David Hay was born at Rhynd, Perthshire, Scotland, on 2 October 1815, the son of Ann McGregor and her husband, John Hay, a weaver. David is said to have trained as a gardener on an estate in Perthshire, where he would have received a thorough education. The three-year apprenticeship began at the age of 12. The boys learnt gardening by day and at night studied subjects ranging from grammar and arithmetic to geometry, surveying, botany, garden architecture and geography. Further experience could be gained by moving from estate to estate. At London, England, on 15 June 1840, Hay married Sarah Baker. They subsequently lived in northern Hertfordshire and in Gloucestershire.
In 1855 David, following his elder brother, William, a carpenter, emigrated to Auckland, New Zealand, with Sarah and their six children. Their last child was born there in March 1856. David Hay acquired upwards of 40 acres at Hobson Bay, of which six acres were deep alluvial soil and the remainder clay. He established the Montpellier Nursery on the best land and let milch cows graze the remainder. Montpellier's first advertisement appeared in the 1860 Chapman's New Zealand Almanac, listing 17 items besides a large stock of fruit trees and flower and vegetable seeds.
David Hay is credited with the first commercial introduction to New Zealand of Pinus radiata ( P. insignis ). In his 1860 catalogue only European conifers are mentioned, but from 1862 this began to change. It is likely that, initially, Hay's radiata pine and other American conifer plants were imported from Shepherd and Company of Sydney, thought to have been the first Australian nursery to stock Pinus radiata. There is no evidence of a direct American link at this stage, but a price drop in the seeds advertised in Hay's 1872 catalogue suggests that he was importing direct from America by this time, as were nurseries in other centres – William Martin and George Matthews in Dunedin, William Wilson and others in Christchurch, William Hale in Nelson, Robert Pharazyn in Wanganui and the Mason brothers in Auckland.
Hay compiled a manuscript on conifers and invited comments from Sir George Grey, who shared his interest. The pine tree in New Zealand, with additions by Grey, was first published in 1865 and by 1873 the book was into its third edition. In 1873 Hay exhibited cones of Pinus radiata in an address to the Auckland Institute. The release of large numbers of American conifers in this period was to change the New Zealand landscape permanently.
The New Zealand Institute published articles by Hay in its Transactions – 'On the cultivation of native trees' (1872), 'On trees suitable for streets and avenues' (1874), and on 'Lime as manure' (1876). In 1863 he wrote a monthly gardening column for the Southern Monthly Magazine. Catalogues were issued annually by G. T. Chapman, who also published Hay's annual garden book. Besides writing, he served on the committee of the Auckland Horticultural Society and was in demand for judging at flower shows.
The range of Hay's nursery stock expanded, but limited transport meant that in the early years customers were mainly from the Auckland area. By 1879 he had taken into partnership his only son, David Alexander. Like many other major nurseries, Montpellier by this time carried a comprehensive selection of plants and now fulfilled orders from further afield. That year D. Hay and Son took over the Victoria Nursery at Mechanics Bay, where trees were worked under the supervision of Edward Lippiatt, curator of the Auckland Acclimatisation Society. Many prominent nurserymen trained at Montpellier, including Hayward Wright (who developed the Meyer lemon), and the Cutler brothers.
David Hay was not generally involved in public affairs, but was well regarded as an informed man of sound judgement. He was a prominent member of St Mary's Church, Parnell, serving on its council for many years. He died at Auckland on 30 December 1883. His wife, Sarah Hay, died in 1903.
After David Hay's death Montpellier continued under his son. In these years the nursery reached its zenith, becoming widely known overseas as well as in New Zealand for quality and variety. Ten further acres were acquired near Orakei basin. Valuable plum varieties were introduced from Luther Burbank at Santa Rosa, California. Many importations came from Europe, while other fruit varieties, such as the Goldmine nectarine and Golden Queen peach, were local in origin. The 1903 catalogue advertised 2,500 plants. David Alexander Hay died in 1933; the nursery was leased for a few years after his death, but in effect Montpellier died with him.