Growing Tips for ASTER Plant your aster seeds in a sunny location with well-drained, fertile soil. Prepare the soil by loosening it to a depth of 6-8 inches and mixing in compost or aged manure. Sow the seeds 1/4 inch deep, spacing them 2-3 inches apart. Water your aster seeds regularly, keeping the soil consistently moist until they germinate in 10-20 days. When the plants are a few inches tall, thin them to 12-18 inches apart. Mulch around the plants to retain soil moisture and suppress weeds. Apply a balanced organic fertilizer at planting time, then again every 4-6 weeks during the growing season. Pinch young plants at 6 inches tall to encourage bushier growth, and deadhead spent blooms regularly. Cut the flowers for bouquets in the early morning when the blooms are freshly open. Protect taller varieties with stakes, and cut plants back after the first frost to prepare for winter regrowth.