Wet winters, climate change, storms on the way. Is it the end for the English country garden?

For 300 years, the yew hedges have flourished at the heart of the water gardens of Westbury Court – a manicured paradise cosseted for the nation by the gardeners of the National Trust.

Then, earlier this year, the custodians of this unique garden in Gloucestershire noticed something strange – the yews were dying, gnawed at their roots by a micro-organism that thrives in the damp. The direct source of the destruction was Phytophthora. But the true cause is a longer-term menace that is wreaking havoc across Britain's most famous gardens – the weather.

The National Trust has suddenly found itself fighting a battle to protect its herbaceous borders from high winds, floods, droughts, rising temperatures and wet winters.

The casualty list in the 200 Trust-owned gardens, beloved of armies of weekend coach parties, is growing rapidly.

As well as the yews of Westbury are hostas planted by Capability Brown in Warwickshire extinguished by summer droughts; white pine in Sussex decapitated by a winter storm; and a colony of rock roses munched to extinction by an explosion of aphids in Cornwall.

The problem is likely to spread to all the Trust's gardens and 69 landscaped parks – visited by an estimated 8 million people a year – and to every other flowerbed and window box in the country.

In short, outdoor Britain and the plants that grow in it are undergoing a revolution driven by climate change, which Britain's leading charity says nobody can afford to ignore.

Mike Calnan, head of gardens and parks for the Trust, said: "The latest scenario is that by 2080 average temperatures will have risen in Britain by three to four degrees Celsius.

"It will be accompanied by more extreme weather events, particularly storms. That has all sorts of consequences for any garden – the survival rate of tree species, the nature of garden collections."

Such is the extent of the problem that the Trust, with a range of agencies including English Nature, the Royal Horticultural Society and the Department for Environment, Food and Rural Affairs, has commissioned a study into how gardeners should deal with the increasingly unruly climate. The report, which will be published in November, will recognise that gardeners must radically change the varieties they use.

As the climate warms and indigenous plants either wilt in summer or drown in winter, drought-loving species from the Mediterranean and further afield will become popular. The English lawn, particularly demanding of water, could rapidly become a thing of the past. But for the National Trust, ripping up lovingly tended clematis and forests of camellia to be replaced by rows of cacti and yucca is virtually unthinkable.

The result for the gardeners responsible for looking after such horticultural gems as Sissinghurst in Kent and Stowe in Buckinghamshire is a practical and financial headache.

Mr Calnan said: "We are charged with maintaining the history and true culture of the gardens we have acquired into perpetuity. But we cannot sit there like King Canute and try to stop the tide – we are faced with a changing climate which is very different from the one these gardens were created for hundreds of years ago.

"Our challenge is to introduce the type of management techniques which will allow us to preserve what makes these properties unique, whether it is a particular arrangement, plant or landscape."

The Trust, which spends about £11m of its £199m annual budget on maintaining and developing its gardens, says it is prepared to go to extra expense, such as installing better irrigation and drainage, to maintain its best collections.

But the reality is that some of the most notable features of Trust gardens could be heading for the compost bin.

The yews at Westbury Court are one such case. They form part of the centrepoint of a peerless Dutch-style water garden, framing an ornamental canal built in 1700 by its designer, a retired soldier, Colonel Maynard Colchester.

The garden, which sits on the Severn flood plain, was acquired by the Trust in 1967 and became the first to be restored by the organisation, now one of the wealthiest charities in Britain.

But a string of wet winters over the past decade has seen parts of the garden regularly flooded by water. Every high tide brings a new influx of Phytophthora. The waterlogged yews, a third of which have already died according to the head gardener Jerry Green, are ideal breeding grounds for the organism, which thrives in the warm and wet conditions and is impossible to eradicate once established.

Dr Nick Haycock, a consultant hydrologist brought in to look at the gardens, has found that the predicted rise in the level of the Severn caused by climate change means Westbury is likely to flood at least 20 times a year.

An internal Trust report in May this year, which laid out suggestions for saving Westbury Court, said the best measure, a flood defence, would be hugely expensive and that the question of its "national importance" may have to be reconsidered.

Add to that the increased risk of inundation by salt water from a sudden tidal rush, and the future of Colonel Colchester's slow-growing yews – and the rest of his elaborate garden – is bleak.

Dr Haycock said: "The options range from putting a huge flood defence bund or earth bank round it to abandoning the garden to its own fate."

English Nature says all conservation groups are trying to tackle threats, whether to individual gardens or to entire species. In the Thames Valley and the Chilterns, it is feared the beech tree, which cannot withstand successive dry summers, could disappear over the next three decades. Waterlogging is an equal threat. A number of 150-year-old cedar trees had to be felled in the National Trust-managed Osterley Park, west London, earlier this year after "drowning" in rain-sodden soil.

Other British plants that thrive in the cold, such as the wood cranesbill, a type of geranium, and the dwarf willow could soon disappear from all but the chilliest regions.

The remedy, according to the Trust, will be long-term planning to protect existing gardens and short-term measures to alleviate problems as and when they occur.

The threat of winter storms, striking from random directions, means shelter butts – areas of woodland planted to protect species below – will have to be extended, a process that takes up to 50 years.

On a more immediate level, Trust property managers are also having to rethink their opening seasons. Warmer springs mean flowers are blooming earlier, requiring some gardens to open earlier in the year. But while the flower beds are blossoming, lawns still heavy with winter rain are easily churned up.

Mr Halnan said: "You end up with a mud bath as visitors try to cross the garden. There is a whole issue of the management of these sites."

He added: "Climate change is going to present us with difficulties but also opportunities – there may well be a wider range of plants we can use."

Quite whether that means the Trust, hitherto a redoubt of heritage from cream teas to rose gardens, will in the future be planting olive groves, lemon orchards and vineyards in a newly parched English landscape remains to be seen.

Gardens at risk

The South-east

Sheffield Park, 120 acres of landscaped 19th-century gardens in Sussex, has fallen victim to both ends of the spectrum of extreme weather. A drought caused a vast algae bloom in one of its lakes, above, which, with 400 tons of accumulated silt, cost £40,000 to remove. Winter brought heavy rain, killing a large number of trees that were waterlogged for six months.

The South-west

Shifts in the Cornish climate mean the camellias in the Trelissick gardens, above, bloom up to three weeks early, forcing the Trust to open its gates in mid-February. Magnolias, unused to extreme winter damp, are suffering from root rot. The mixture of warmth and wet has led to a large increase in aphids and honey fungus, which attack trees and shrubs, including the rock rose.

The Midlands

Summer droughts have wiped out the hostas planted by the 18th-century landscaping genius Capability Brown in the woodland garden at Charlecote Park, above, a Tudor mansion in Warwickshire. Two forms of blight, encouraged by wet weather, have also infected a new garden in front of the house, killing dozens of plants at a cost of £1,300.

The North-east

Normally used to harsh winters, the gardens of Cragside House, above, in Northumberland are now populated year-round by plants associated with warmer climates because of a run of mild weather between November and February. But a cold snap two years ago wiped out two flower species entirely. Heavy snows have also weakened the roots of trees.