Zone: Typically 4 - 8, but variable
Soil: Variable - check label
Light: Full sun to part sun
Bloom colour: Pink, purple, blue, white
Bloom period: August into fall
Height: Typically 1 - 6 feet, but up to 8 ft.
Moisture: Varies, see notes
Attracts: A variety of bees and butterflies.
Notes: Asters are beautiful plants with a bountiful supply of flowers while in bloom. They are simply a must in a wildflower garden because they flower in the latter part of summer and fall when there is a high demand for pollen and nectar. They act as host plants for Northern and Pearl Crescents. They also attract Monarchs, Skippers, Painted Ladies and Sulphur butterflies. In general, asters are preferred by bees over goldenrods. When goldenrods have finished, asters are still around to provide foraging bees with food. Asters reseed easily and start popping up all over the garden, but they are still easy to control.
Asters under modern plant classification systems are placed in several genera and if you go to a good nursery, then you should be able to find specimens that suit your particular soil and moisture requirements.
For moist or clay soils, Symphyotrichum puniceus is a tall and attractive specimen. Compared to other asters its size, large flowers and heavy bloom with its purple stems set it apart as a garden worthy plant. It really does need constant moisture and plenty of sun to be at its best. It will bloom earlier than most other asters. In a loam soil, with regular watering, it will do just fine. In nature, its height is variable, but in more consistent garden conditions you should expect the plant to be between 5 to 6 feet high.
However, New England Aster is also one of the best looking species as well as being one of the most attractive for wildlife. New England aster grows to around 6 ft and holds it own when naturalised with goldenrods. When this plant is in full bloom, it is top heavy with flowers and is going to need some support. This plant has a long bloom period and will continue to provide nectar to bumblebees and sweat bees well into October in Ontario. Cutting the plant down by half at the end of June will stunt the growth of the plant and prevent it from becoming lanky while enabling the plant to stay upright on its own. This is well before the blooming period and will not affect flowering. The best looking cultivar of this plant is the compact Purple Dome. Unfortunately, the semi-double flowers do not serve pollinators well and this cultivar is not recommended. If you do the "Chelsea chop" on the on the straight species you will have have great looking plant with a large number of pollinators.
The flat topped white aster is another species to consider for moist soil. While it is not as attractive as New England aster, flat topped white aster is an excellent plant to attract wildlife. It flowers much earlier than other asters when a greater diversity of pollinators are around to take advantage. The nectaries are easily accessible so many beneficial wasps as well as bees will feed on this plant.
Boltonia asteroides (false aster) can grow in a wide range of conditions but it does prefer moist conditions. The species, growing to about 5 feet, is taller than the cultivars and may need support. The picture here shows a cultivar called snowbank. It is a dense plant covered with a spray of white flowers. It may still need support after heavy rains. This plant is not native to Ontario or Quebec, but it is found in most of Eastern North America
A white Aster to consider for drier soils that is native to Ontario is Arrowleaf Aster. It prefers part sun and can grow in a range of soils. The bloom is more upright than other white Asters which gives it a nice form. It grows to about 3 feet high and will flower in late summer
Smooth Aster, at 4 ft (120 cm), is more compact than New England Aster and it is excellent for attracting bees. While the individual flowers are not as pretty as the New England Aster, the sheer mass of flowers that it produces are impressive. It does well in dry sandy soils and is good for xeriscaping. I use tomato cages to support these plants. Sky blue aster grows in similar conditions to smooth aster and only grows to about 30 inches high (75cm). It does not require support. Sky blue aster has a basal rosette of leaves. The stems have much smaller leaves with a large number of flowers. These characteristics make this plant a worthy addition to the garden.
Heath aster is another species that does well in dry sandy soil and is covered with such an abundance of small white flowers that the stems are difficult to discern. It is also only about 30 inches (75 cm) high and does not need support. While New England and smooth asters are frequented by bumblebees, hover flies and sweat bees, heath aster attracts beneficial wasps. Heath aster frequently hybridises with New England aster to produce white or lilac flowers. It reproduces quite easily and in wild areas, it may be the dominant plant. Pull out all the daughter plants as soon as you notice them. Another white aster that grows in sunny dry soils is the arrow-leaved aster. It is harder to find than heath aster but the stature and the heavy bloom of flowers makes it a more attractive option.
Asters provide some great flowers to brighten up the shade garden in the fall. Heart-leaved aster (Symphyotrichum cordifolius) matures into a plant with a decent spray of flowers that is more attractive to bumblebees than any other shade plant that I have grown. It grows up to 3 feet high but typically has much less stature. The individual flowers are stunning. The plant has light blue ray flowers. The disc flowers in the centre start off at yellow and turn to a deep purple shade as the flower matures. A much rarer shade plant of similar size is Short's Aster which has flowers that are nearly twice the size of Heart-leaved aster. Combine it with zig-zag goldenrod or blue-stemmed goldenrod. It grows to about 30 inches high (75 cm).
You might also consider Large-leaved Aster and White Wood aster, which both tolerate shady woods with the former plant doing well in more more mesic conditions. These asters are smaller in stature and should be planted in front of other shade plants. Large leaved aster flowers earlier than other asters and has a long blooming season. White wood aster is the plant to choose for dry shade. It forms a compact flat spray of flowers. It will not stand out if you just plant a few, but it will look absolutely fantastic if you plant it en masse. The bloom is so dense that it is hard to see the foliage underneath. White Wood Aster combines well with Virginia Blue Bells, which dies back in early summer. This aster will fill in the spot and give a second round of blooming
Here are some recommendations (many of those mentioned are now classified in the genus Symphyotrichum):
Full sun and medium moisture:
Aster novi-belgii, Aster novae-anglicae, Doellingeria umbellata or the much taller Symphyotrichum puniceus.
For sun and dry soil:
Symphyotrichum laevis, Symphyotrichum dumosus, Symphyotrichum ericoides, Symphyotrichum urophyllum and Symphyotrichum oolentangiensis.
For rock gardens, try Symphyotrichum alpinus. It is rare wild plant in Ontario.
Symphyotrichum shortii, Symphyotrichum cordifolius, Eurybia divaricatus and Eurybia macrophyllus.