Gardening: Silent stream

I imagine I'm not the only person who has looked out at a waterlogged garden through rain-spattered windows over the past few weeks, finding consolation only in grimly ironic thoughts about "the new Mediterranean climate". But there are more positive ways of reacting - and getting stuck into a big project in the garden is one of the best. Particularly if it involves a bit of eccentric fun, hints optimistically at the need for a cool and shady retreat and calls for shell- or fossil-hunting expeditions.

You've done Project Pergola, Project Pool, Project Gazebo; but this is no time for retirement, as the best is yet to come: Project Grotto. (And "grotto" is such a nice word to roll around your mouth that half the pleasure of making one must come from being able to give frequent exercise to the word in conversation.)

The first grottoes were found rather than made: caverns adjacent to or containing a sacred spring, where nymphs and muses resided and sibyls offered their consultancy services. But the ancients were enthusiastic builders of artificial grottoes and fountains in their gardens, too, although tastes were divided between the rustic look (lumpy tufa, shells and coral) and the architectural look (hewn stone, vaults and mosaics). This division seems to have permeated all the great eras and centres of grotto-building - Renaissance Italy, Baroque France, Rococo Germany, and England, where the grotto flourished as both a neo-classical and a Victorian Gothic phenomenon. But it's not a terribly important division; whichever taste you adhered to, the decoration was to be as fantastic as your purse would allow.

"It is a place capable of giving you so much pleasure and delight," wrote John Woolridge in 1677, "that you may bestow not undeservedly what cost you please on it." There were indeed a number of no-expense-spared grottoes produced around that time, in which the spiritual dimension became slightly submerged by all the extravagant artistry. Or, taking off on another kind of tangent, Pope's grotto in his garden at Twickenham ended up as a kind of mineral museum, with rare stalactites and chunks of the Giants Causeway lining the walls.

Perhaps these should be read as cautionary tales, because grottomania is on the brink of making a comeback. Possibly it's because the message has got through that gardens are for fun, for experiment and imagination, in terms of both the plants and the architecture they support. The fabulous new grotto that opened in 1987 at Leeds Castle, in Kent, started the ball rolling. The entrance is at the centre of a maze and the subterranean chamber leads you back to the outside of it, a clever conceit. And I keep hearing of gardens in private ownership where grottoes are under consideration or even construction. The funny thing is that people are often quite coy about wanting to have a grotto; inevitably, I've heard it said that a grotto marks you down as an extravagant eccentric, and the associations with spiritual mystery still cling from the earliest days.

I'm not so sure; it would take only the tiniest bit of spin-doctoring to present the grotto as a cool (in both senses of the word) adjunct to the New Garden. Take Candace Dahouth's work, for example.

Dahouth is an American designer, whose mosaics and needlepoint tapestries have won her great acclaim. She is currently engaged on the interior decoration of a newly completed architectural grotto for a private client. The construction is hexagonal, focusing on a mirrored basin into which flows water from surrounding temples. The vaulted ceiling is medieval blue, decorated with gold stars and clusters of shells at the base of the ribs. Fragments of mirror have been fitted amongst the pebbles and shells (her clients have amassed thousands from all over the world in the past 10 years) which cover the walls in an abstract, textural pattern; the floor is made from artificial black ammonites. Candace Dahouth has spent the best part of a year working on this extraordinary project, which is now nearing completion. By candlelight, the tiny shards of mirror in the encrusted interior should make for a Baroque experience for which even Coleridge might have struggled to find words.

You could see that the two farmers standing on the opposite bank of the river that forms the boundary of Margaret Morgan-Grenville's garden were struggling to find the right words to describe what, in its undecorated condition, resembled a pedimented sentry-box, constructed from breeze blocks, fitted snugly into the bank on her side of the river. "We've been wondering what you're building there," one of them finally hazarded across the water, "and we've come to the conclusion that it's a fishing-hut."

Margaret, a self-confessed grotto-lover, decided to build her own as an alternative to writing a book about them. She had started to research the subject when the idea came to her that it would be "much more fun" to build one and decorate it herself, than to write about other people's grottoes.

Her grotto faces west and has been placed to make the most of the evening sun, rather than to provide a shady retreat. The fossils with which she is covering the walls have come from local beaches (Lyme Regis is a short drive away) and piles of broken and whole ammonites and belemnites are being transformed into a crusty mosaic. All that it lacks at present is a motto, to be woven into the pattern. Any thoughts?

Candace Dahouth, Ebenezer, Pilton, Somerset BA4 4BR (01749 890 433)

Anna Pavord returns next week

Weekend work

Plant or transplant lily of the valley: the leaves may turn brown within a week, but they will produce the goods next year.

Organise stakes and supports for perennials prone to floppiness or exposed to wind.

Trim back silvery green-leaved shrubs such as santolina or lavender artemisia, to avoid untidy, leggy growth later in the season.

Lawns that have been conspicuously waterlogged over the past few weeks will need spiking.