(Aster dumosus x A. novi-belgii), Zone 4.
Vigorous, disease-resistant dwarf mounds blanketed with flowers in September and October. Some of the finest: 'Audrey': Mauve-blue single, 30 x 45 centimetres. One of the best blues. 'Jenny': Double rose-red, 40 x 60 centimetres. Bright and cheerful. 'Little Pink Beauty': Semi-double bright pink, 40 x 50 centimetres. Reputed to be the best semi-double pink. 'Lady-in-Blue': Semi-double blue, 25 x 45 centimetres. Compact, early-blooming and free-flowering. 'Professor Anton Kippenburg': Semi-double bright purple-blue, 30 x 45 centimetres. Reliable favourite.
(Heath aster), Zone 3
A North American species native from Maine to B.C. and as far south as Georgia to Arizona. Upright, bushy clumps of tiny leaves topped with even tinier white flowers. Pastel blue and pink cultivars are available. All 90 x 60 centimetres. Blooms August to October. Tolerates drier soils; mildew-resistant.
Aster x Frikartii
'Moench': Large, clear lavender-blue single flowers bloom profusely from July to October. Sturdy clumps grow 75 x 60 centimetres; tolerates drier soils without developing mildew. Heights vary significantly (from 35 to 75 centimetres) according to growing conditions, with rich, moist soil producing taller plants. An outstanding perennial.
Aster novae angliae
'September Ruby': Deep, glowing raspberry-red, 120 x 45 centimetres. Needs staking. Lower leaves wither early (use at back of border) and plant is subject to mildew, but colour is spectacular. Blooms August to October; tolerates wet soils.