National Gardens Weekend is the perfect opportunity to pinch ideas for your own patch

It's National Gardens Weekend, says the NGS, whose famous yellow book lists nearly 4,000 gardens open to visitors; together, in the past 10 years, they've raised £22m for charity. Reading the entries in the guidebook, the come-on seems to be as much about lemon drizzle as lupins. Cakes are a must. Rain and wind might be lashing your borders but must never prevent you producing a decent coffee sponge. The latest one-upmanship is to bake the cakes with eggs from your own hens.

In this country (though probably nowhere else) it's a particular badge of honour to call your patch a "plantsman's garden". As gardeners, it's been our downfall as well as our delight to be able to grow plants from so many other places in the world. We have a temperate climate and plants are very forgiving. But our emphasis on plants has often been at the expense of the spaces in which they are put. We're not as comfortable with design as we are with decoration.

That applies to the gardens we visit, too. The writer Rory Stuart wonders why we don't approach gardens in the same way as we might an exhibition of sculpture, why we don't take more trouble to analyse why some things work and some, in our opinion, don't.

In his most recent book, What are Gardens For? (Frances Lincoln, £16.99) he's asking a question that he's not actually trying to answer. Instead, he's giving us a masterclass in garden visiting. He goes through some of the reasons that send us out at the weekends to prowl round other people's patches: we want to escape into a setting better than the one we live in; we need the particular kind of tranquillity and beauty that a good garden can give; we want to learn about plants and ways to put them together. He does not mention cake.

Most important, he feels, is the beneficial effect that visiting other gardens can have on our own. Stuart quotes the American garden designer Joe Eck: "The more one looks at gardens, actually or in books, and the more one thinks about them and tries to isolate what is pleasing about them (or not), the better one's own garden is likely to be."

The problem, and Stuart acknowledges it, is that gardens don't stand still. They work in a fourth dimension – time – as well as the usual three. All kinds of evanescent effects, the dry sound of a breeze rattling the leaves of a Magnolia delavayi, the shadow of a cloud moving over grass, the quality of the light, which in high summer is very different from the low, milky light cast in early autumn, all these things change the way we feel about a garden. And the way we are made, too, means we respond to some elements more easily than others. I'm predisposed to under-gardened gardens, ones where the owner isn't insisting too loudly where we must look.

But, whatever the style or size, there are certain questions we can ask ourselves of any garden we visit. How does it relate to the house (if at all)? How is the space divided up? How are the divisions between the spaces handled? Solid yew hedges? Roses on ropes? Are there enough spaces to breathe? I'd be screaming at Hidcote if there wasn't the big, calm green space of the Theatre Lawn to escape to, after the claustrophobia of the garden "rooms". How are the borders handled? "It is not enough merely to mass together plants of the same colour;" writes Stuart, "repeating the same plant to establish a theme is also important, and paying attention to the shape of the planting."

What Stuart does is coax us to think. He's had plenty of practice. Most of his working life was spent as a schoolmaster, until a maiden aunt (where have they gone, all these useful maiden aunts?) left him her cottage in the Cotswolds and he began to garden. He was 37. He left Westminster School, where he had been teaching English, and took up a part-time job in Cheltenham so that he could study garden design at Pershore. He's now based in Italy and earns his living as a garden designer and writer.

He's seen a lot, not only in Europe, but in India and China, too, and the gardens he uses as examples are mostly well-known ones such as Hidcote, Villa Lante in Italy, Wang Xi Yuan (the Master of the Fishing Nets garden) in Suzhou. Those aren't the kind of gardens most of us have, I observed, when we met earlier this year. "It doesn't matter," he replied robustly. "Even if you are just growing lettuces on a balcony, you can still try to make the most of the lettuces. Choose different colours. Different leaf shapes. Arrange the pots in a different way. Symmetry or asymmetry? Asking questions of the gardens you visit will alert you to many possibilities (and defects) in your own."

His own garden, he points out, is not big, lying on a steepish slope outside Rome, with a view of sunsets and the city. He understands the constraints that lack of space can impose. If he'd had more room, it would have been less formal, he said. He inherited two big Roman pines, but in laying out the rest, he has no doubt that he has benefited from all he has seen. "It's useful asking yourself why something works in a garden," he argues. "You might not be able (or even want) to copy it exactly but there will be a principle there that you can assimilate."

So if, today or tomorrow, you visit one of the 800 gardens that will be celebrating National Gardens Weekend, don't just make straight for the tea. Consider the plot (Stuart likes theatrical comparisons – at Uppingham he famously played Macbeth with Stephen Fry as First Witch). Is there drama? Is there suspense? Is there surprise? "That's all very well," grumbled the keeper of a National Trust garden to whom I was extolling the virtues of Stuart's book. "But with our visitors, the first question will inevitably be, 'Is there a loo?'."