Milkweeds (Asclepias) get their name from the sticky white sap that oozes from the leaves when they are damaged. Butterfly weed is commonly planted in formal garden borders and in meadow and prairie gardens. This wildflower does not transplant well as it has a deep woody taproot. It is easily propagated from seed. The petite, star-shaped flowers of milkweed are perfectly designed for pollination. Equally well designed are the large seed pods that develop from the fertilized flowers. In the fall, these proficient self-sowers split open to release hundreds of seeds.
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. It’s important to note that milkweed plants have a taproot and do not transplant well.
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,” recommends Kelly Ballard of Joyful Butterfly, a supplier of butterfly plants and seeds.
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.