Picture perfect: Try picking up a camera to find the weak spots in your garden

I don't have 50 varieties of clematis, or great wafts of lavender and roses, or a herbaceous border that looks as if it was planted by Gertrude Jekyll or Vita Sackville-West. My garden, I hope, does something rather different. It's designed to make people think they're on holiday, to take them into another world.

To that extent, it is a conceptual garden, but I'm wary of using that word because it tends to send gardeners running for the gate. It has come to mean 'wacky' or 'avant-garde' or in some way inimicable. Personally I agree with landscape critic Tim Richardson's definition, which is: a design in which a single idea, or concept, is used to underpin everything.

My garden is unashamedly exotic and un-English. It's not specifically Australasian, or Oriental, or South American, though it contains plants and ideas from all those places. Purists might complain that there is no biogeographical logic to it – plants from China sit alongside those from New Zealand and Mexico.

The native plant lobby might complain that such heavy use of exotics is potentially damaging to the ecosystem, but all of my plants are chosen to suit the conditions in my garden, which is invariably full of birds and bees, butterflies and frogs.

I like the physical act of gardening, and I do conventional gardening things such as taking cuttings and growing things from seed. However, my passion is not so much for growing plants as for using them to create an atmosphere – like designing a set for a theatrical production.

The aim is a Rousseau-esque jungle of foliage in shades of green, chartreuse, copper, blue-grey, and the huge burgundy leaves of Ensete ventricosum 'Maurelii', or red banana. The end of the garden is dominated by a huge dark-leaved flowering cherry, so while it's not the most classic colour scheme, I find myself coming back to it again and again. The brooding presence of the cherry needs the injection of bright leaf colour, to give the impression of dappled sunlight rather than unrelieved shady gloom.

There are flowers, of course – the brilliant orange of cannas and dahlias, and the firecracker colours of crocosmia and geraniums. But it is the foliage that underpins the garden, in the way that the string section underpins an orchestra.

One of the lessons my garden has taught me is that you don't necessarily have to be completely in love with everything you plant – although that frequently ends up being the case. I often hear people say that they don't like grasses, or topiary, or conifers. But sometimes, you need to think outside your prejudices in order to provide contrast and texture. Just as in the theatre, there are star performers and a supporting cast.

Being a 'set designer' gardener involves a lot of looking. You have to develop what Mary Keen, writing for the website thinkingardens.co.uk, calls the "basilisk stare", which means scrutinising the overall effect to ensure that nothing is out of place, or out of proportion – or just plain crooked.

Of all the tools in the batterie de jardin, the camera is the most useful for this. I like to feel that from every angle of my garden, there is something to look at: a vista, if you like. However, my eye is not so much basilisk as Photoshop. It's very good at editing out things that my brain regards as tedious or irrelevant, such as weeds or dead leaves. The camera doesn't do this. It simply shows you what is there, not always in a very flattering light (though I find that if you underexpose, or lower the ISO, at least the light looks better).

Better still, you can tinker with the photo in order to look at other aspects, such as texture. My favourite tip is from the garden writer Noel Kingsbury, who recommends greyscaling a picture of your garden. This takes away the distraction of colour and allows you to see what sort of contrast there is between the plants.

Are there lots and lots of small leaves and petals, which have the effect of looking a bit like one big blur? Or is there a good mixture of leaf and flower shapes – feathery, spiky, small and big? I often print out the picture on a sheet of paper and scribble on it, experimenting with shapes and heights to see what works best.

Visiting other gardens is an important part of developing the ability to see, in the same way that art students visit exhibitions. I try not to be distracted by comparisons with my garden ("Goodness, my eucomis are in much better shape than theirs"), but look at the way plants are combined, or containers are used, or how focal points are achieved.

I found another piece of good advice in Catherine Horwood's book, Gardening Women (Virago Press, £20). She quotes Lady Heathcoat-Amory, née Joyce Wethered, who with her husband created the garden at Knightshayes Court in Devon.

"A rule we have always kept, which I think is a good one," said the woman who coincidentally was one of Britain's greatest female golfers, "is that if a group of plants is continually passed, or bypassed without being looked at, then this area is obviously dull and must be altered to attract attention."

It sounds simple, but it's very difficult to do in your own garden, particularly if you are stubbornly devoted to a particular arrangement or colour scheme. For ages, I planted one bit of my garden with yellow-and-green striped cannas, blue agapanthus and yellow crocosmia. In my head, it sounded good. In photographs, it always looked underwhelming.

I've now changed it and have 'Durban' cannas, with their red- and burgundy-striped leaves, sunset-coloured 'David Howard' dahlias, blue agapanthus, lilac Verbena bonariensis and the vibrant purple flowers of Buddleja davidii 'Black Knight'. It may sound horrible, but it looks good.

Part of the difficulty with developing a "basilisk stare" is that we British tend to be polite about gardens, rather than critical. In the five years that I have opened my garden for the National Gardens Scheme, no visitor has ever said anything disparaging about it. I'm glad they haven't, because after the effort of getting it ready, and baking 15 cakes to serve with tea, I'd probably burst into tears.

On the other hand, I don't think for one minute that all the visitors are uncritical. I know that when I visit gardens that are open to the public, I'm conscious of an internal running commentary that analyses whether focal points work, whether the 'garden art' is verging on the over-whimsical or twee, or whether the planting is heart-stopping or just ho-hum.

How can we employ the internal critic to improve our own gardens? Just as the camera is never mentioned in lists of essential garden tools, I think emotion is often overlooked as a source of inspiration and a method of analysis.

A garden is more than just a collection of plants in earth, in the same way that a painting is more than just a collection of brushstrokes on a canvas. Some gardens are peaceful and relaxing places, some – designed to show off their owner's wealth and power – can you make you feel almost intimidated. Others can make you laugh out loud.

Think about how your garden makes you feel. If a certain part of it – all of it? – makes your heart sink, then that's a strong indication that you need to change it. If it makes you feel better just to step out into it, it obviously works for you.

I hope my garden works for you, too.

Victoria Summerley's garden is open under the National Gardens Scheme tomorrow from 2-6pm at 28 Multon Road, London SW18 3LH. Admission is £3, children free. All proceeds go to the NGS, which raises money for a range of charities including Macmillan Cancer Support.