With the onset of September comes the end of summer; that first nip in the early morning air; heavy dew on the grass and the children are back to school.

But that first feel of autumn doesn't mean it's all over in the garden; this is when the season can be extended with those valuable late-season perennials kicking to hopefully flower until the first frosts.

The aster, or Michaelmas daisy, (named after Michaelmas day on September 29) is the most widely known plant in this group and is available in a wide range of colours, commonly purple though incorporating mauve to pinks and whites. At Rosemoor we grow the self-supporting Aster umbellatus with small creamy white flowers and a yellow eye; this is a back-of-the-border plant of great merit.

An aster for the middle of the border is A. novae-angliae 'Mrs S T Wright' – a more traditional Michaelmas daisy with a large mauve flower and a yellow central eye. It is very upright in its habit, grows on substantial stems and easily bulks up to provide solid clumps of colour.

A shorter aster suitable for the front of the border, with a multitude of starry white flowers is A. lateriflorus var. horizontalis AGM. The foliage of this plant turns darker as the season progresses, contributing to the autumnal look of the border.

Another tall perennial for the back of the border is the aconitum, or Monk's Hood, so named because the flowers resemble the hood of a monk's habit. We have most success with self-supporting Aconitum carmichaelii (Arendsii Group) 'Arendsii' AGM. When the deep-blue flowers have finished, the seed heads dry on the plant, standing well into winter and turning a rich bronze colour. This plant associates well with the airy Bidens aurea with its simple, single, creamy yellow flowers; it is a bit of a spreader, but gives that natural feel to a border. A shorter cultivar is available called B. aurea 'Hannay's Lemon Drop' which has a golden yellow flower with white-tipped petals.

The dahlias flower well into autumn, especially if they are regularly dead-headed. Our hot garden couldn't be without the popular 'Bishop of Llandaff' AGM, with its clear red flowers carried above bronze foliage. Another star to emerge on our long borders is dahlia 'Chat Noir' – taller than the bishop and with velvety, dark rusty red flowers and a near-black centre over green foliage.

The kniphofias, or red hot pokers, are one of those plants you either love or hate. I wasn't a great fan of them, but they've grown on me over the years. They have a long flowering period and the first one to flower in early summer is Kniphofia 'Atlanta' then K. 'Wrexham Buttercup' and K. 'Prince Igor', followed by K. rooperi AGM, which is one of the last to flower in September. This is a stately, robust plant with large, globe-like flowers of orange and yellow and needs space to do itself justice; an architectural eye-catcher.

The persicarias, or knot weeds, are ideal for a more relaxed planting for a semi-shaded border or streamside planting. The self-supporting Persicaria amplexicaulis 'Firetail' AGM, as the name suggests, throws up spikes of red flowers over clumps of lush green foliage. This also comes in a subtle pink form – P. amplexicaulis 'Rosea' which associates well with a wide range of colours and is easy to place in the garden.

Another perennial that associates well with a more natural planting scheme is the toad lily, or Tricyrtis formosana AGM, which has erect stems of glossy foliage, topped with upward-facing pale pink flowers, spotted dark maroon like a spotted toad, hence the name. A very stately, classy plant.

Last but not least are the sedums and anemones. The sedum that first springs to mind is 'Autumn Joy', now called Sedum (Herbstfreude Group) 'Herbstfreude' AGM (I prefer the old name!) It is a clump-forming perennial with fleshy glaucous leaves topped with flat, plate-like clusters of star-shaped, deep pink flowers and is loved by butterflies.

Japanese anemones are erect perennials with suckering shoots and they work well both in a formal border or in a semi-shaded natural setting under trees, but they seem to flower best in the more open sunny positions. They are available in pink and white. This plant can be invasive in light soils, but here at Rosemoor we do not have that problem! The clumps of foliage are overtopped by wiry stems of simple flowers. Anemone x hybrida 'Honorine Jobert' AGM has a pure white, single flower with a golden centre; 'Queen Charlotte' has semi-double pale pink flowers.

So as August comes to an end, a new array of interesting plants spring into life in the garden. Let's hope the frosts hold off well into October to allow them to do themselves justice.