The survey is part of the National Trust's three-year, £1 million project to survey 80 of the charity's gardens.
Head gardener Phil Rollinson and interpretative gardener Carolyn Estcourt have begun identifying and recording the estimated 20,000 plants in the Mount Stewart garden.
With the help of garden staff and volunteers they will be photographing each plant and using GPS mapping technology to record a grid reference, which will be entered into a database.
The information will be used to identify which plants, trees and heritage vegetables need to be propagated at Knightshayes Court in Devon to ensure their survival and to help the trust make plants available to other properties.
Trust representative Mike Collins said: "Mount Stewart is a jewel in the National Trust crown. It is home to plants collected from around the world by plant hunters on expeditions, and is valuable because this part of Northern Ireland has a very mild climate and the garden provides us with a glimpse into what gardens might look like in the UK as a result of a changing climate."