Dahlia days: Autumn has come early this year, but there's one gloriously garish plant that's guaranteed to liven up even the drabbest of borders, says Emma Townshend

"Excuse me," says a lady visitor to Wisley, "can you show me the way to the dahlia test beds?" It's not your average Sunday afternoon out, but the idea of test beds is to see lots of different dahlias all in one place without the distractions of a garden to sway your decision-making. I was thinking of having a look myself, but got sidetracked enjoying the skill with which Wisley's gardeners have incorporated many of my favourite gaudy plants into their spectacular early autumn borders.

This is the time of year when subtropical plants come into their own. I stop to gawp at the absolutely scarlet canna Tchad because I have a passion for the big-leaved healthy greenness of tender plants. They always make me think of Eric Carle's children's classic The Very Hungry Caterpillar, and all the merits of a nice green leaf. As autumn seems to have started this year without asking permission, the plants' splendour is doubly welcome. Subtropical plants will still produce their tightly bound, optimistic flower spikes right into autumn, offering a real spiritual lift to those of us who feel low at the idea of the year ebbing away.

In my garden, the canna lilies are just getting going, and elsewhere huge hand-sized dahlias are showing off. I can't help it; I know I should like tasteful things but I just don't get any satisfaction out of them.

And so to the dahlias. In the continued absence of a What Dahlia? magazine, you have two choices for checking out plants. If you prefer the test-bed approach, you could start with Wisley. But those gluttons for whom Wisley is simply not enough should consider a trip to Aylett Nurseries's annual dahlia festival, which starts today and runs until 23 September. Aylett is near St Albans and the M25, and the festival is well worth a visit.

But many gardeners prefer to see how plants work in proper flowerbeds. And lots of us would confess privately not just to pinching ideas about single plants, but even whole groupings of colour and shape. Last weekend I made the trek to the south coast to visit Meon Orchard, a private garden whose few openings for the NGS each year are worthwhile enough to earn the garden almost two pages in the RHS Garden Finder. There were plenty of good ideas to steal. Meon Orchard's owners, Dr and Mrs Smith, have a national collection of eucalyptus, consisting of many different species of the tree growing for you to compare. They also grow plenty of canna, dahlias and hedychium, the so-called "hardy" gingers. Every corner is crammed with plants, rammed in their pots into flowerbeds straight out of the greenhouse, and perched on walls and stonework. There is lots of helpful labelling, and Dr Smith bounds up with the energy of a happy spaniel to answer queries.

Walking underneath the eucalyptus canopy and brushing against banana leaves, you could imagine you were thousands of miles away, though the garden is on a miniature scale and ends with views of some very English fields.

Meon Orchard also hosted a small rare plant sale, and I got into conversation with Julian Sutton of Desirable Plants in Devon about a plant I'd bought someone else. My rhizome of Hedychium gardnerianum came from a source I will not name at a May RHS fair, and it still hasn't sprouted. "Oh dear," says Sutton, "I don't think it should have been sold in that state. Without any roots? Sadly it was probably imported with thousands of others in a shipping container, dug up by some poor farmer in India. The plant's probably still in shock."

Julian Sutton manages to be scathing about the sales technique of his competitor and deeply informative about my plant, giving me careful cultivation instructions when I hadn't even bought anything from him. However, that was about to change, and I took home a healthy ginger lily with buds about to open. On the way back down the drive, I passed some other garden visitors on their way home. There's nothing like the wry smile you meet on the face of someone else taking home plants from a plant sale. "Ah, we've done it again," they seem to be saying, "and so have you. How will we ever find the room?"n