One of the classic harbingers of autumn is a bed of purple-mauve Michaelmas daisies. Red and white varieties are available, but the purple ones with yellow centres are the kind everyone knows. They are, indeed, likely to be flowering on Michaelmas (29 September) though they are not, in fact, daisies but a variety of aster. There is precious little choice of easy-going perennials that flower at this time of year, but all the same, approach these with care. Gertrude Jekyll wrote that her own Michaelmas daisies "have, as they well deserve to have, a garden to themselves... where the flowers are in glad spring-like profusion, when all else is on the verge of death and decay". Glad spring-like profusion is all very well if you have a stately-home sized plot to play with. In more limited acreage, turn your back and these will take over, getting spindlier every year. Keep them firmly in their place.