A rare collection of portraits of domestic staff at a Welsh country house that spans more than 100 years has gained a modern addition with a striking photograph of its recently retired head gardener.

The collection at Erddig in Wrexham highlights a new appreciation of the back-breaking work carried out by servants and staff in the upstairs-downstairs world of country houses and stately homes. Kensington Palace recently hosted an exhibition of portraits of overlooked, and often overworked, back-room staff in royal households.

The latest addition to the Erddig collection is a portrait of Glyn Smith, who retired last month after 38 years spent caring for the estate’s Grade I-listed gardens. Smith said he felthonoured to join the long line of commemorated employees.

Philip Yorke I, an earl who inherited the house in 1767, started what became a family tradition by commissioning a set of six portraits of servants in the 1790s. A seventh painting commemorates two black coachmen who worked at Erddig before Yorke was born.

Altogether, more than 40 of Erddig’s domestic staff were documented in painting, photographs and poems between the 1790s and 1920. The verses, written by Yorke and his successors, and displayed alongside the portraits in the servants’ hall, provided further details of the workers’ lives.

“We know [Yorke] was interested in social history but we don’t know why he decided to document his staff in this way,” said Susanne Gronnow, Erddig’s curator. “It was very unusual. I wonder what the artist made of Philip’s request – and also what the staff thought.”

One-off portraits had been painted of country house staff elsewhere, but this collection was remarkable for its record of successive generations of domestic staff, she said.

One of the first to be painted was Jane Ebbrell, who started work as a housemaid and spider-brusher in the reign of King George I. By the time she was immortalised in oil by a local Welsh artist at the age of 87, she was the doyenne of downstairs.

Ebbrell – wife and mother to two Erddig coachmen as well as fulfilling her own demanding role in the house – is painted sitting outside her cottage, with a dog at her feet, mops and brooms propped against the wall, a scroll on her lap and a dour expression on her face. The accompanying poem describes her as “the Mother of us all” who “by the Virtues of her mop, To all uncleanness, put a Stop”.

Until now, the last person to join the servants’ gallery was gamekeeper Alfred Thomas, whose photograph was hung in 1920 alongside a poem by Philip Yorke II. Social changes in the aftermath of the first world warresulted in the decline of country houses serviced by large teams of workers. Erddig has been cared for by the National Trust since the 1970s.

The new portrait of Smith pictures him smiling, wearing a tweed jacket and cap, a gardening fork in one hand and watering can in the other. The accompanying poem, written by a colleague, extols his “legacy in every plant, in every tree”.

Smith said: “In many ways, [Erddig] has been my garden, though I realise I have just been the custodian for a few of its years. I’m honoured to have been a part of its fascinating history and to play my part in conserving it for future generations, just as the servants I’m now pictured alongside once did for the Yorkes.”

After almost four decades, he said he was ready to hand over to his successor, the National Trust’s first female head gardener – but was planning to join 20 volunteers who work in the garden along with three staff gardeners.

The garden was a “very rare survivor of an early formal garden design, and one of our most important historic landscapes”, said Patrick Swan, the trust’s gardens and parklands consultant. “It needs specialist knowledge and a keen eye to care for it and for the last 38 years Glyn Smith has led the way with remarkable precision and attention to detail.”

Smith’s portrait and poem will be on display at Erddig until the end of May.