In the 1790s, he went, with a friend, to the Isle of Wight to study the local flora and they subsequently published a paper called ‘A List of Rare Hampshire Plants'. Shortly afterwards, Joseph Banks, a fellow Oxonian, invited him to become a fellow of the Linnaean Society. After settling in Bishopstoke in 1807, Garnier's natural love of flowers, and acquired knowledge of horticulture caused him to devote himself to the acquisition and collection of all the most novel and rare specimens of plants and flowers from every part of the world. His herbarium is lodged at the Natural History Museum.
In 1826 he built a long wall against which he planted rare and semi-tender plants, including magnolia and camellia. At the base were belladonna lilies and ixias, rising from a groundwork of mesembryanthemums. Beside the path stood tall urns with flowers, and on the lawn, sloping slightly southwards to some huge elms, were 50 different sorts of rhododendrons, and many rare and lovely plants, including Escalonia Floribunda, Cedrus Deodaria, Pinus Pallula from Mexico, and Araucaria Imbricata.
At one end of this terrace was a roomy, rustic summer-house, and at the other stood a handsome conservatory for exotics. Another feature was a circular bed of bush roses and dwarf georginas (dahlias) ringed by two beds of standard roses, the whole encircled by herbaceous plants, each of the three rings divided by four staggered entrances to the grass paths between the beds.
Garnier's gardens were featured in 1834 in Loudon's Gardener's Magazine. The articles give glowing descriptions, together with plans and planting lists.
In addition to being well planned and maintained, Garnier's garden was particularly well-known for its grass which was compared to soft green velvet, so carefully was it tended. This gem of English gardens made it one of the show places of England.
Garnier's garden at Bishopstoke was considered at its peak in 1851 when H.R.H. the Prince Consort made a special trip to visit it. A garden dedicated to Dean Garnier's memory and to his accomplishments as a gardener was created in the Winchester Cathedral close in 1996.