National Trust researchers will look at how every British garden from the historic park to the suburban plot will withstand the effects of the hotter, more unstable climate scientists believe will affect Britain by 2050, if not before.
The forecasts range from drought to heavier rainfall, to more frequent, more violent storms and a significant increase in pests and diseases. The traditional plants and trees of the British garden may face attack on several fronts at once.
Parched conditions could pose severe difficulties for many flowers and shrubs such as rhododendrons, which need a high moisture content in the soil near the roots, the trust believes, unless enough extra water is available.
But keeping parched gardens going all over the country when water for domestic use is at a premium may well be very difficult, if not impossible. The lush lawn of emerald green could become a memory.
Trees such the oak and the ash, which govern the character of many historic parks, will not only suffer from drought, the trust fears; they will be vulnerable to a greater number of pests that have survived through warmer winters - from insects and fungal infections to more grey squirrels and rabbits - because frost may have gone from southern England by 2050.
They might then be frequent storms of the intensity of the hurricanes of 1987 and 1990.
More vulnerable species such as roses are even more likely to suffer from such multiple assaults.
The trust's head of gardens, Mike Calnan, points to the old walled rose garden of Mottisfont Abbey on the River Test in Hampshire as the sort of place under threat.
The garden contains French moss, damask and bourbon roses from the 18th and 19th centuries, which are not bred for hardihood like the modern varieties.
"They are very fragile plants which are practically antiques in the garden world," he says. "They may well be more at risk from drought, and they may well be more at risk from increased pests and diseases. And if we have the heavier rainfall that is predicted they may suffer from that as well." Although much work has already been done on how agriculture will fare in a different atmosphere caused by global climate change - how will wheat grow, and where? - horticulture has so far benefited from little such research.
The National Trust is spearheading the new inquiry because it is the world's biggest owner of gardens, caring for 200, mostly historic and many, such as Sissinghust in Kent, internationally famous. It will be joined in the study by the Royal Horticultural Society and the UK Climate Impacts Programme, a government-sponsored research centre at Oxford.
The trust's executives are particularly interested because they have to think in very long timescales, says Mr Calnan. "Some of our gardens have evolved over 300 years, and we are dealing with planting today, of things like oak trees, that will take us into the 24th century. We need to be reassured that we are planting the right things that will last for the future.
"We look after gardens from the north-east to the south-west of Britain that experience a variety of growing conditions - and we do know these are likely to change."
The international community now accepts that the world's atmosphere is warming because of the increased concentration of gases such as carbon dioxide from power stations and motor vehicles.
Scientists believe the change is already detectable in the temperature rises recorded in the past decade.
Last year was the hottest recorded for the world, and may have been the hottest for 1,000 years.