Ivor Benoy, Trewithen's first Historic & Botanic Gardens Bursary Scheme placement trainee, has been reinstating the Cock Pit as a feature of the garden.
When researching the history of the pit, Benoy discovered that Thomas Hawkins, who owned the estate 300 years ago, used it for cock fighting.
However, it was when Johnstone inherited the estate 200 years later that he planted it out as an added attraction in the garden.
The second phase, now complete, included rebuilding all of the terracing and re-creating planting bays either side of the pit.
Benoy said: "The main aim was to make the area more accessible to our visitors because it has been somewhat overlooked. This is a fantastic project to start with."
He has planted Podophyllum hexandrum var chinense, Tricyrtis hirta and various types of fern to complement the existing six Dicksonia antarctica Award of Garden Merit that were planted in 1906.
Trewithen head gardener Gary Long said: "This is Ivor's first project for which he will achieve marks towards his final qualification. The process has been really beneficial and, come next spring when the gardens reopen to the public, the Cock Pit will look stunning."