Why asters earn their place in a late-season border

By the second week of September, most of the border has shot its bolt. The roses are down to a few tired blooms, the hardy geraniums have gone brown at the edges, and everything seems to be quietly packing up for winter. Then the asters open, and the whole picture changes.

Asters — still called Michaelmas daisies by most of us, because they flower around Michaelmas on 29 September — are the great givers of the autumn garden. A single clump of Symphyotrichum novi-belgii in full flower will carry a border on its own, throwing out hundreds of daisy-like blooms in violet, lavender, pink and white. They hold their colour for six weeks or more, they stand up well to a wet British autumn, and they are one of the last proper feeding stops for bees, hoverflies and late butterflies before the cold sets in.

They are not difficult plants. But they do have opinions, and if you get the site wrong you will end up with a mildewed, floppy mess by mid-August. Choose well and they will reward you for years.

Choosing the right aster for your border

Garden asters have been through a botanical reshuffle, and most of the old Aster names are now Symphyotrichum on the label. Don’t let that put you off — the plants are the same, and the good cultivars are still widely sold.

Think about height before anything else, because the range is wide. A few reliable choices:

• Aster x frikartii ‘Mönch’ — about 60cm, lavender-blue with golden centres, flowering from July right through to October. The most mildew-resistant aster you can grow.
• Symphyotrichum novi-belgii ‘Little Carlow’ — 90cm of clear violet-blue on dark stems. Compact enough for the middle of a border and far better behaved than the old tall sorts.
• Symphyotrichum novae-angliae ‘Andenken an Alma Pötschke’ — a tall one at 1.2m, with startling cerise-pink flowers. Needs the back of the border and a stout stake.
• Aster amellus ‘Veilchenkönigin’ — just 45cm, deep violet, and happy at the very front where you can see the flowers properly.

If you only have room for one, take ‘Mönch’ or ‘Little Carlow’. Both are compact, both resist mildew, and both will flower their socks off in a decent year.

Pick a sunny, sheltered spot with free drainage

Asters want sun — a good six hours a day, ideally more. In shade they grow weak and leggy, lean towards the light, and go grey with powdery mildew before they have even opened a bud.

Shelter matters just as much as sunshine. The tall cultivars carry a lot of flower on relatively slender stems, and a gusty exposed corner will have them flat on the soil after the first October gale. A south- or west-facing border with a hedge, fence or wall a few metres away is close to ideal.

Soil-wise, they are unfussy about fertility but fussy about drainage. They will tolerate clay, but they will not tolerate sitting in water over winter, which rots the crown. Before planting, fork the bed over and work in a couple of buckets of well-rotted garden compost or leafmould per square metre, plus a handful of horticultural grit if your soil is heavy. Aim for a pH around neutral; they sulk a little on very acid ground.

Planting and getting them established

Plant in spring, once the soil has warmed, or in early autumn so roots can settle before the cold. Autumn planting works well in most of the UK, provided you keep an eye on watering.

• Space plants 45–60cm apart. Asters spread, and crowded clumps are the first to mildew.
• Set the crown level with the soil surface — no deeper — and firm gently with your knuckles.
• Water in thoroughly, even if rain is forecast.
• Sprinkle a few slug pellets, or set a nematode drench, if slugs are a problem in your garden. Young shoots are their favourite food.

Newly planted asters need consistent moisture for their first season. After that they are reasonably drought-tolerant, though they will always flower better in a soil that does not dry out completely.

Watering, feeding and the Chelsea chop

During dry spells in July and August, water deeply once a week rather than little and often. A soak that reaches 15cm down is worth far more than a quick flick of the hose, and it encourages roots downwards. A mulch of compost in early summer will lock that moisture in.

Go easy on feeding. Rich soil produces soft, sappy growth that flops and mildews. If your plants look pale, a liquid feed high in potash — a tomato feed will do — once a fortnight from midsummer is plenty.

To keep tall cultivars compact and delay flowering slightly, try the Chelsea chop: cut the front half of each clump back by a third in late May to early June. Those stems will branch and flower a fortnight later, extending the season and removing the need for staking.

For the ones you leave alone, push twiggy pea sticks or a grid of string around the clump in June, while the stems are still short. They will grow up through it and hide the support entirely.

Keeping asters healthy and long-lived

Powdery mildew is the main complaint — a grey-white dusting on the leaves from midsummer. It is worst on dry roots and crowded plants, so water, mulch, and thin out congested clumps. Cutting the whole plant back by half in early June also helps, as the fresh regrowth is tougher.

Asters benefit from lifting and dividing every two or three years, in early spring as the new shoots appear. Discard the woody centre and replant the vigorous outer pieces into refreshed soil. This keeps the clumps flowering freely and stops them wandering into their neighbours.

Leave the stems standing over winter rather than cutting them down in October. The seedheads look handsome in frost, goldfinches will work through them, and the hollow stems shelter overwintering insects. Tidy up in early March, just before the new growth pushes through.