How do I prevent powdery mildew on asters?

Choose mildew-resistant cultivars, ensure good air circulation by spacing plants 45-60cm apart, water at the base rather than overhead, and maintain consistent soil moisture as drought stress increases susceptibility. The Chelsea Chop in late May also improves air flow through bushier growth.

When should I divide my asters?

Divide asters every 2-3 years in early spring (March-April) when new shoots are just emerging. The Loam app can send you timely reminders based on when you last divided your plants and track their performance year-on-year to help you maintain vigorous flowering.

Why are my asters flopping over?

Asters flop due to insufficient light, overly rich soil causing soft growth, or lack of the Chelsea Chop pruning in late May. Cut leggy stems back by half in late spring to encourage sturdier, self-supporting growth, or provide discreet support with linking stakes or pea sticks inserted early in the season.

Aster

Symphyotrichum novi-belgii

Perennials

Watering: Weekly during dry spells
Sunlight: Full sun to partial shade
Soil: Moist, well-drained, fertile
Humans: Safe
Cats: Safe
Dogs: Safe
Height: 60–120 cm
Spread: 45–90 cm
Spacing: 45–60 cm apart
RHS recognition: Award of Garden Merit

About Aster

Asters are beloved late-season perennials that produce masses of daisy-like flowers in shades of pink, purple, blue, and white from late summer through autumn. Native to North America but naturalised in parts of the UK, these hardy plants are essential for extending colour in British borders when many other perennials have finished flowering. Their nectar-rich blooms are particularly valuable for pollinators preparing for winter, making them a favourite among wildlife gardeners.

UK fact: Many Symphyotrichum novi-belgii cultivars have received the RHS Award of Garden Merit, and these asters have become so well-established in British gardens that they're often found naturalised along riverbanks and in damp meadows across southern England.

Watering Aster

Keep soil consistently moist during the growing season, particularly from spring through to flowering, as asters dislike drought. Water at the base of plants rather than overhead to reduce the risk of powdery mildew, which thrives in humid conditions. Established plants in moisture-retentive soil may need watering only during prolonged dry periods, but newly planted asters require regular watering throughout their first season. During typical British summers, weekly watering is usually sufficient unless conditions are exceptionally dry.

Sunlight requirements

Asters perform best in full sun, which encourages compact growth and abundant flowering, though they tolerate partial shade particularly in southern England. In shadier positions, plants may become leggy and require staking. South or west-facing borders are ideal, though avoid very hot, dry spots where powdery mildew becomes more problematic.

Soil and feeding

Asters thrive in fertile, moisture-retentive soil that doesn't dry out completely but also doesn't become waterlogged. They prefer a neutral to slightly alkaline pH (6.5-7.5), though they tolerate a range of soil types. Improve heavy clay with organic matter to aid drainage, and enrich sandy soils with compost or well-rotted manure to improve moisture retention. Mulching in spring helps conserve moisture and suppress weeds around the shallow root system.

Apply a balanced general-purpose fertiliser in early spring as new growth emerges to encourage healthy foliage and abundant flowering. A second feed in early summer supports flower bud development. Avoid high-nitrogen feeds later in the season as these promote soft growth susceptible to mildew.

Pruning Aster

Cut back stems to ground level in late autumn after flowering finishes or leave seedheads for winter interest and wildlife, cutting back in early spring before new growth appears. To encourage bushier plants and reduce the need for staking, cut stems back by half in late May (the 'Chelsea Chop'), which delays flowering slightly but produces sturdier growth. Deadheading spent flowers prolongs the display, though this is labour-intensive on mature clumps. Divide congested clumps every 2-3 years in spring to maintain vigour and flowering performance.

Propagating Aster

Asters are easily propagated by division in spring, which also rejuvenates older clumps that have become woody in the centre. Lift established plants and separate healthy outer sections with roots attached, discarding the tired central portion. Basal cuttings taken in spring from new shoots also root readily in a mix of compost and grit. Seed can be sown in spring, though cultivars won't come true to type and division is far more reliable for maintaining desirable characteristics.

Common problems

Powdery mildew is the most common problem, appearing as white powdery patches on leaves, particularly in dry conditions or where air circulation is poor. Choose mildew-resistant cultivars and ensure adequate spacing and watering to minimise this issue. Slugs and snails may damage young spring growth, whilst aphids occasionally colonise shoot tips in early summer. Aster wilt, a soil-borne fungal disease, causes plants to collapse suddenly; affected plants should be removed and destroyed, and asters should not be replanted in the same spot.

Spot a problem with your plant? Photograph the affected area using the Loam app and it will suggest the most likely cause and treatment.

Safety

Humans: Safe
Cats: Safe
Dogs: Safe

Toxicity information is for general guidance only.

Seasonal care calendar

Monthly care guide for Aster in the UK, based on average conditions.

Monthly care guide for Aster
Month	Care tasks
January	No action needed; plants fully dormant.
February	Clear away old stems if not done in autumn.
March	Cut back remaining stems as new growth emerges. Divide congested clumps. Apply mulch and balanced fertiliser.
April	Watch for slug damage on emerging shoots. Water during dry spells.
May	Perform the Chelsea Chop (cut stems by half) for bushier growth. Apply second feed.
June	Water regularly during dry weather. Monitor for aphids and powdery mildew.
July	Continue watering. Stake taller varieties if needed.
August	Flowering begins on late varieties. Maintain watering regime.
September	Peak flowering period. Deadhead to prolong display. Enjoy the pollinator activity.
October	Continue deadheading. Flowering continues until first frosts.
November	Cut back finished stems or leave for winter structure and wildlife.
December	No action needed; plants dormant.

Frequently asked questions

How do I prevent powdery mildew on asters?: Choose mildew-resistant cultivars, ensure good air circulation by spacing plants 45-60cm apart, water at the base rather than overhead, and maintain consistent soil moisture as drought stress increases susceptibility. The Chelsea Chop in late May also improves air flow through bushier growth.
When should I divide my asters?: Divide asters every 2-3 years in early spring (March-April) when new shoots are just emerging. The Loam app can send you timely reminders based on when you last divided your plants and track their performance year-on-year to help you maintain vigorous flowering.
Why are my asters flopping over?: Asters flop due to insufficient light, overly rich soil causing soft growth, or lack of the Chelsea Chop pruning in late May. Cut leggy stems back by half in late spring to encourage sturdier, self-supporting growth, or provide discreet support with linking stakes or pea sticks inserted early in the season.

Got a question about growing Aster that isn't covered here? Ask the Garden Advisor in the Loam app. It knows your saved plants and local conditions.

This page was compiled with AI assistance and reviewed by our team. .