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Home & garden.

16 August 2021

How to grow asters

Asters add vibrant colour in late summer and autumn, and attract a wide range of pollinators. Discover our best varieties and tips for how to grow them.
CT
Ceri Thomas
Aster

The name of some types of aster was changed recently to symphyotrichum, but it seems likely these cottage-garden favourites will be fondly known as asters for sometime to come. They give an exuberant display of colour in late summer and autumn for very little effort, and they come in so many shapes and sizes there’s a good choice for any size of garden. 

They’re often associated with the fungal disease powdery mildew, but the reality is that by choosing the right varieties, or by growing susceptible ones in the right conditions, the problem can largely be avoided. 

They are great for pollinators, too; attracting a diverse range that includes butterflies, and many different types of bee and hoverfly.

Which? Gardening magazine grew a range of popular varieties in the north and south of the UK over two years to see which would give us the best display and be hardy enough to get through UK winters.


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Key facts:

PLANT TYPE Herbaceous perennial

POSITION Sunny, or part-shade for some

SOIL Well-drained but moisture retentive

How to grow asters: month by month

January
February
March
April
May
June


DIVIDE/PLANT/MULCH
DIVIDE/PLANT/MULCH


July
August
September
October
November
December

FLOWERING
FLOWERING
FLOWERING
CUT BACK

Best aster varieties

Which members can log in now to see the full results and which are our Best Buy varieties. If you're not a member, join Which? to get instant access.

Best Buy asters
What it looks like
Variety name
Height x spread
80 x 60cm
These sturdy plants were a more easily manageable size than some and needed no support at all, even when they were covered in a mass of blooms. The flowers were simple daisies, which were quite large with long petals, and their colour, an unusual purple-pink, looked lovely in the sun and on duller days, when they shone out. While it wasn’t the best variety for insects, it still got plenty of attention from pollinators. Peak flowering: Aug-Oct
What it looks like
Variety name
Height x spread
100 x 50cm
A thick ruff of long, gently curving petals in delicate baby-pink gave these flowers a decidedly feminine look. In the first year, the plants stood up well with no support, but in the second year, they grew taller and flopped a little after they had been in full flower for a while. The mass of flowers attracted huge numbers of bees and other pollinators that helped make this plant a joy to grow. Peak flowering: Sep-Oct
What it looks like
Variety name
Height x spread
60 x 50cm
This variety, which holds an RHS Award of Garden Merit (AGM), grew particularly well in Glasgow, where it was in full flower for six weeks. It showed good resistance to the mildew that affected other varieties there and was very healthy. It was one of the shorter varieties we grew, which may explain why it didn’t need any support, and it gave a great display of blooms from August onwards. Hoverflies loved it, too. Peak flowering: Aug-Oct
What it looks like
Variety name
Height x spread
120 x 90cm
This tall variety threw up new shoots over a wide area, so it needs a large border to accommodate it. In the right location it would look fantastic with its small but abundant flowers changing colour as they aged from pink to white. Unsurprisingly, it needed some support, but it was easy to put these in place and allow the stems to grow through them. It was healthy and resisted the mildew experienced in Glasgow.  Peak flowering: Sep-Oct
What it looks like
Variety name
Height x spread
95 x 95cm
Due to a hitch with the delivery of the plants, this variety was only grown in north London. It remains a Best Buy, though, as it grew so well there and is also very resistant to mildew. It started flowering earlier than many varieties, producing a sea of eye-catching blooms for many weeks and was buzzing with insects, including butterflies, hoverflies and many different types of bee. It’s also an AGM holder Peak flowering: Jul-Oct

How we test asters

We chose 24 varieties of asters. These included recommendations from specialist grower Helen Picton at Old Court Nurseries, existing Best Buys, well-established and widely sold varieties, and promising newcomers. We grew three plants of each variety in the well-drained soil at Capel Manor in north London, and in Glasgow Green park, where the soil is slightly heavier, rainfall higher and weather generally cooler than in north London. The varieties were planted in spring and we assessed them over two years for: impact, display, duration and insect attraction;  survival, shape and whether the plants need staking; and whether the plants suffered from common problems. In early winter, when flowering had finished, the top growth was cut down to ground level. The plants in north London were regularly irrigated during the hot, dry weeks of early to mid-summer, while in Glasgow they were only occasionally watered.

Caring for your plants

Planting

Plant in spring in well-drained but moisture-retentive soil. Most asters do best in sun, but some will flower well in part shade, too. Water regularly until plants are well established. Mulch around the roots after planting.

Watering

Mulch in spring to retain soil moisture. Water plants as needed in the summer to stop the soil from drying out.

Cutting back

Cut back all finished flower stems to ground level as soon as they fade to prevent them from setting seed. Cut back the whole plant in late autumn-late winter to make space for the new growth in spring.

Propagating

Divide clumps of New England and New York asters regularly to help keep them healthy. Either dig up the whole clump every two to five years, divide and replant the divisions, or dig out some of the outer shoots (with roots) of spreading varieties every year or two and replant elsewhere.

Common growing problems

Powdery mildew

This causes a white, powdery coating on stems and leaves, and weakens plants. To help prevent it from taking hold, don’t let the soil dry out during the summer and don’t overcrowd asters with other plants. Cut out infected stems and spray susceptible varieties with a fungicide early in the season.

Not all asters are prone to mildew. All the varieties of the traditional Michaelmas daisy or New York aster, (S. novi-belgii) are prone to mildew and need to be given the right care to keep them healthy. Other types of aster, such as New England asters (S. novae-angliae) and some small-flowered varieties, are more resistant but can become infected during hot, dry weather so they can need a bit of extra care if circumstances encourage the disease to strike. Some asters (A. amellus, A. x frikartii, A. piraneus) are completely resistant and don’t suffer from mildew at all, so these can be easier to grow.

Read more about powdery mildew

Verticillium wilt

Verticillium wilt is a soil-borne fungus that can cause leaves to wilt and sections of the plant to die back. Unfortunately, there are no chemical controls to use against it, but plants might respond well to feeding in the following spring.