Garthmyl Hall (nprn 283) lies on the north-west side of the Severn Valley and is set in a small park (300347). Gardens lie to the west and north of the house. The earliest evidence for garden is an illustration of 1798. But by 1886, and probably by 1883, the gardens had taken on their present form. The pleasure grounds were densely planted by this time and the fishpond, or lake, was developed from an earlier farm pond. It was given a stone lining, with a plug, and a small island.

The gardens cover about 2 acres (0.8 ha) (excluding the kitchen garden) and mainly comprise two large areas of lawn with peripheral plantings of trees and shrubs. The western boundary is defined by a shelter belt of mature specimen trees which include cedars (possibly dating from the late eighteenth century), a redwood and a copper beech; these dominate the garden.
The main walk is a path from the forecourt at the south-west corner of the house leading north, past the site of a conservatory, through the west and north gardens towards the walled kitchen garden (700348). To the west of this path is a lawn stretching to the garden boundary and the tree shelter belt, with four circular beds of azaleas and a magnolia in the lawn. On the east side of the path, a narrow border runs between the path and the west external wall of the service court, against which creepers and a wisteria grow. Beyond the conservatory site the main walk continues down a flight of steps, through the centre of the north garden and around a wide, raised, circular basin containing an iron fountain.
On the west there is a large level lawn dominated by a mature cedar of Lebanon and, on the east, there is a square rose garden, consisting of seventeen cut beds. 

To the east of the house there is an additional area of about an acre (about 0.5 ha) of relict pleasure grounds which contains a small lake. Densely planted by the 1880s, the area around the lake has been redeveloped as a modern private garden for a new house, 'Lakewood'.

Sources:
Cadw 1999: Register of Parks and Gardens of Special Historic Interest in Wales, Powys, 72-6 (ref: PGW Po58(POW)).
Ordnance Survey second-edition 25-inch map: sheet Montgomeryshire XXX.14 (1902).

RCAHMW, 30 June 2022