To grow milkweed, start seedlings indoors four to eight weeks before the last expected frost of the season, and transplant the seedlings to a location with full sun when the plants are 3 to 6 inches tall. After transplanting, spread mulch around the base of the seedlings, and apply water-soluble fertilizer three to four times throughout the growing season. Water the milkweed frequently until it is well-established.
When starting seeds indoors, place the gardening flats containing the seeds in a sunny location, such as a sunny window, and maintain a temperature of approximately 75 degrees Fahrenheit to encourage germination. Amend the soil as needed, and acclimate the seedlings before planting them outdoors.
To acclimate the seedlings, place the trays containing the plants outdoors for increasing amounts of time over the course of several days. After several days, plant the seedlings 6 to 24 inches apart.
Seeds are also sown directly in the garden by spreading the seeds over loose, well-draining soil after the last frost of the season. Refer to the seed packet for specific information on planting the seeds, as the instructions vary based on the species of milkweed. Thin the seedlings several times through the growing season beginning one to two weeks after germination, and harvest seeds for the following year during autumn.