Parc Cefn Onn is a small, informal woodland park located on the northern edge of Cardiff, on the north side of the M4 motorway; there is no house attached to the park. It was laid out in the early twentieth century with some fine exotic specimen trees and shrubs, in particular conifers, rhododendrons and azaleas. The park has historical associations with Ernest Albert Prosser (1867-1933) general manager of the Taff Vale Railways, who laid out the Dingle garden from c.1911 and particularly from 1919 onwards with his head gardener Tom Jenkins.

The park is linear in shape occupying the sides and floor of a small north-south valley to the south of the Graig Llanishen scarp. It is bounded on the east by the Taff Vale railway, on the south by the M4, and on the north by farmland. To the west is rolling open ground now in use as a golf course for Llanishen Golf Club. The main feature of the park is the planting of ornamental trees and shrubs, in particular some fine conifers, rhododendrons and azaleas. These have been planted informally, next to paths, in glades, and interspersed throughout the native woodland. Although some of the detail of the park has been lost or changed since it was made it retains much of its original spirit. The layout is one of contrived rustic simplicity, with winding paths interlaced with water channels and a stream, channelled in places into small cascades and rills, opening to ponds and crossed by small rustic stone bridges. E.A Prosser built a summer house in the garden for his son, who was suffering from tuberculosis, with a pool below for bathing. A small pillar at the base of the summerhouse bears the initials of Ernest, Cecil and Donald Prosser and Tom Jenkins.

The park is entered at the south end and is divided into a northern and southern half, with iron railings and a public right of way separating the two sections. The northern half, known as The Dingle, is the original park, laid out by E.A Prosser as the setting for the proposed new house at Cwm Farm. The southern half was originally owned by the Plymouth estates. Both came to Cardiff City Council in 1944, after which date the southern part was planted to match the northern. Much of the original layout and planting of both sections was the work of the head gardener, Tom Jenkins, who was at Cefn Onn for over 30 years. He continued to be responsible for the park after 1944.

Sources:
Cadw 2000: Register of Parks and Gardens of Special Historic Interest in Wales, Glamorgan, p.65 (ref: PGW(Gm)22(CDF).
Parks & Sports Service Cardiff Council, Parc Cefn Onn Conservation Management Plan (2016).

RCAHMW, 15 July 2022