The groundwork of the garden was laid down by Reverend John Moore following the completion of the house in 1861. It involved reclaiming a barren hillside of rock and gorse and creating a series of plantations, a walled garden and an extensive pleasure ground. Reverend Moore died in 1903 and bequeathed the property to his nephew, Hugh Armytage Moore, who was the man responsible for creating the gardens as we know them today. With his rare gift for planning and great eye for plants, he developed Rowallane using many of the novel plants of his day. Armytage Moore had close contacts with the Edinburgh Botanic Gardens among other places and raised many of his plants from seed collected by the great plant hunters of the early twentieth century, such as Wilson, Forrest and Kingdon Ward. He also developed a number of his own hybrids and was awarded the Victoria Medal of Honour by the Royal Horticultural Society in 1942 for his work. In 1955 the National Trust took over the running of his gardens, which are still maintained much as they were in Armytage Moore's time. The house is now the headquarters of the National Trust in Northern Ireland.

The avenue leading up to the house passes through much fine woodland, with arboreal rhododendrons and mossy rocks on either side and a series of idiosyncratic stone seats, cairns and ornaments built by Reverend Moore in the nineteenth century. From the car park visitors walk up to the house, passing the Home Wood where there are mainly large-leafed, late-flowering rhododendrons sheltered below beech and Scots pine. Species to delight in here include the yellow-flowering Rhododendron macabeanum, R. basilicum and the glabrous evergreen Trochodendron aralioides.

The Walled Garden beside the house contains a splendid collection of flowering herbaceous plants and some excellent shrub roses. Along the north side grow great clumps of Meconopsis, including the most popular Himalayan blue poppy M. grandis and a local variety, M. x sheldonii 'Slieve Donard'. In midsummer the various primulas put on a great display, especially the vigorous natural hybrid Primula 'Rowallane Rose' which seeded itself her in the days of Armytage Moore. Early-flowering shrubs in the area include magnolias, Vibernum plicatum tomentosum 'Rowallane' and many other tender subjects. The national collection of large-flowered penstemons is held at Rowallane and makes an excellent end-of-season show. Varieties to note include 'Garnet', 'Burgandy' and 'Sour Grapes', with delicious shades of crimson. Also eager to impress are a group of showy gloxina-like varieties characterised by clear white-throated flowers in reds and crimson.

The outer Walled Garden, formerly a nursery area, provides shelter for a variety of hostas and a collection of hydrangeas, notably the large H. sargentiana. Visitors will find the original Chaenomeles x superba 'Rowallane' here as well as the famous Hypericum 'Rowallane' hybrid which began as a seedling self-sown in the Rock Garden. It has a large yellow cup and glossy green foliage and is considered the most beautiful St John's wort in cultivation.

Passing through the Haggard area where the bursting seed capsules of a Populus maximowiczii resemble snow in July and August, the visitor arrives in the Spring Ground - a series of undulating slopes whose banks are covered with beds of azaleas and rhododendrons. Wildflowers are encouraged and the area has a high content of devilsbit scabious producing a beautiful blue haze in summer. During autumn there is much colour from maples and azaleas and tints of Prunus sargentii.

North east a narrow valley called the Stream Ground, which G. Forrest once styled as 'a bit of Yunnan', curves downward. It contains many triflorum rhododendrons as well as a boggy pool with royal ferns and bog arums. Beyond stretches the Hospital where visitors will come across a fine handkerchief tree, Davidia involucrata, and an immense Desfontainea spinosa. South east through the Old Wood lies the recently restored Rock Garden - a vast upheaval of natural whinstone accommodating a wide range of alpines, heathers and dwarf rhododendrons. More dwarf rhododendrons surround the Bishop's Rock, while further to the west the New Ground produces large rhododendrons and a number of fine trees, including a magnificent Chamaecyparis nootkatensis. Beyond lies Trio Hill with its brilliant orange red-flowering Embotherium coccineum. Among the many conifers here are fine specimens of Cupressus lusitanica 'Glauca Pendula' and Tsuga yunnanensis. Back across the Spring Ground the visitor should inspect the Paddock with its fine species of Sorbus and magnificent southern beeches before returning to the stable-yard.

The Pleasure Grounds - a lovely parkland that extends behind the house - are well worth a visit before leaving. Many of the fine trees here were planted by Reverend Moore in the last century and include Tsuga mertensiana and Glyptostrobus pensilis. Wildflowers are profuse here and grow in abundance; the variety of orchids is truly glorious; twayblade, greater butterfly and species of Dactylorrhiza. It's a butterfly paradise.