Why I'm crazy for this daisy: Michaelmas are a vital late-season source of nectar for insects, says Monty Don

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Michaelmas daisies may not be gorgeous divas, holding the flowerbed in their thrall, but at this time of year they are a lovely cloud of flowers, enriching the autumnal borders in every shade of blue, mauve, pink and white. 

Butterflies and bees adore them and they are a late-season source of nectar for insects facing a long winter.

The plants are also known as asters and the two most common kinds are the New Belgian varieties (Aster novi-belgii) and the New England cultivars (A. novae-angliae). Despite their names, both come from the United States and both prefer cool, slightly damp growing conditions in full sunshine.

Butterflies and bees adore them and they are a late-season source of nectar for insects facing a long winter

The novi-belgii cultivars have an especially good range of colours from the lilac ‘Ada Ballard’ and the double blue ‘Marie Ballard’ to the deep plum colour of ‘Winston S. Churchill’. 

However they are prone to mildew, whereas the New England varieties are less susceptible but have a less vivacious selection of colours to choose from. ‘Barr’s Violet’ is a good purple, as is ‘Violetta’. ‘Lou Williams’ and ‘Septemberrubin’ are on the pink side of plum.

Aster amellus has flowers that are looser with more open florets. A. amellus ‘King George’ is a short form, growing perhaps to only 46cm (18in) and is a bright mauve. A taller and altogether bigger cultivar is ‘Nocturne’ which has deep lilac flowers on stems that are about a metre (3ft) tall, flowering very late. 

All the amellus asters like good drainage, so add plenty of grit to the planting hole. Keeping them well-drained makes them much more resistant to verticillium wilt. They also like an alkaline soil.

Aster x frikartii like the same alkaline, well-drained conditions as amellus but are otherwise problem-free. ‘Jungfrau’ is a compact rich lilac good for small borders. ‘Mönch’ is pale lavender with flowers from July to early October.

Powdery mildew can be a problem for all Michaelmas daisies. The fungus starts on the underside of the leaves and then spreads rapidly, blown by the wind. Keeping weeds down helps as they act as hosts to the mildew. 

Removing all top growth at the end of the flowering season and shredding and composting it will reduce the likelihood of spores surviving the winter. Keeping the plants watered and mulched helps too as mildew spreads more quickly in drought.

Lifting the plants every three years and dividing them before replanting the exterior sections in a fresh site and discarding the interior will mean healthy growth which will inherently be more mildew-resistant. Always do this in spring, just when you see new shoots appearing.

Recently I have been looking for and planting asters that will tolerate shade. Some species such as the lovely little A. divaricatus which only grows about 2-3ft tall but is spangled with white flowers from August through to early October is a trusted and utterly reliable performer for a dark corner. 

‘Little Carlow’ will flower in light shade, and I have also planted ‘Elegans’ (a tall cultivar with small pale violet flowers) and cordifolius ‘Chieftain’ in my shady orchard borders and both of them are doing well.

All asters can be easily propagated by division. A dry day in early spring is ideal. Lift a clump and split them with a spade into segments about 15cm (6in) across. 

Plant these new divisions in small groups of three, five or seven directly into soil that has been enriched with garden compost, with each piece about 2ft from its neighbours. Water them well and mulch thickly. You can also take cuttings in spring from fresh growth, making the cut right at the base of the shoot.