the Asters (Aster) is with over 180 known types not only part of the big family of the Korbblütler, but even its name-giver (Asteraceae). The original forms of the plants come from America and Africa as well as from the Eurasian region. The colourful composite flowers decorate the garden at the end of the season in a flowering abundance that is unusual for this season and are not even particularly demanding.
Attention: Due to their large genetic differences to the Eurasian species, the North American Aster species are now grouped together under their own genus names in taxonomical terms and are no longer strictly speaking classified as Asters. Affected by this are among others the known garden forms of the smooth leaf aster, robber leaf aster, myrtle aster and cushion aster, which now carry the generic name “Symphyotrichum” instead of “aster”, as well as the summer aster (former Aster chinensis, today Callistephus chinensis) and the gold aster (former Aster linosyris, today Galatella linosyris). This division, however, has not yet established itself either in the hobby gardener sector or in the plant trade. Therefore, this portrait of the genus includes all garden varieties traditionally known as “asters”.
Appearance and growth
Due to the variety of species, a wide range of sizes, colours and growth forms can be found among the asters, which are predominantly perennials. From a few centimeters up to three metres high, the “starflowers” have it all. The shape, placement and appearance of the leaves also vary depending on the species. However, all asters have in common their characteristic basket-shaped flowers, in which the long tongue florets in white, pink and blue tones stand around the small yellow tubular flowers arranged in the middle. They appear individually or in groups at the end of a slightly hairy stem. Asters are available as annual or biennial ornamental plants, most forms forming a rhizome.
Location and soil
Asters generally appreciate soils with a medium nutrient content, too lean sites should therefore be improved with compost. Mountain aster (Aster amellus) and summer aster (Callistephus chinensis) attach importance to a calcareous soil, but they are also satisfied with nutrient-poor places. Mountain asters are suitable for rock gardens and also survive dry phases well. Most Aster species like to stand in the sun, but Large Leaf Aster (Aster macrophyllus), Wild Aster (Aster ageratoides) and White Forest Aster (Eurybia divaricata) are also suitable for semi-shady locations. The small varieties can also be planted extensively as autumn flowering groundcovers, even in difficult locations under trees.
Planting and care
Best planting times for asters are from March to May and from September to November. Mountain aster and summer aster should be planted better in spring, then they grow better. Maintenance after ingrowth is largely limited to pruning in March (simply break out dry stems) and occasional fertilisation and moderate irrigation. Only the high-growing species are fertilized with compost in spring. Not only for propagation, but also to preserve the flowering joy of the asters over the years, the plants should be regularly taken out of the soil, divided and replanted.
In spring, cut off the flowered stems about a hand’s width above the ground. Healthy plant parts are allowed on the compost. If the asters are affected by mildew, the pruning is disposed of as residual waste. The cushion aster (Symphyotrichum dumosus) grows into dense cushions through runners. To keep them compact, cut the plants back by half after flowering and sprinkle them with mature compost. Otherwise a contour cut is not necessary for asters.
Asters are typical farm garden plants, but also look good in tubs or as balcony flowers – but only until spring. Then the most compressed asters reach their normal height and belong in the bed. There they join fat stonecrop, autumn anemone, coneflower, dahlia and autumn chrysanthemum. Also ornamental grasses are pretty companions. Thanks to their diversity, asters never seem boring when combined with each other. The different shades of red and violet harmonize wonderfully. If you choose the right location and observe a few simple care principles, you will enjoy the colourful perennials for many years, as will numerous butterflies and bees, which especially in autumn enjoy the flowers rich in nectar and pollen.
The low pillow asters enjoy their appearance in small groups in the front row and result in beautiful bed borders. High species can also stand alone. Your thin stems need a stable support. Attention: Some varieties of asters tend to lose their leaves from below and to become glabrous. This can only be partially prevented by regular watering. The ejection of the leaves is a normal protective mechanism, often already with little dryness. It is therefore advisable to plant high varieties in the bed further back and to place smaller, but opaque plants in front of them, which hide the dry stems.
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If you get to know the world of asters better, you will be surprised how many species there are whose varieties inspire us with their pile in summer and autumn. The annual summer asters (Callistephus chinensis) grow 20 to over 100 centimeters high and flower in white, pink, violet or yellow as early as July. They love warm locations in full sun. The flowers of the summer asters are usually filled and can reach a diameter of over ten centimeters, which makes them a beautiful cut flower.
Predatory leaf, smooth leaf and cushion asters are the most important groups of September and October bloomers. Predatory leaf and smoothleaf asters (Symphyotrichum novae-angliae and Symphyotrichum novi-belgii) rise with impressive heights of up to 1.20 metres above fences and their neighbours in the bed. Support the high nests in time! An airy stand reduces their susceptibility to fungal diseases. Pillow asters live up to their name and form up to 50 centimeters high, richly flowering cushions on the side of the path or the flower bed. Blue and violet tones dominate here. A small shopping aid: In a comparative perennial shrub screening, the cushion aster varieties ‘Apollo’, ‘Blauer Gletscher’, ‘Herbstgruß vom Bresserhof’, ‘Niobe’, ‘Rosenwichtel’ and ‘Zwergenhimmel’ were classified as ‘excellent’.
The Erika aster (Symphyotrichum ericoides, also called Myrten aster) grows as a richly branched bush and adorns itself until November with small, but innumerable flowers in white, pink or lilac. Important: Avoid waterlogging! Alpine asters (Aster alpinus) are only up to 20 centimeters high. They are ideal for small rock gardens. Autumn planting gives these plants a flower head start in spring. Despite similar name, they are not to be confused with the mountain-asters (Aster amellus). They like it if you sprinkle lime around the plants from time to time, because they do not tolerate acid soil (hence the second name lime aster). Mountain asters belong together with summer asters to the early starters, because the compact, upright nests blossom already from July. Especially deep blue-violet and compact is the mountain aster ‘Violet Queen’. A special feature among the asters is the gold (hair) aster (Galatella linosyris). It differs from its relatives above all by its golden-yellow head of flowers, which it carries from August to September on stems up to 50 centimeters high. The bloom carries no tongue, but exclusively tube-florets, what lets the bloom-head seem virtually pushelig. The Gold Aster likes to stand in the sun and is very robust against prolonged drought.
Autumn asterisks are best divided in spring. Use a spade to cut diagonally into the soil and carefully lift out the root runners. Then cut off pieces with two to three eyes for a new shoot. The sections will be replanted in other sunny and nutrient-rich locations. Remove uncontrolled growth beforehand. Place the parts as deep into the soil as the mother plant did before. Good pouring supports the rooting in in the first weeks after the division.
Diseases and pests
If the plant shows flaccid leaves and black shoots, it suffers from aster wilt and should be removed along with the roots. It is best to pay attention to wilt-resistant breeds directly when buying. Over-fertilisation, heat and drought stress promote mildew infestation. For warm, dry locations in the garden, it is therefore advisable to choose varieties that are tolerant of heat as a precaution. Among others, the varieties of the predatory leaf aster (Symphyotrichum novae-angliae), the alpine aster (Aster alpinus) and the early summer aster (Aster tongolensis) are certainly free of mildew. Powdery mildew control with mains sulphur is only possible at the very beginning of the infestation in order to contain the spread.