For the quickest flowers, start seed in late August.
Fresh seed requires a period of cool, damp stratification.
Provide this by placing the seed in a bag of moist peat moss and keeping it in the refrigerator for ten to twelve weeks. Older seed will not require this cold period to break its dormancy. Direct seed butterfly weed outside, or grow the seedlings in a nursery bed until the plants have five or six leaves, at which time it should be moved to its permanent location. Fall-grown plants will provide moderate bloom the following year, with improved bloom in successive seasons. If you prefer, you can start butterfly weed in the spring.