If you arrive as a new owner of an old garden, do not be too quick to condemn existing plants. They may need pruning. They probably need feeding. But at least they are proving that they can grow. The fact that you have not put them there is not, on its own, a good enough reason for taking them to the tip. In many ways it is easier to have one or two ingredients to work with than face the daunting bareness of a brand new patch.
When choosing plants to furnish the garden, keep a few principles in mind. Will the plants have a reasonable chance of surviving in the place you have in mind (no rhododendrons in lime-ridden patches)? Will your chosen clutch perform at different seasons of the year, rather than racing out together in spring?
With the rising of the sap in spring, there is a corresponding stampede to the garden centre. The overflow car park was heaving at our local centre this week. Plants beckoned on every side. Without a plan, you tend to pick up only the things with flowers on them. The danger is that after a blazing spring, your garden will have no space for anything to perform for the rest of the year.
Plants already flowering in their containers are not, in any case, best buys. All plants find it easier to establish themselves below ground if they do not have to sustain a display above ground at the same time. If you can bear it, choose plants for their general shapeliness and balance rather than because they happen to be flowering.
Aim for balance, too, when you are planting your garden: balance between evergreens and deciduous plants; between plants that have their heads in the clouds and those that sprawl at ground level. Think about the form and texture of the foliage as well. Flowers may arouse the greatest passions, but foliage is more sustaining.
The showiest plants do not necessarily provide the best basic furnishing. Forsythia catches all eyes in spring, but its habit is ungainly and its leaves coarse and boring. What is this shrub going to do for you once its one and only trick is over? Zilch. The smaller the garden, the more critical you need to be of a plant's overall performance.
Some plants need to be like the good sofa that interior-design gurus tell us will hold the rest of the room together. The fact that my favourite sofa is quietly exploding in our sitting room does not blind me to the merits of unpretentious evergreen shrubs such as osmanthus, covered now with sweet smelling flowers. It is always handsome, clippable (if that is what you want), and presents a fine background for later slashes of brilliant colour from lilies, zinnias or red hot pokers.
What you want most of all, especially in a small garden, are plants that pay rent more than once a year. This might be with berries or fruit, as showy as the flowers have been in their season. There may be seedheads, like the allium's drumheads, that only a vandal would cut down rather than keep. The plant might provide a wave of autumn colour before it finally packs itself away for the year. It may have bark (as with willows, dogwoods and acers) that gives the garden a dramatic new dimension in winter.
But how does one learn about these paragons? The answers to many questions lie in my colleague Ursula Buchan's new book, Plants for All Seasons, where you will find more than 80 plants chosen for their chameleon qualities. It covers trees, shrubs, climbers and perennials. This is a wide brief, but makes the book all the more useful for gardeners. At one moment you are considering the merits of a tree such as Cornus `Eddie's White Wonder', which will eventually spread at least 15ft wide. At the next you may be rolling round the possibilities of Paeonia mlokosewitschii, which rarely makes a clump bigger than two feet high and wide.
Ms Buchan is a practical gardener, so she also points out what her chosen plants need by way of soil, aspect and climate. The photographs that accompany her text made me fall in love all over again with `Eddie's White Wonder'. In May it is covered in showy white flowers that are actually bracts. The real flowers are in the centre of the bracts, like knobby little hairpins holding the bracts together. In autumn the foliage blazes up into a final fire of crimson, yellow and gold.
Sensibly, Ms Buchan warns that "they are not suitable for all situations". They need shelter from wind and late spring frosts. They will only perform well on fertile, well drained, neutral-to-acid soil. It is the last, intractable problem that has prevented me from planting this cornus in the past. I was glad to be reminded by the author that it would be cruel in the extreme to introduce it to our cold, heavy, alkaline clay.
The peony I already have. There, the problem lies not in growing it successfully, but in steeling oneself to say its name properly. Like most other people I know who have it, I cheat and call it "Molly the Witch". The foliage now is extraordinary: a greyish, pinkish, greenish bronze, with a bloom on the back of the leaves that makes them look as precious as a Persian silk carpet. The flowers (they'll appear later this month) are a particularly bitter yellow. Even those finely tuned souls who generally shudder at yellow have to admit that this is different. And very good.
The directory of plants takes up the largest part of the book, but the first third covers more general guidelines to choosing plants. Think about foliage, writes Ms Buchan. Think about texture.
Push your mind beyond a plant's flowering period to ask yourself what happens after. Think about a plant's habit of growth and whether that is likely to change as it ages. This section is rich in the kind of advice that is only given by gardeners who do their own gardening.
Groundcover is a case in point. The coverall term can apply as easily to wintergreen (Gaultheria procumbens) as it can to barrenworts (the Epimedium tribe).
But, as Ms Buchan points out in her book, "the former creates a darker, more sombre atmosphere than the latter. They are not interchangeable plants just because they like the same conditions."
Nor does she forget that a good garden also needs some purely evanescent plantings, such as annual flowers and bulbs provide. You could make a garden entirely of annual flowers, but this would be the equivalent of furnishing a room only with brightly coloured cushions. It is gay, but most appropriate for those just passing through, as students do in rented flats.
Nevertheless, annuals, small seasonal perennials, and bulbs all have an important part to play in a garden, furnishing the bottom layer of a three-tiered planting scheme. Use them to build up contrasts of colour around the more solid furniture of shrubs and small trees. Now attack the garden centre. But take this book with you.
`Plants for All Seasons' by Ursula Buchan with photographs by Howard Rice is published by Mitchell Beazley, pounds 16.99Reuse content