Asters are the flower that saves the autumn garden: just when everything else is fading, they burst into clouds of daisy-like blooms in purple, pink, blue and white, flowering for weeks and feeding the last bees and butterflies of the year. The New England and New York asters are the tall, classic types, while the smaller Michaelmas daisies suit the front of a border.
Asters are hardy perennials, so they come back year after year and slowly form bigger clumps, and they are easy plants as long as they get sun and soil that does not stay wet in winter. They do need one job every year: dividing congested clumps and pinching young plants in early summer, which keeps them bushy and stops them flopping under the weight of their own flowers.
Asters at a glance
| Growing factor | What asters need |
|---|---|
| Sun | full sun |
| Soil pH | 6.0–7.0 |
| Days to germination | 7–14 |
| Days to maturity | 70–90 |
| Spacing | 12–18 inches apart |
| Water needs | moderate |
| Cold tolerance | hardy |
| Family | Asteraceae |
Starting seeds
Asters give you a choice: start seeds indoors for earlier flowers, or sow direct once the soil warms. For the earliest display, sow indoors a few weeks before your last frost — late winter in Zones 3–5, early spring in Zones 6–8, late winter in Zones 9–10 — and transplant out after the frosts end. For direct sowing, wait until the soil is workable and warming: late spring in Zones 3–5, mid-spring in Zones 6–8, early spring in Zones 9–10. Zones 11–13 are frost-free, so both methods work through much of the year; check your local extension service for the best local dates. Seeds germinate in 7–14 days. Keep seed mix or soil evenly moist until seedlings are up, and give indoor seedlings as much light as you can. Our seed starting calculator plans your indoor sowing date, and the germination time calculator estimates when to expect seedlings.
Transplanting and planting out
Transplant seedlings out after the last frost, on a cloudy day or in the evening so they do not wilt in the sun. Space plants 12–18 inches apart — that spacing is what keeps air moving and disease down — and water each plant in well. Our plant spacing calculator works out exactly how many plants fit your bed, and the square foot garden planner helps lay them out.
Soil and feeding
Asters grow best in fertile, well-drained soil with a pH of 6.0–7.0. Dig in compost before planting — it feeds the soil and helps it hold moisture without getting soggy — and mulch after planting to keep the roots cool and the weeds down. Feed during the growing season with a balanced fertiliser at the label rate, roughly once a month while plants are actively growing, and stop feeding as the season ends. Our fertiliser calculator works out the right amount for your bed, and the compost ratio calculator helps you make good compost to dig in.
Watering
Asters have moderate water needs: keep the soil evenly moist while plants are establishing and flowering, and water deeply rather than little and often so the roots grow down. Water at the base in the morning so the foliage dries during the day, mulch to hold moisture in, and watch for wilting in hot, dry spells. Our watering calculator gives a starting point for your bed size, and the watering schedule generator can build a routine around it.
Sunlight and temperature
Asters need full sun — at least six hours of direct sun a day. Asters are hardy and tolerate frost and cold weather well, so they can be planted out earlier and left in the ground over winter in most zones. In frost-free Zones 11–13, heat and a lack of winter chill can limit performance, so give them the coolest spot you have. Check your zone with the hardiness zone finder before choosing varieties. Our sunlight calculator can check whether your spot gets enough sun.
Pests and diseases
The pests and diseases to watch for on asters are aphids, slugs, thrips. Aphids can cluster on young growth, slugs may eat young shoots in spring, and thrips can mark the petals. The main disease is powdery mildew, which is worse in still, damp air. Check plants weekly, looking under leaves as well as on top, and act early. Hand-pick larger pests, knock aphids off with a strong spray of water, and remove badly affected leaves. Keep the area weed-free and avoid wetting the foliage when you water, since damp leaves invite fungal disease. Our pest identifier helper can help identify what you are seeing, and the plant disease checker helps narrow down leaf and stem problems.
Companion plants
Good companions for asters are lavender, marigolds, garlic, chives. Lavender, marigolds, garlic and chives are good companions, and asters pair well with late-flowering grasses and sedums, which support them and extend the autumn display. Our companion planting finder can suggest more pairings for your bed plan.
Harvesting
Cut asters for the vase when the flowers are fully open, in the morning, and put stems straight into water — they last about a week. Deadhead spent flowers to keep the plants tidy, but leave some seed heads for the birds over winter. In late autumn, cut the stems down to the ground once they have died back, and mulch the crown to protect it over winter. Divide congested clumps every three or four years in spring. Our harvest date calculator can estimate your picking date from your sowing date, and the vegetable yield estimator helps you work out how much a bed will produce.
Common problems and fixes
| Problem | Likely cause | Fix |
|---|---|---|
| Powdery mildew | damp, still air | Water at the base, space plants well, and remove affected leaves. |
| Plants flop over | tall growth and no support | Pinch young plants in early summer and stake tall varieties. |
| Centre of the clump dies out | old, congested clump | Lift and divide the clump in spring, replanting the young outer pieces. |
Powdery mildew
White powdery patches on aster leaves are the most common problem, and they spread fast in still, damp air. Water at the base in the morning, keep to the spacing in the table so air moves through the clump, and pick off the worst leaves as soon as you see them.
Plants flop over
Tall asters flop under their own weight in flower, especially after rain. Pinch the tips of young plants in early summer to make them bushier and shorter, and put stakes or a grow-through support in place before the plants need it.
Centre of the clump dies out
An old aster clump often dies out in the centre, leaving a ring of growth around a bare middle. This is normal for a perennial that has not been divided: lift the clump in spring, discard the woody centre, and replant the vigorous young pieces from the outside.
Frequently asked questions
How long does it take to grow asters?
From planting to maturity takes 70–90 days. Seed germination takes 7–14 days, and the total time depends on whether you start from seed, a nursery plant, or a bulb or cutting.
How far apart should I plant asters?
Space asters 12–18 inches apart. That spacing gives each plant room for air movement, which keeps disease down, and room to reach its full size without crowding its neighbours.
What soil pH do asters need?
Asters prefer a soil pH of 6.0–7.0. Test your soil before amending it — most garden soils are close enough, and guessing with lime or sulphur can do more harm than good.
When do asters flower?
Asters are autumn flowers, blooming from late summer into autumn. The data puts maturity at 70–90 days from planting, and an established clump flowers every year in late summer and autumn.
Do asters need dividing?
Yes, every three or four years. Lift the clump in spring, divide it into pieces with roots and shoots, and replant the young outer pieces. Dividing keeps the plant vigorous and flowering well.