Planting and soil

Perennial asters do best planted in spring in fresh, gritty compost — 1 plant per 5 L pot or 3 per 20 L tub. Annual asters are sown in March under cover, or direct outside in May.

All asters want sun, free-draining soil, and a position with good airflow — crowding invites mildew. Mulch with compost (NOT against the stems) in early spring.

Avoid heavy clay; in pots, mix 1/3 horticultural grit into the compost. Wet feet over winter is the second most common way to kill an aster.