Diggers at the National Trust for Scotland site dated their find seventh to ninth century AD. The dig was part of a seven-year refurbishment at Crarae Garden in Argyll.
The archaeological work was part of a major programme of re-development at Crarae garden, which began in 2002.
The £500,000 project mapped and numbered the plant collection, improved paths and bridges.
Crarae's mix of rhododendrons, camellias, magnolias and azaleas are said to give the site the feel of a Himalayan gorge.
Head gardener Nigel Price said: "Now that our extensive refurbishment is complete, the garden is looking better than ever."