INTERNATIONAL advisers have laid plans for an irrigation system to help replant two Banyan trees threatened by the Royal Hong Kong Jockey Club expansion in Happy Valley. French firm Gregory International recommended an irrigation system for the trees, which have already been set in giant 'flowerpots' with underground cases built around their roots. The trees are scheduled to be removed from Sports Road in January 1995. Pipes for the irrigation system were installed last week and excavation work has begun around the tree roots. Monitors in the soil will gauge the water content, triggering sprinklers in the trees' branches when more water is needed. Pruning has almost been completed on the trees, which will eventually lose all their branches to reduce their weight, so they will be easier to move. A large container is being installed around the roots of each tree so they can be removed from the ground. ''It's just like a big flower pot, only this is a huge one,'' said Kenneth Wan Kwok-hing, project manager of external works and civil works for the Royal Hong Kong Jockey Club. So far the containers have been placed about two metres into the ground. They will be extended downwards gradually over 18 months. ''We can't cut too many roots off at the same time, otherwise we will damage the trees,'' Mr Wan said. Gregory International is the only firm in the world with experience in moving banyan trees. Advisers from the company extended a recent stay in Hong Kong to be with the trees during last month's typhoon, but returned recently to France.