Michaelmas daisies are tall flowers that spread and bloom in the fall. They grow best in USDA hardiness zones 4 through 8. They acquired their name from Britain, where they bloom around the Old English Christian holiday of Michaelmas, but they are more commonly known as purple asters in the United States, where they are commonly included in butterfly gardens. If you are thinking about growing Michaelmas daisies in your area, make sure that you have well-drained soil and a location with lots of sun, or the flowers will not perform well.
Plant the seeds at approximately 1/2 inch deep and cover them with soil. The seeds take 15 to 30 days to germinate, so be patient. You can plant more seeds than you actually desire. This way, you can thin the plants out to keep only the healthiest seedlings. When you thin the plants, allow approximately 1 foot between them because Michaelmas daisies grow into a bush and need the space.
Keep the soil moist, but do not overwater. Whenever the soil begins to look dry, water just enough to re-moisten it. Too much water prevents root growth and leaches helpful nutrients from the soil.
Cut back the plants in early summer to approximately half their size to create bushier plants with more blooms in the fall. It is also necessary to divide the clumps every year or two to prevent overcrowding since Michaelmas daisies spread rapidly.
Providing good air circulation, by fan if necessary, will help prevent diseases.