Daisy-like asters (Aster spp.) feature bright yellow central disks surrounded by rays of brightly colored petals. Flower color differs among the different types, but white, pink, purple and blue are common. Asters thrive as perennials in U.S. Department of Agriculture hardiness zones 9 and below or as annuals outside their preferred climates, although hardiness can differ between the various cultivars. The flowers grow readily from seed, often flowering by summer’s end. Start aster seeds indoors or outside in a cold frame about six weeks before the last expected frost. Plant them outside in late spring or early summer.
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Asters grow and flower well as potted plants. Move the seedlings to a 12-inch diameter pot instead of transplanting them to the garden. Maintenance needs vary by aster variety, but most require 1 inch of water per week and respond well to light trimming and deadheading. Fill 2-inch-diameter seedling pots with a sterile potting mix formulated for starting seeds. Sterile mixes minimize disease problems on new seedlings.
Set the pots on a drainage tray if you are starting the asters indoors, or place them directly in the cold frame outside. Water the soil until moisture drains from the pot bottoms and the soil is thoroughly moistened.
Sow two seeds 1/8-inch deep in each prepared pot. Mist the soil surface with water to moisten it after planting. Cover seeds kept indoors with a plastic bag to retain moisture so the pots don’t require watering until after germination. Mist the soil in pots kept in a cold frame daily to retain soil moisture.
Check the pots daily for sprouting. Typically the seeds sprout in about 10 days. Remove the plastic once sprouts appear. Move indoor plants to a window that receives sun all day. Monitor the temperature in the cold frame and open the lid for ventilation if the temperature climbs above 75 degrees Fahrenheit.
Thin the asters in each pot so only the strongest plant remains after the seedlings produce their second set of leaves. Cut the weaker seedling off at soil level with a small pair of scissors.
Transplant the asters to a flower bed in partial shade or full sun once the seedlings are 6 inches tall and frost danger is past. Plant the seedlings at the same depth in the bed they were in the seedling pots and space them approximately 18 inches apart in all directions.
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