Exactly when to plant asters depends on how you are propagating them, however all varieties do best when planted or divided in spring. The two primary methods of propagating asters are division of established plants and growing from seed.
Method to Use
If you want to know in advance exactly what you’re going to get, division of established plants is the method to use. Your new plants will become exactly like the parent plant from which they were divided. Division should be done in early spring, as soon as the ground can be worked after the last frost. Because asters grow rapidly and need to be divided every two or three years, chances are you can find a gardener who has some to share. Dig up as much as two-thirds of an established plant clump, then divide it into two or three smaller clumps. Replant each new clump in your desired location.
Propagating asters from seed can be an adventure. Since most varieties of asters commonly found in gardens are hybrids, their seeds do not breed true. The new plants will very likely not resemble the parent plant. You have two options when starting from seed. If you want to control the spacing and location of the plants, start them in pots, indoors, a few weeks before the last frost. After the last frost, you can plant the established seedlings outdoors in your selected spot. This method will also yield earlier blooms. If you’re really adventurous, or if you want a wild, meadow appearance, simply scatter the seeds into your flower bed and rake them into the soil.