Chances are that you turned on the television sometime in the last week and found yourself watching a garden makeover programme. In double quick time, a sensational, action-packed transformation takes place. The erstwhile, scrappy back garden becomes a tropical paradise, a formal French garden or a bit of inland seaside, complete with Victorian bathing hut. The programme-makers - if pressed - would admit that this is more about entertainment than gardening. But for many viewers, it raises hopes that it is possible to magically acquire a perfect, mature garden, installed overnight by a band of big-hearted garden professionals.

Real life is somewhat different. Most garden designers have a two-month waiting list at least, while some are booked up into March or April of next year. And the instant garden, as seen on television, is often designed to be experienced through a camera lens, for a limited time only.

Koraley Northen, the administrator of the Garden and Landscape Designers Association, says, "a good designer could make a garden that will look mature in a year. But he or she would need to be an expert plantsman and know what the rules are and how to bend them".

During an initial consultation, the designer will "ascertain your likes and dislikes, carry out a design survey, take a note of the soil and prevailing winds, check out the boundaries and changes of level, analyse shadow patterns, and log and evaluate existing plantings". Usually, a designer will present a concept plan, and possibly a planting list. Some clients take over at this point, choosing to do the work themselves.

This can be a remarkably rewarding experience, says Koraley. "Watching a plant grow that you put in yourself is very satisfying, almost like watching a family grow." The financial cost of such an exercise is likely to be less, and can be spread out over a long period. Most garden designers charge around £250 or £300 a day, plus expenses and VAT. As a rough guide, a detailed concept plan with a planting list may cost from £600 to £1,000, for an average suburban front garden. An alternative approach is to get the garden designer to oversee the entire job and to liaise with the landscaping contractor. Usually in this case, the designer charges a fee of around 10 or 20 per cent of the project. Garden designer Angela Binchy specialises in large projects such as stud farms and country houses. Owners of stud farms, she says, often opt for planting mature trees and shrubs. "They might have clients coming at a certain time of the year, and they want to make an instant impression." And in the domestic garden, the use of instant hedging and semi-mature trees is becoming more common, she says. "Certainly, if you are visualising selling on in the near future, it's well worth having the garden properly designed from the start, and putting in more mature plants."

The cost of using mature trees and shrubs will add about an extra 50 per cent on to the final price of a garden project, says Angela.

Gabriele Sanio, who redesigns readers' gardens in the pages of The Irish Garden magazine, cautions that "the mature garden doesn't stop there. It keeps maturing!" There's no such thing as a no-maintenance garden, she says.

At the National Garden Exhibition Centre in Kilquade, Co Wicklow, there are 20 display gardens by a large selection of Irish designers. Suzanne Wallis, who owns the centre with her husband, Tim, says homeowners are "very much into using bigger, better quality plants now". Large, architectural specimens, such as chusan palms and cordylines, are especially in demand, and cost £150 each. "Money doesn't seem to be an object at the moment!"

It's worth noting though, that many mature specimens are often outstripped by more vigorous, younger plants, so that if you're prepared to wait a few years, you will achieve a nicely balanced, mellow garden. And during that waiting period, says Koraley Northen of the GLDA, "there comes an enormous sense of achievement, satisfaction and pride. These are not off-the-shelf commodities. Like love, they can't be bought."

The Garden and Landscape Designers Association, 73 Deerpark Road, Mount Merrion, Co Dublin. Telephone: 01-2781824; fax: 01-2835724.

The National Garden Exhibition Centre, Kilquade, Co Wicklow (turn off the N11 at the signpost at Kilpedder). Telephone: 01-2819890.