Fuschias are among the plant species hit by the severe winter weather
Severe winter weather has damaged more than half the plants in some National Trust gardens, the charity says.
South-west England's usually-mild climate allows Mediterranean and southern hemisphere plants to grow.
But after the coldest January in Devon and Cornwall for more than a decade, the trust says many of these "tender species" have been badly hit.
Volunteers examined 14 estates to find that frost and snow had also delayed the early blooming of spring flowers.
At the sub-tropical Glendurgan gardens, near Falmouth, a third of the plants have been lost or damaged. Succulents, tree ferns and fuschias were worst hit.
About 60% of the garden at Knightshayes in Devon, has been affected by the weight of snow on the plants, including a magnolia tree which split in half.
While the number of plants flowering now is less than half that at the same time last year, the trust says it is optimistic that the gardens will recover over time.
It warned in December that UK wildlife was struggling to cope, as erratic and unseasonal weather took its toll for a second consecutive year.
Birds, mammals and particularly insects had all suffered from a cold, late spring, a wet summer with little sunshine and a long, dry autumn, it said.
The trust said species under threat include puffins, marsh fritillary butterflies and lesser horseshoe bats and warned another wet summer in 2009 could be a disaster for insects.
December's dry weather and the cold January had "wiped out a generation" of frogs in Cornwall's Lizard peninsula, the charity has also revealed.
The Cornish chough had also struggled to feed, as the frost prevented it poking its beak into the ground to feed on small invertebrates.