Asters: growing, pruning & winter care

Asters are eye-catching flowers that make a statement in any garden or park. From May to November, the countless species and varieties brighten up the garden with their flowers.

Asters, also known as Michaelmas daisies, have been delighting us in our gardens for years with their beautiful flowers. The smooth leaves and late flowering of the New York aster (Symphyotrichum novi-belgii) distinguish it from the bushy aster (Aster dumosus), which has sprawling growth, and the New England aster (Symphyotrichum novae-angliae), which has rough leaves and an abundance of colourful flowers. Moreover, asters are very easy to care for, which is why its success as a perennial is not surprising. Keep reading to learn more on how to properly care for asters.

Asters: origin and characteristics

Asters belong to and are named after the daisy family (Asteraceae). The plants originally came from America and Africa but also from Eurasia.

Fun fact: Following a reorganisation of the botanical system within the Asteraceae family, species such as the New York aster and the New England aster no longer belong to the genus Aster. However, they are still commonly referred to as asters. There are also many other related plants that are called asters due to their resemblance. You can read more about them in our article on aster species and varieties.

Asters come in a wide variety of species, and so they can look very different from each other. Their total height can vary from a few centimetres to three metres. Aster leaves can be basal or alternate, and petiolate (stalked) or sessile (stalkless) on the stem. But the structure of the flowers is quite uniform in all aster species and varieties. The daisy-like flowers have a yellow tubular corolla which is surrounded by a ring of white, pink, purple or blue petals.

How long do asters flower? The flowering time of asters also varies depending on the species or variety:

• Spring asters: Flowering time in May; includes species Aster alpinus and Aster tongolensis.
• Summer asters: Flowering time from July to August; includes species China aster, Aster ageratoides, Aster amellus, Aster divaricatus, Aster linosyris and more.
• Autumn asters: Flowering time from September to October; includes species New York aster, New England aster, Aster ageratoides, Aster cordifolius, Aster divaricatus, Aster dumosus, Aster ericoides, Aster laevis, Aster lateriflorus, Aster tataricus and more.

Asters are beneficial to bees because of the wide range of flowering times that can result from planting different types. Late-flowering ones, in particular, provide an important food source for insects when there are few other flowers left.

Are asters annuals or perennials? The majority of asters are perennials. The China asters (Callistephus chinensis) are actually annuals, but they do not, in fact, belong to the genus Aster.

Planting asters: where, when and how

Aster flowers thrive in sunny locations with loose, well-drained soil that contains a medium amount of nutrients. Nevertheless, there are also species that tolerate semi-shady locations well.

The best time to plant an aster is from March to May or from September to November. When planting, it is important not to place the perennial deeper in the soil than it was in its pot. Small species such as the bushy aster can be planted at a distance of about 25 cm from other plants, whilst medium-sized species require a distance of 45 cm and large specimens a distance of 50 to 60 cm.

Sowing: From mid-May onwards, scatter the seeds in a suitable location directly in the bed. After germination, prick out the seedlings and space them apart according to their species-specific requirements. You can start asters indoors as early as February: sow the seeds one centimetre deep in trays filled with seedling compost. Keep the substrate moist and at a temperature of around 15 °C. It takes at least four months for the seedlings to be strong enough to be planted outdoors.

Water and feed asters on a regular basis if they are grown in pots. This is only recommended for a few species such as the New England aster. Use a well-structured and well-draining soil. Our Plantura Organic Flower Compost, for example, is particularly suitable for planting due to the organic long-lasting fertilisation, as asters react sensitively to excessive nutrient concentrations. The crushed expanded clay in the soil also adds more structure.

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How to care for asters

Care measures for asters, both outdoors and in pots, include pruning in March, occasional fertilising and moderate watering.

Watering and fertilising

Feed potted asters once a fortnight from April to August. To meet these perennials’ nutrient needs, we recommend using a plant-based fertiliser, such as our Plantura Flower Food, which is mixed in when potting and only needs to be applied once or twice a year. You can also use this slow-release fertiliser on outdoor asters in spring.

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Water the aster plants well and regularly, especially during flower development, otherwise the flowering may be rather sparse. In the event of prolonged drought, water asters grown in the bed regularly. Keep the substrate in the pots moderately moist and do not let it dry out nor become waterlogged.

Pruning asters

Deadhead withered flowers regularly to encourage the aster to keep flowering. Prune back asters close to the ground towards the end of winter, around March, to encourage new shoots. The withered parts of the plant still provide protection during the winter. To prevent self-seeding, cut back the upper third of the plant after flowering.

Should you cut back winter hardy asters? Even hardy asters are not cut back until early spring to protect the root ball from low temperatures.

Pests and diseases

The most common diseases are aster wilt and powdery mildew. Brownish discoloration of the foliage and limp leaves are symptoms of aster wilt. By growing resistant varieties, this can be avoided from the start. When purchasing a perennial from a gardening centre, ask an expert for advice on recommended varieties.
Powdery mildew infestation can also be kept in check by growing robust varieties, but unfortunately not always. Spraying with a horsetail decoction can also help.

Which asters are hardy?

Except for the annual China asters, all asters are hardy. To provide additional protection, cover the ground around them with leaves, brushwood or fir branches. Place potted asters against a sheltered house wall and wrap the pot in fleece to protect the root ball. Water occasionally, otherwise the plants will die from drying out.

Propagation

Perennial asters are easily propagated by division. Divide the plants every few years in spring or autumn. Dig up the aster without damaging the roots. Separate the plant into two to three new plants using the sharp edge of the spade. Make sure each piece has plenty of healthy roots and stems. Replant in a new sunny location. Another, but rather difficult, method of propagating perennial asters is via shoot cuttings.

Annual asters, such as the China aster, can be propagated by seed. After flowering, the asters produce seed capsules. Let the seeds ripen and then harvest them. Sow the seeds in a tray and grow them inside on the windowsill in spring or sow them directly outdoors from mid-May.

Are asters poisonous?

The flowers of the well-known China aster are edible, i.e., non-toxic to humans, and they also pose no danger to animals. However, because there are so many different species and cultivars, and because they can be confused with poisonous chrysanthemums, you should only eat them if you can identify the genus and species with certainty and rule out any risk of intolerance.

If you want to offer the insects in your garden even more food sources in autumn and enjoy the vibrant flowers too, then it is worth planting more autumn flowers. Find out the five most beautiful ones for your garden.