Gardening: Stars of the variety show - How do you find the best weigela? No problem, says Anna Pavord. There is a useful new plant guide which tells you

One of my mother-in-law's best stories is about the day she went to buy some lavatory paper and came back with a second-hand car. She never makes the mistake of trying to justify such incidents, serenely accepting her position as queen of the impulse purchase. The car, as it happens, went beautifully until a bollard in the doctor's car park got in its way. 'Such a ridiculous place to put a bollard,' said my mother-in-law.

She sits at the opposite extreme to the consumer freaks who read several years' worth of Which? magazines before chancing their arms on a transistor radio. Most of us swill around somewhere between the two. The cheaper the product, the more likely we are to allow impulse to dictate the outcome.

When buying plants, impulse should if possible be tempered with forethought - enough to ensure that the plant is going to be as happy as you are. You need to know that you have the right kind of soil to grow it and sufficient space for it to have some hope of reaching maturity.

After that, choice becomes more complicated. Is mock orange preferable to lilac? Should there be an apple or a pear tree in the middle of the lawn? Each of us carries around a ragbag of memories - perhaps the brilliant outline of ceanothus blossom against a cloud, or the smell of a ripe plum - that predisposes us to like one thing above another.

I have a deeply rooted dislike of weigelas, which I remember pressing through the wire-mesh fence of a house where I sometimes had to deliver messages. A particularly terrifying pink-nosed bull terrier used to charge all-comers there, and the pink of weigela blossom still reminds me of the pink of that hideous dog.

But say that you had survived your childhood without acquiring this irrational hatred; say that you had decided you must have a weigela in your front garden. How do you then decide which weigela?

The garden centre may decide for you, by offering only one variety. 'We don't get a call for anything else,' is the usual response if you suggest that life has more to offer than Weigela 'Bristol Ruby'.

The Plant Finder lists 49 varieties of weigela available in the UK, but your task would hardly be made any easier if all 49 were lined up in front of you. Some, surely, must behave better in gardens than others? Some must be less prone to fall ill? They do. They are.

To find out which, you need a copy of the Royal Horticultural Society's new list of plants that have won its Award of Garden Merit (AGM).

The list, which will be updated annually, should have appeared long ago, for it is badly needed. The RHS is impartial, in the commercial sense, and the plant trials it has organised for almost as long as it has existed have important implications for the amateur gardener. Until now, the results have not been easily available.

With this list, you can quickly whittle down the 49 available types of weigela to the six that the RHS recommends for garden use. 'Abel Carriere' has dark buds that open to only slightly less dark pink flowers, fading as they age. 'Foliis Purpureis' is an extremely popular, compact type with purplish-green leaves setting off pale-pink flowers. 'Mont Blanc', raised in France in 1898, is the choice among the white-flowered weigelas, and the RHS includes two with variegated leaves, W. florida 'Variegata' and W. praecox 'Variegata', the variegation in both slightly too yellow for the pink flowers.

Its sixth choice, 'Fiesta', is a hybrid between 'Eva Rathke' and 'Newport Red', with long, lax branches thickly covered in deep pink-red flowers. Oddly, it is not listed in The Plant Finder, but exists in the national collection of weigelas at the City of Sheffield Botanic Garden. (I should go there to be cured of my aversion.)

Availability is one of the criteria by which plants are judged for the award. With 'Fiesta', the RHS is stretching the word to include availability for propagation. But even if some enterprising nurseryman decided to offer it, it would take a while for the plants to percolate to nurseries and garden centres.

The most important yardstick in the trials is general excellence. The plants on the list are the ones the RHS considers the best of their kind for general garden use. They have strong constitutions, are not particularly susceptible to pest or disease and are unlikely to revert. Nor do they need highly specialised care. They will not necessarily be the plants that a seasoned gardener would consider the most beautiful of their kind, but they will be healthy and reliable.

The list is heavy on rhododendrons, a reflection of a long bias at the RHS in favour of woodland gardens, and surprisingly light on hybrid tea roses. Of the hundreds available, only 18 are listed. I will not lose any sleep over that.

Half of the recommended varieties were bred in the Eighties; only two of any age have kept their place among the best. 'Mrs Oakley Fisher' was bred by Cants in 1921 and has large clusters of single flowers, deep buff yellow with amber stamens. The foliage is lustrous and healthy. No one can remember who its parents were.

And 'Peace' is still there, probably the one HT rose that everyone can recognise at a hundred paces, for its size if nothing else. It grows to at least 4ft and has huge flowers of clear pale yellow tinged with pink. It does not always flower freely, nor is the smell anything to remember, but it is certainly vigorous. Its name celebrates the year it was bred - at the end of the Second World War.

If you have your own firm, even cranky, opinions about particular plants, the RHS list is unlikely to shift them. But it is invaluable in helping to choose a member of a family about which you know nothing.

Say you had decided that the one tree in your garden was going to be a crab apple (a good choice since you get autumn fruit as well as spring blossom). Trial and error might bring you to the same conclusions as the RHS selectors, but what a lot of time you would have wasted, growing varieties that perhaps were susceptible to scab or of weak constitution.

The list immediately leads you to the 13 varieties that experts judge to be the best. Where space is tight, you may go for 'Evereste', which grows to only one-third the usual size and is smothered in spring with pinkish-white flowers.

All types of plants are covered in the list: trees and shrubs, climbers, conifers, bamboos, herbaceous perennials, alpine plants, plants for conservatories, fruit and vegetables. The fact that a plant does not appear on the list does not mean, of course, that you should never grow it.

None of my favourite tulips has been awarded the AGM accolade, but I would not be without them. Their colours are worth suffering for.

You may feel the same about a tomato which, although lacking in vigour, grows fruit sweeter than anything you have tried. Where your own tastes deviate from the list, it should be for a specific reason. New gardeners will find it an indispensable prop.

The Award of Garden Merit Plants, published by the RHS ( pounds 3.50 in paperback), is available from the RHS garden's shop at Wisley, Surrey, or by mail order (0483 211113).