How to grow Asters
Asters are a large group of plants, consisting of many different species, each one offering something different in height, colour and design potential. There are many cultivars available to provide you with good displays of late summer and autumn colour, with daisy-type flowers in shades of white, pink, purple, blue and red. Most are easy to grow, except on very heavy clay soils or soils that dry out in summer.
Many plants that were named Aster have now been reclassified and have a different genus name – we’ve kept them altogether here as the cultivation is still the same.
- Most asters flower in summer and autumn
- They thrive in full sun, but many flower in partial shade
- Asters grow well in moisture-retentive fertile soils
- Heights range from 20cm-2m (8in-6⅔ft)
- Single daisy flowers are good for pollinators
- Most asters are perennials, lasting many years
- Suitable for a range of garden styles, including borders, prairie-style plantings and rock gardens
All you need to know
What’s in a name?
If you pop into a garden centre and look under A for Aster, you might or might not find what you are looking for. The reason for this is that some asters have had a name change, so here’s our guide to finding what you are really after. But long and short of it is, you might need to check under it's old and new name to be sure if they have it for sale.
In recent years, botanists and scientists have been looking closely (to DNA level) at the groupings of plants in the daisy family (Asteraceae). Name changes, particularly within a popular genus like Aster, are only made and introduced when there are good reasons to do so. The genus Aster previously contained not particularly closely related plant species from all over the world. To address this problem some regrouping was necessary.
Some species retain the name Aster, for example, Aster amellus and Aster × frikartii have not changed at all. Some have a new genus name, with slight alterations to the species name, for example Aster turbinellus is now known as Symphyotrichum turbinellum.
Symphyotrichum is one of the new names that will become better known to UK gardeners because it has been given to two of the most popular types of Michaelmas daisies. New England asters (Aster novae-angliae) and New York asters (Aster novi-belgii) are now called Symphyotrichum novae-angliae and Symphyotrichum novi-belgii respectively.
As with other name changes, it will take a while until the new names are widely used. If you would like to know more about the name changes, an informative article on the subject is available here - The Splitting of Aster.
Choosing an aster
There are many asters to choose from, so to help with your selection it’s easiest to begin by narrowing down your options:
- What will grow well in your garden’s growing conditions?
- Which garden style would you like your new plants to fit in with?
- Any size and colour preferences?
- Perennial or annual?
As a result, we’ve made lists of asters to help you select which are suitable for
Asters for containers
Asters for cut flowers
Asters for shade
- Two of the most popular types of asters grown in UK gardens are Symphyotrichum novae-angliae (New England asters) and Symphyotrichum novi-belgii (New York asters), with dozens of cultivars of different size and colour listed in RHS Find a Plant.
- Most asters grown in the UK are hardy herbaceous perennials. Asters do not like prolonged periods of wet soil in winter. A free-draining soil is particularly important for asters that originate from Europe, for example Aster amellus.
- Callistephus chinensis and cultivars of it are annuals, ideal for summer bedding, containers and cut flowers.
To help you choose from the many available asters, look for those that have received the RHS Award of Garden Merit, as these have been assessed and are known to perform well. Use the RHS Find a Plant tool to browse images and focus your search on criteria for your particular requirements.
There are National Collections of asters, more information on the collections and when they can be viewed can be found on the Plant Heritage website.
Buying an aster
Garden centres often stock plants when they are in full flower and looking at their best. Buying asters in late summer and early autumn when they are flowering has the advantage of allowing you to see exactly what colour the flowers are, although the plants will be very thirsty when at their peak of flowering, so will require regular watering after planting to help them establish. It is also worth bearing in mind that container- grown plants are unlikely to have reached their full height potential, so the plant could be a lot taller after a
The period of time when an individual plant is in active growth. This will depend on the local climate and light levels, and can vary between different plants, although it is broadly from spring to autumn.
You will most likely see container-grown asters for sale, but it is possible to buy herbaceous
Perennials are any plant living for at least three years. The term is also commonly used for herbaceous perennials which grow for many years (To compare: annual = one year, biennial = two years).
These have been lifted from the ground while dormant, with little or no soil around their roots. Various plants may be available bare root, including fruit trees, hedging plants and some perennials. They are generally cheaper than plants in containers, but are only available in winter/early spring, while dormant
Annual aster, Callistephus chinensis, can be grown from seed in spring. Some garden centres stock seed; alternatively, they can be bought from online seed catalogues (eg. Chiltern Seeds or Mr Fothergills).
When to plantEarly spring (March to early May) is a good time to plant asters, this gives them the best part of a growing season to establish before they flower.
Container-grown asters are sold throughout the year at garden centres and nurseries. You can plant at any time as long as the ground is not frozen or waterlogged. If you plant in summer, they will require additional watering to keep the soil around the roots moist.
Plant bare-root asters soon after delivery. If weather conditions are not favourable for planting it is important to ensure that the roots do not dry out; you can stand the roots in a bucket of moist compost, or lay them diagonally in a shallow trench and back-fill with soil to cover the roots (heeling-in). Plant out as soon as weather conditions improve.
Most asters grow well in full sun; they will flower okay in light shade, but the shadier the location the less flowers. Some types are suitable for slightly shady borders.
Where to plant
Many asters grow well in fertile soil that holds plenty of moisture in spring and summer. Symphyotrichum novi-belgii, with its shallow root system, particularly benefits from a moisture retentive soil. So, if you have a light sandy soil, dig in some well-rotted organic matter when planting (see How to Plant section below), and add a 5cm (2in) layer as mulch to the soil surface in spring to help retain moisture around the roots.
Asters originating from the prairies of North America (eg. Symphyotrichum turbinellum) or exposed hillsides in Europe and Asia (eg. Aster amellus and Aster thomsonii), so they are able to grow well in exposed, windy gardens, although taller types will need support by staking.
In general, European and Asiatic asters like free-draining soils, especially in winter. They also grow happily in sheltered conditions, particularly those suited to woodland conditions (eg. Eurybia divaricata).
Neutral to alkaline soils are ideal for asters, although most will perform well on slightly acidic soils. If you garden on soil with a low pH level (acidic soil), it would be best to avoid the European asters that are a bit more fussy.
In the ground
How to plant
Begin by preparing your soil. If you make your own garden compost, dig in a bucketful per sq m (per sq yd) to a spade’s depth over the planting area. If you do not make compost, buy soil conditioner or well-rotted manure, and apply the same amount in the same way.
Although it can be tempting to add grit, sand or gravel to improve heavier, clay soils, you need extremely large quantities to make a noticeable difference. Digging in organic matter not only improves structure and drainage, but boosts fertility too.
Annual asters, Callistephus chinensis, are often sold in bedding plant multi-packs, these can be planted out when any risk of frost has passed. If you have grown your own plants from seed please see the propagating section below.
In a container
Choose a container that is at least 20–23cm (8–9in) in diameter
Use peat-free multipurpose compost
Position in a warm, sunny spot
WateringAsters in the ground
Water well after planting. On free-draining soil or during prolonged dry spells, they will need additional water to keep the soil around the roots slightly moist, but not soggy. Aim to water well and occasionally, rather than little and often.
The height of an aster can vary from year to year according to the amount of water it has received.
Asters in containers
These need more frequent watering than plants growing in the ground. Water as often as needed, which could be daily in hot weather. Try not to let the compost dry out, but do not let it get waterlogged for more than a day or two either.
Asters generally do not need regular applications of fertiliser in most garden soils. Mulching plants in borders with well-rotted manure or garden compost each spring should be sufficient. If your garden soil is light and sandy your asters can benefit from a feed in spring when they start to come into growth. Use a general-purpose fertiliser such as Growmore or Vitax Q4, applying a handful per sq m/yd.
For containers, use a liquid fertiliser, such as Phostrogen or seaweed feed, diluting according to the instructions on the bottle.
RHS guide to fertilisers
RHS guide on nutrient deficiencies
The majority of perennial asters are hardy throughout the UK and can withstand low temperatures. Most species and cultivars of Symphyotrichum can survive temperatures down to -20°C (-4°F).
Winter wet is more likely to be a problem than cold.
Remove annual Callistephus chinensis when they finish flowering in late autumn or leave overwinter. Add the old plants to your garden compost heaps.
Like many herbaceous perennials, asters benefit from being lifted and divided every three to five years to maintain health and vigour. Lift and divide asters in early spring (March/April). (See the propagating section below).
If you deadhead when flowers have faded this will encourage plants to keep flowering. However, because most asters produce a profusion of flowers, deadheading is not usually necessary for a good display. You might choose to leave the seedheads and stems in place for winter interest as these can look attractive and provide food for birds and shelter for wildlife.
For China asters (Callistephus chinensis) grown as summer annuals, it is definitely worth regularly deadheading to get the maximum amount of flowers into the autumn.
Some tall asters will need staking. If the aster you choose will reach more than 1m (3⅓ft) then it is definitely worth planning how you are going to stake it, and putting supports in place before mid-summer.
Some asters, particularly Symphyotrichum novi-belgii, respond well to the ‘Chelsea chop’. Carrying out the ‘Chelsea chop’ will reduce the eventual height and can do away with the need for staking; however, results can be very variable between different types of aster and even from year to year depending on the weather.
Dividing clumpsMost asters will benefit from being lifted and divided every three to five years This is best done in early spring (March/April). Digging up an established aster and dividing it into smaller clumps will maintain the health and vigour of the plant, this is also the best way to propagate cultivars as all the new clumps will be the same as the original plant.
Sowing seedAsters can be propagated from seed. This is most useful when growing annual China asters (Callistephus chinensis), sowing purchased seed in spring is common practice. Seed can also be bought for some of the species asters (those that have not been hybridised, eg. Eurybia divaricata). Seed can be collected from your asters; however, the resulting plants are likely to be very variable so it’s usually best to buy named varieties unless you’re particularly keen to have a go.
Fusarium wilts (Aster amellus seems more susceptible than most) and grey moulds may be occasional problems.
Slugs and snails may attack young growth.
If you are a member of the RHS, you can use our online Gardening Advice service for all your gardening questions.
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