IF you are visiting the gardens at Glenveagh Castle in Donegal or Glanleam House in Valentia Island next month, don't go looking for the young head gardeners at either of the estates.

Both Sean O Gaoithin of Glenveagh and Seamus O'Brien from Glanleam will be far, far away - in a land of mist shrouded mountains and water, where giant leeches drop from the trees onto travellers below and where water snakes patrol the lakes.

The two young men are the Irish members of a 10 person botanical expedition to China's mountainous Yunnan province. Led by Alan Clark, a rhododendron expert and curator of Muncaster Castle Gardens in Cumbria, the expedition has been set up in conjunction with the Botanic Institute in Kunming, the capital of Yunnan.

For four weeks the two Irishmen, along with eight other botanists, horticulturists and geologists - six British and two Americans - will travel around Yunnan by jeep, on foot and with Sherpas and ponies, depending on the terrain. Their quarry is the wild rhododendron, and other plant species which live in this most southwesterly part of China "the Kerry of China", as Sean O Gaoithin puts it.

"It's the trip of a lifetime", says O Gaoithin. "We're going into a part of China which is very difficult to get to. We'll be going high high up in the Himalayas, and into places where westerners haven't been allowed since the 1940s." O Gaoithin (32), who comes from Kilmacud in Co Dublin, can't believe his luck. Head gardener at Glenveagh for Just 15 months, he finished his studies at the Botanic Gardens in Glasnevin only a couple years ago. And now this. "I met Alan Clark at the PlantNet (a joint British Irish plant organisation) conference in Easter and he asked me to go out on the trip because I have a good grounding in the rhododendron genus."

Clark asked could he recommend another Irish gardener with the right qualifications. He could: Seamus O'Brien, from Baltinglass in Co Wicklow, head gardener at Glanleam Subtropical Gardens.

O'Brien (26) admits he is "a chicken compared to the other people on the trip". But, it is the youthfulness of the two Irish gardeners that is one of their important assets. "Because we're the younger members of the team," O Gaoithin says, "we'll be depended upon to do a lot of the climbing and finding of plants."

The group will set off from Kunming - known as "the city of the eternal spring" because of its mild climate - along the historic Burma Road. In their bags will be a sophisticated satellite navigation system which will help them record the distribution of rhododendron species throughout Yunnan province. Their findings will be beamed via satellite into a computer somewhere in the Kunming Institute. "It's fairly high tech," says O'Brien. "It certainly beats pen and paper!"

In return for this service, the Chinese authorities will allow the expedition to collect seed to bring out of China. Besides rhododendron seed, the group will collect from many other plants - conifers, magnolias, oaks, primulas, poppies, arisaemas and lilies - growing in this floristically rich area. Last year Alan Clark led an expedition which gathered seed from 540 different species.

From this expedition, O'Brien hopes to bring back to Glanleam seed of tender epiphytic rhododendrons: tiny moss like species that grow on the branches of trees and which should thrive in Glanleam's beautifully mild and moist climate. Glanleam is also a conifer conservation site, acting as a "species bank" for endangered conifers. So O'Brien will be keeping his eyes peeled for specimens of Cunninghamia Ianceolata, the rare Chinese fir, and Keteleeria davidiana, a pine introduced to cultivation in 1888 by the Irish plant hunter Augustine Henry.

O GAOITHIN'S goal is to bring back seed from the nine foot tall giant lily, Cardiocrinum giganteum var. yunnanense, which takes five years to flower. Most of the seeds that the expedition collects will be from species already in cultivation in Europe. But by bringing in wild seed, "new blood" will be introduced, creating genetic diversity and thus stronger, better plants. And, according to O Gaoithin, "although a lot of collecting has been done over the last 150 years, they still reckon that there are a lot of species and better forms of species that haven't been brought back. That's the really exciting part!" Seed that is brought back to Ireland will be distributed among the various State run gardens, including Glenveagh. The privately owned Glanleam will also get a share of the spoils.

"This is the time of year says O Gaoithin, "because it is the seed season. We'll see little in flower, but we should be able to identify a good proportion of the stuff by the fact that we're already familiar with it." Unknown seed will be grown on in Ireland to be identified when it blooms. "Alpine" seed will be propagated at the Botanic Gardens in Glasnevin, and the plants will go on show in the brand new alpine house there.

For the two Irishmen the trip offers a chance to see how our "garden plants" grow in the wild. "We should be able to learn from that and practise better horticulture in Ireland," says O Gaoithin. O'Brien, however, has heard that "plants in Irish gardens are nearly better than what's growing in the wild". But to see them in their spectacular natural settings should be a real thrill, he insists.

Even if it means putting up with altitude sickness, mosquitoes, snakes, the odd bear - and those giant leeches.