Herfstaster

Aster novi-belgii

Engels: Michaelmas Daisy

perennialAsteraceae

Herfstaster (Aster novi-belgii) is a perennial from the Asteraceae family that grows up to 120cm tall. This plant thrives in full sun to partial shade and requires moderate maintenance. Blooms in late summer and autumn with purple, blue, pink, white flowers and attracts bees and butterflies.

Hoogte

40–120 cm

Breedte

30–60 cm

Zonligging

full sun, partial shade

Waterbehoefte

moderate

Grondsoort

loam, clay soil

Onderhoud

moderate maintenance

Bloeitijd

late summer, autumn

Bloemkleuren

purple, blue, pink, white

Ecologische waarde

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Verzorgingskalender

Taak	Jan	Feb	Mrt	Apr	Mei	Jun	Jul	Aug	Sep	Okt	Nov	Dec
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Care tips

Planting

Michaelmas daisies thrive in full sun or partial shade, though flowering is most prolific in a bright, open position. They tolerate a range of soils but perform best in moisture-retentive loam or clay that doesn't dry out completely in summer. Avoid very light, sandy soils unless you're prepared to water regularly. Plant container-grown asters in spring (March to May) or autumn (September to October). Spring planting gives them a full season to establish before their first flowering, while autumn planting works well if the soil isn't waterlogged. Dig over the planting area and incorporate a bucketful of well-rotted compost or manure per square metre to improve soil structure and moisture retention, especially if your soil is on the lighter side. Space plants 45 cm apart to allow for their mature spread and to ensure good air circulation, which helps reduce mildew problems later. Dig a hole slightly wider than the root ball and plant at the same depth the aster was growing in its pot—burying the crown too deeply can lead to rot. Firm the soil gently around the roots to eliminate air pockets. Water thoroughly after planting, even if the soil feels damp, to settle the roots in. Apply a 5 cm layer of organic mulch such as garden compost or bark chips around the base, keeping it a few centimetres clear of the stems. This conserves moisture and suppresses weeds while the plant establishes. In dry spells during the first growing season, water weekly until the roots have spread into the surrounding soil.

Pruning

Michaelmas daisies don't require complex pruning, but a couple of well-timed cuts make a significant difference to their performance. The main pruning task is cutting back spent stems after flowering or in early spring. In November, once flowering has finished and the foliage begins to collapse, cut all stems down to ground level. This tidies the plant, removes potential overwintering sites for pests, and prevents mildew-affected foliage from spreading spores. Alternatively, if you prefer to leave some winter structure for wildlife or frost interest, delay this job until March, cutting back all old growth just as new shoots emerge at the base. Use clean, sharp secateurs or hedging shears for the job—tall clumps can be cut back quickly with shears. Remove all the top growth but leave the woody crown intact at soil level; new shoots will appear from here in spring. Many gardeners also practise the "Chelsea chop" in late May: cutting back half or all of the stems by one-third to one-half of their height. This technique delays flowering slightly on the cut stems, extends the overall display, and produces bushier, sturdier plants less prone to flopping. It's optional but particularly useful for taller varieties that tend to become leggy. Deadheading individual spent blooms during the flowering season isn't practical given the sheer number of flowers, and it has little impact on performance. Focus instead on the annual cutback and, if needed, the Chelsea chop to keep your asters compact and floriferous.

Maintenance

Michaelmas daisies have moderate water needs. During spring and summer, water during dry spells to keep the soil evenly moist but not saturated—aim for a thorough soak once a week rather than frequent light sprinklings. Clay and loam soils hold moisture well, but in prolonged dry weather the plants will flag and become more susceptible to mildew. Reduce watering in autumn as growth slows. Feed in early spring (March or April) with a general-purpose granular fertiliser such as blood, fish and bone or Growmore, scattered around the base at roughly a handful per plant and lightly forked in. A second, lighter feed in early summer can boost flowering on poorer soils, but avoid overfeeding, which encourages soft, floppy growth. Powdery mildew is the most common problem, especially in dry conditions or where air circulation is poor. Lower leaves develop a white, powdery coating from late summer onward. Improve spacing, mulch to retain soil moisture, and remove affected foliage promptly. Resistant varieties are available if mildew is persistent in your garden. Slugs and snails may nibble emerging shoots in spring; use your preferred control method early in the season. Michaelmas daisies are fully hardy and need no winter protection. The clumps can become congested and flower less freely after three or four years. Lift and divide them in March or April, replanting vigorous outer sections and discarding the woody centre. Refresh the mulch layer each spring to suppress weeds and conserve moisture through the growing season.