The £4.5m project, led by Pembrokeshire County Council in partnership with local authorities and charitable trusts in the area, will be part-funded by a £1.9m grant from the EU European Regional Development Fund.
The project aims to increase visitor numbers and create jobs through a series of renovations and enhancements to gardens and an integrated marketing campaign.
At Margam Country Park in Bridgend, the formal pleasure gardens will be restored and water features will be developed, including works to the Japanese Garden water feature, the orangery fountain and the development of a sustainable irrigation system.
The Victorian walled garden at Scolton Manor, Pembrokeshire, is to be restored to give visitors some insights into that era's horticulture.Improvements will also be made to facilities at the Bryngarw Country Park in Bridgend and Cwmdonkin Park and the Penllergare Valley Woods in Swansea.
Pembrokeshire councillor Rob Lewis explained: "Garden tourism has grown considerably in recent years and this opportunity will allow us to make significant enhancements to historic garden sites across south-west Wales by improving visitor facilities and expanding the historic interpretation of these important gardens."
He continued: "We anticipate this initiative will see around 2.2 million additional visitors and 60 direct and indirect jobs created by 2014." The project is part of a larger scheme by Visit Wales to develop several centres of excellence in the area.