Gardening: Colourful notes from the violas: Some are soloists, others play an accompanying role. Anna Pavord on the subtle virtues of a small treasure

Like many of life's enduring pleasures, violas might not grab you instantly. Growing little more than six inches off the ground, they are not built to be grabbers. They quietly enmesh you. And at the moment, nothing in the garden is giving me more pleasure than six big mats of violas which have taken over a space previously occupied by clumps of tulips.

The flowers sit like a well-drilled miniature rent-a-crowd, all gazing in the same direction, each bloom well-mannered enough not to get in the way of the one behind. They might be watching the Queen go by, a few rubber-neckers craning round the edge of the group for a better view.

Though small, violas are tenacious. The flowers, which start in May, will continue at full pelt until the end of the summer. All you have to do is to deadhead them occasionally.

All violas have five petals, two making rounded ears at the top, two making cheeks at the sides and one pouting into a chin at the bottom, but the way that the petals are put together gives each variety a completely different character. Some are moon-faced, the petals running together into a gentle, rounded shape. Others have narrower petals that make long rather peaky faces, as with my favourite, 'Ardross Gem'.

This viola spreads out a low mat of foliage before raising its flowers well above the leaves on wiry, 7in stems. The effect is quite different from that of the violas which like to have their leaves close to their flowers and which pull their foliage up around them like shawls.

'Ardross Gem' is also a particularly good colour; a rich mauve- blue splashed with yellow on the chin. The blue is clearer and brighter than that of most violas, and the cheeks are rayed with darker lines leading to the centre.

Violas seem to like the heavy, damp clay that underpins most of my garden. The best clumps grow on a bank sloping to the south, half-shaded, though not overhung by trees. 'Ardross Gem' flourishes there with a silver-leaved, mossy saxifrage and a dwarf pink geranium, G sanguineum striatum.

The plant I set on the bank has grown much bigger than one I planted at the same time in a 7in clay pot outside the back door. This is what you would expect, but even so, the potted plant has 35 flowers on it.

'Ardross Gem' works well in a pot. The mat of leaves spreads to cover the soil and stops it drying out; the flowers bob about in the clear space above. I would not keep a plant in a pot smaller than seven inches - it would dry out too quickly. Nor should the pot be in full sun. My back door faces north and the violas there brighten up the spaces between monochrome potted succulents such as Aeonium goochiae and Echeveria pallida.

Brighten is, perhaps, not the right word, because the colours of violas are generally smudged; the blues always on the purplish side, the yellows often overlaid on the backs of the petals with purple, which sucks the intensity out of the primary colour. Few of the flowers are pure selfs - the same colour all over. Even 'Haslemere', which seems to be a uniform dirty-mauve pink (a phenomenal colour), has dark-rayed whiskers leading to a tiny yellow eye.

'Haslemere' is good with the foliage of a hellebore such as 'Boughton Beauty'. Both enjoy the same kind of growing conditions and both have been painted from the same palette, the weird pink of the viola appearing in a deeper shade on the stems of the hellebore. 'Vita' is similar, perhaps a shade browner.

Violas are easy garden plants because they are tolerant of a wide range of growing conditions and make accommodating companions for plants that, as with the hellebores, have completely different flowering seasons. They can, for instance, usefully disguise the collapsing foliage of colchicums, which are looking at their worst just now.

But the colchicums' time will come when you shear back the violas for their winter rest, and the one show will drift seamlessly through to the next. Or should. I always leave the shearing-back too late, not wanting to waste the violas' autumn flowers. It is the only other job, besides the deadheading, that is vital to keep the plants on their feet.

The fact that you can never quite describe the exact colour of any particular viola is an advantage. Like chameleons' skins, their smudgy colours adapt to their company. The silvery mauve flowers of 'Maggie Mott' look cool with grey artemisias, but change their character completely when combined with the warmer shades of verbenas.

The original Maggie Mott lived at a house called Scotswood, in Sunningdale, Berkshire, where her family had a gardener who was a viola fancier. He named a seedling after his employer's daughter and showed it successfully at the Royal Horticultural Society. It appeared in the Society's viola trials in 1904 and has been in cultivation ever since.

It flourished in India in the 1930s, when the garden designer John Codrington used it in the grounds of the old Residency in Delhi. Sir Philip Chetwode, Commander-in-Chief of the British forces, who lived there, had grandiose ideas for a large circular pool at the end of a vista. Building the pool proved too expensive, so Codrington planted a huge circle of 'Maggie Mott' instead, the silvery colour giving the illusion from the old Residency windows that there really was water to complete the view.

By contrast, the darkest of the violas, perhaps the blackest of all flowers, is 'Molly Sanderson', which has petals with the sheen of viciously expensive satin. It is not as easy to place as other violas. Not long ago there was a vogue (fortunately brief) for all-black gardens, when you saw 'Molly Sanderson' combined with the black grass, ophiopogon, and the dark chocolate-scented cosmos. None gained from the association.

Don't be tempted to use this dark, saturated shade with white- flowered plants such as the low- growing Achillea huteri. The contrast is too obvious. Try it, instead, with the misty blue of a veronica such as 'Spode Blue' or 'Ionian Skies', or the magenta of a verbena.

When you are planting violas, think cool, think damp. They will not flourish in dry shade under trees, nor will they be happy in light soil in full sun. Mulch around plants improves them wonderfully, for it retains moisture in the soil and keeps the roots cool. Liquid feeds help too, particularly in bolstering up plants after you have sheared them down in autumn.

Unfortunately, slugs have a great fancy for violas, a fatal flaw in the so-called balance of nature. If they ate bindweed I would find it much easier to believe that we are all part of some grand design.

Violas are available by mail order from Elizabeth MacGregor, Ellenbank, Tongland Road, Kirkcudbright DG6 4UU (0557 330620). She sends plants out from March-May (catalogue 80p). Bouts Cottage Nurseries, Bouts Lane, Inkberrow, Worcestershire WR7 4HP (0386 792923) dispatch from September-October and also in April (send a stamped, addressed envelope for a catalogue).