Given conditions to their liking, though, few perennials are as reliably hardy or as easy to grow. (After all, they covered most of the continent for eons without assistance.) All they ask is sun, a steady supply of water, and a little compost (or a balanced fertilizer such as 10-10-10) once or twice early in the season. Withhold fertilizer once flower buds form or you'll get rank growth at the expense of flower production. Be sure to plant them before July 1 to allow them time to get established.
To keep taller types more manageable and encourage even more flowers, pinch your asters back as they grow, ideally at about 20 centimetres and again at about 30 centimetres. In practice, pruning clumps to half their height in mid-June works just fine. Cut each stem just above a leaf for best results.
Ideally, asters should be lifted and divided every two or three years. But if you don't mind your clumps migrating a bit here and there, quickly survey the site every spring just as new growth is beginning. Pull out older, exhausted clumps, thin out congested ones and space any seedlings you want to keep. With an appropriate planting site and this modicum of attention, fall asters will produce their multitudes of starry flowers for many years to come.
Powdery mildew is the common cold of the plant world. A disease of opportunity rather than an indication of a sickly plant, it shows up whenever external conditions favour its development. Dry soil, moist air, overcrowding, shade and stagnant air all contribute to mildew. When all five conditions occur together – as commonly happens in early fall--mildew is almost guaranteed. To keep your fall asters mildew-free, take these preventive steps:
• Plant in moisture-retentive soil and do not allow it to dry out.
• Water at ground level to avoid wetting foliage.
• Plant in full sun, making sure clumps won't be shaded later by taller perennials.
• Allow plenty of space between clumps for good air circulation. Avoid planting asters close to buildings or fences that can block air flow.
• Replace older, mildew-prone plants with resistant varieties. Mildew tends to be more prevalent on cultivars of A. novae angliae, A. novi-belgii and the Dumosus hybrids than on other species. Asters have fewer problems with powdery mildews if they are grown in moist, rich soil.
• Clean up garden debris and deadhead spent perennials to deny mildew a starting place.
• If mildew does show up, spray with wettable sulfur, a natural fungicide that can be purchased in either powdered or pre-mixed, liquid suspension form. Like the common cold, powdery mildew is highly contagious, and while it's rarely fatal, repeated or severe cases can seriously weaken plants. Sulfur can also be used as a mildew-preventive; spray every seven to 10 days.