include native bees, honey bees, many other types of butterflies, and hummingbirds. Read more about the best perennials for pollinators.
Where to plant:
Most milkweeds require full sun (at least 6 to 8 hours a day). Because they self-seed readily, locate your plants in a part of the garden where you can better control their rampant spread, such as at the back of the border or in a corner. A spot that’s protected from the wind will also help prevent the spread of seeds while providing a more hospitable environment for butterflies.
When to plant:
If you’re planting milkweed from seed, sow the seeds outdoors in the fall, which will give them the period of stratification (exposure to cold, moist conditions) they need to encourage spring germination and ensure a good display of flowers the following summer. If you purchase starter plants, plant them in the spring after the danger of frost has passed.
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.
How to plant:
To ensure successful germination of milkweed seeds, plant them in a smooth, clump-free soil bed worked to a fine consistency using a rake or rototiller. After you’ve sown the seeds, compact them into the soil (but don’t cover them) to provide good soil-to-seed contact. Keep the planting bed moist until the seedlings become established. As your plants begin to take off, thin out any plants that are spaced too closely together so they don’t compete for sun and soil nutrients.
To attract multitudes of monarchs to your garden, plant milkweed in groups of six or more, spacing plants or thinning seedlings to about 6 to 24 inches apart, depending on the species. Monarchs are very good at finding a milkweed plant, but the more you have in your yard, the more likely they will find it and lay their little eggs all over it. Plant as many plants as you have room for.
Many milkweed species can readily be grown from root or rhizome cuttings as well as by seed. Take the cuttings during the late fall or early spring when the plant is dormant and has more energy reserves. New sprouts will form from the cuttings when the weather warms and will often produce flowers the first year.
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.
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, pruning the flowers soon after they have withered will result in new buds and may extend the blooming period for several weeks. Clipping spent flowers to stimulate new growth will also prolong the availability of nectar for monarchs and other pollinators.
Some plant pests such as aphids, whiteflies and milkweed bugs are immune to the toxic effects of milkweed and may feed on the leaves and seed pods, but they rarely cause significant damage. Also remove leaf litter and spent stalks in the fall to eliminate overwintering sites.
How to control spreading:
If you don’t want milkweed to take charge of your garden, remove the seed pods in the fall before they split open and release their contents or tie them closed with string. For plants with rhizomes, thin them out by hand by pulling the entire plant, including the roots, removing as much of the rhizome as possible. This will be easier to do when the plants are young and before the roots are well established.
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.