Within the genus Aster there are around 250 species of annuals, biennials, perennials and sub-shrubs. They hail from around the world and need every kind of habitat from arid or well-drained mountain slopes to moist woodland edges. Many favourites come from North America and the shrubby specimens are generally from South Africa.
Most of the asters we grow need a moist place in the sun and some are irresistibly beautiful. For charming subtlety, Little Carlow is hard to beat. This bushy, herbaceous perennial boasts thousands of clear blue flowers and fully merits its space.
Unfortunately, asters often fall victim to powdery mildew. This fungus is more prevalent later in the summer as the ground dries and moisture-loving asters will become stressed and are more likely to succumb to the disease.
These magnets for powdery mildew were especially unpopular with gardeners three or four decades ago. Robin Lane Fox, writing in 2010, said: "Thirty years ago, sensitive gardeners never even mentioned them. They were felt to be plants which were best reserved for the sort of bungalow which surrounds itself with purple dahlias."
Some aster hybrids are profoundly vulgar and would sit nicely with purple dahlias. Look no further than Moonshine Mixed, a gaudy rabble, each flower boasting distressingly unhappy colour combinations.
How can plant breeders bear to produce monstrosities like Moonshine Mixed from a family that includes the likes of Aster novi-belgii Audrey? This low-growing beauty produces a charming mass of small lavender-blue flowers, while tiny Shneekissen is carpeted in white. Despite their name, the massively popular novi-belgii varieties have no connection with Belgium. It was an early stab at botanical Latin, attempting a translation for New York. Novi-belgii varieties are still found growing wild along the eastern seaboard of the USA.
Although moisture-loving novi-belgii varieties can easily succumb to powdery mildew, other asters, like the Himalayan A thomsonii species, can tolerate much drier conditions than is often thought. A hundred years ago, the Swiss nurseryman Karl Frikart had also introduced this drought tolerance to a group of hybrids by crossing Aster thomsonii with A amellus.
Frikart named his new varieties after Swiss mountains and the best known is probably Aster x frikartii Monch. This classic Michaelmas daisy, with its pale lavender petals is an exuberant spreader. Last winter I had to dig up half the clump and dumped it in a dry corner beneath a willow. Against all the odds, it's still growing strongly and, incidentally, shows no sign of mildew.
Most asters are like Monch and have stout stems. They grow to around 90cm, but there are varieties to suit most parts of the garden. As its name suggests, A lateriflorus horizontalis provides good ground cover. Its wide-spreading branched sprays are covered with little white petals surrounding crimson centres.
Several other varieties are just as compact and low-growing as horizontalis and are, incidentally, ideal for containers. A thomsonii Nanus makes a bushy compact cushion, 45cm tall, with soft, ferny leaves and lots of small, star-like lilac flowers. But, if you've space for a larger specimen, try A ericoides Blue Star, the Heath aster from the American prairies, that has narrow, bright green leaves and tiny pale lavender flowers.
But, ever versatile, there are asters for the large border - ones that are tall enough to make a statement. An impressive perennial, A cordifolius Chieftain grows to nearly 5ft (1.5m). This tall, erect plant throws up delightful sprays of tiny pale blue flowers. You may just get away without staking Chieftain, but A novi-belgii Ada Ballard does have softer stems. Growing to almost 6ft (1.8m), fast-growing Ada needs support but rewards us with masses of violet-blue daisies. A delight in any border.