Gardening: The colour purple comes to the Great Autumn Show: Anna Pavord invites readers to join us at a private view of the RHS's premier autumn event, and notes the best-dressed gardens are wearing mauve

The difficulty with trends is that by the time you notice them, the people who set them have usually moved on to something else. So I am cautious about this business of purple, but there does seem to be an awful lot of it about this summer. The colour has always been around, of course, but we were not supposed to like it. Pastels have been the thing. Drifts of grey. Billows of pink. Enough white to fill Sissinghurst a thousand times over. Somnambulist gardening.

White appeals to those who like everything to be neat and tidy, according to the National Garden Bureau in the United States, which has been exploring the psychology of colour in gardens. It has nothing to say about purple, but it may have been censored. There is something altogether more passionate about purple than pink.

The two go well together, provided they are deep and saturated. Both, too, are good with rich royal blues. An excellent combination of these colours was used in Dan Pearson's garden for the London Evening Standard at Chelsea this year. He had stands of hairy anchusa with searingly blue flowers combined with the deep, velvety, purple flowers of the old moss rose 'William Lobb'. The anchusa is an unreliable perennial, but when you see well- grown plants, as these were, they are not easily forgotten.

Mr Pearson also used a purple bearded iris, so deep it was almost black. 'Langport Wren' would give the right effect or, if you want something taller, 'Swazi Princess' or 'Titan's Glory'.

Giant alliums were the other important ingredient in this group, footballs of purple flowers on stiff 4ft stems, with wisps of deep bronze fennel filling in the few gaps. In the foreground were clumps of French lavender, Lavandula stoechas, which is only worth growing in warm, fairly dry gardens. The flower stems are short but the spikes have much showier, deep purple flowers than English lavender.

At this time of the year, you need to look to different flowers to create the same rich effect. There is buddleia, although when you mention it, you are immediately cut off by the impatient groans of those who consider it fit only for railway embankments.

But you do not have to make excuses for buddleia and say you planted it only for the butterflies. 'Black Knight' has rich, lustrous, deep purple cones of flower, a great asset at this time of year. There is nothing much to extol in a buddleia's habit or foliage, but nor is there in salvias, which are everybody's darlings.

In front of the buddleia, you could have a stand of tall Verbena bonariensis which is purple, but paler than the buddleia, all stem and no leaf. The thin stems branch in an angular way, and at the end of each branch is a tuft of flowers. They start coming out at the end of June and are still performing in September. Although it is tall (up to 5ft), it is so wiry and insubstantial that you can see through it easily, and it is just as effective at the front of a border as further back.

Among the stems of verbena you could use one of the scrambling purple-magenta geraniums, such as 'Russell Prichard' or 'Ann Folkard', which would thicken up the mass but not get in the way of the verbena's flowers. Few garden plants have such a long season of flower as 'Russell Prichard'. It starts in June and continues until autumn, when the long, scrambling side branches die back to the compact central crown.

Where there is space for it to spread its wide wands of flower, use dierama by the verbena, or instead of it. You must get the tall kind, D. pulcherrimum with flowering stems that make great arcs bending away from the central clump of grassy leaves. The flowers are magenta bells, though they are purple in bud and purple again as they die. The buds emerge from dry, buff paper cases and hang on threads so fine that you cannot see them.

Purple leaves can be even more sumptuous than purple flowers, though I am not sure I could ever bring myself to plant a purple sycamore or even a beech. The beech looks glorious in spring when the leaves have a bronze translucence about them, but the colour can seem heavy and dead as the summer wears on.

Shrubs such as purple cotinus, berberis and the purple nut, Corylus maxima 'Purpurea', are likely to be the most useful because they all respond well to heavy pruning. This stimulates them to produce fresh growth and new foliage, and stops them taking up too much space.

'Useful' is a damning phrase. I do not mean it like that. Well, not entirely. But none of that trio can hold a candle to the purple vine, Vitis vinifera 'Purpurea', the colour of dusty old velvet curtains fading in the sun. It will cover an arbour, climb a pergola pole, straddle itself against a wall, thread its way through a decorative tripod.

Although there is nothing wrong with using it on its own, it also makes a splendid foil for purple clematis, perhaps 'Etoile Violette' whose flowers are just the right size for the vine leaves. The vine becomes more purple as the season advances, then suddenly by autumn it flares in a wild blaze of crimson. When the leaves have fallen, you can shape it by pruning.

This is one of the plants that Paul Whittaker of P W Plants, in Kenninghall, Norfolk, will be bringing to the Royal Horticultural Society's Great Autumn Show next month. Last year we held a private view for Independent readers on the first evening. Tickets were limited and there were several hundred too many applicants.

This year we have organised another private view, again on the first evening of the show, and I hope those people who were disappointed last year will have better luck this time. Apply early, using the coupon below. All those things you were told as a child about early birds and worms are unfortunately true.

The show will provide the perfect milieu for finding out whether purple is indeed on the up. Mr Whittaker is not sure that it is. 'I'm more of a green man myself,' he says. But that is because he is nuts about bamboos, of which he will be bringing plenty to the show. Unlike at Chelsea, you can buy here as well as look.

He will also be bringing some of his favourite grasses. 'Stipa arundinacea - everyone ought to have it. Miscanthus 'Strictus' - so much more refined than 'Zebrina'.' It is always comforting to be in the company of people with strong opinions, and his are worth listening to. He won a gold medal for his stand at the Hampton Court Show last month.

There will also be grasses on the Hop Shop's stand at the Great Autumn Show, but dried rather than alive. Caroline Alexander, who grows vast quantities of flowers, herbs and grasses for drying at Castle Farm, near Sevenoaks in Kent, will be bringing about 70 different varieties: dried peonies and sunflowers, artichokes, helichrysum, millet, roses and, of course, hops - all of which are air- dried in kilns at the farm.

The Hop Shop's Chelsea display, the first time that dried flowers have been allowed inside the hallowed Great Marquee, was awarded a gold medal this year.

Bibliophiles should look out for Mike Park's stand of second-hand books. After Anthony Huxley's death last December, Mr Park bought most of his extensive library. Mr Huxley was a great plantsman, a prolific writer and editor of the RHS's massive Encyclopaedia of Gardening.

This stall is where you will most probably find me, loitering with intent, looking, as always, for more books on tulips.

(Photograph omitted)