Clarke, John (1889–1980), potato breeder, was born on 1 February 1889 in his mother's homeplace of Lemnagh Beg in the parish of Ballintoy, north Co. Antrim, eldest child of Daniel Clarke (d. 1940) and his wife Rose Clarke (née McLernon) (d. 1897). Daniel Clarke owned a small farm in the townland of Broughgammon, thirty acres of cutover bog, with eleven acres of deep bog. When John was eight, his mother died of pyaemia, three weeks after giving birth to her third child and second daughter, who was named Rose and brought up by her grandmother. John went to St Mary's national school, Ballinlea, but his formal education finished when he was 12, and he worked thereafter on the home farm. His father was interested in potato-growing, and John applied to have the farm inspected and certified for the production of seed potatoes. In 1924 he met an official of the newly formed Northern Ireland Department of Agriculture, John Bankhead, himself later a noted potato breeder, who gave him a pamphlet produced by the Scottish agriculture board, in which there was some information about how to set about breeding new varieties. Despite his brief formal education, Clarke would in his spare time travel several miles by bicycle to a local library to read technical material on plant genetics, and in 1928 he started experimenting at first with just six potato seedlings, growing in tin cans in makeshift cold frames at Broughgammon. From 1932 he worked exclusively on the production of seed potatoes and of new potato varieties.
The potato's long history of undergoing human breeding interventions as an agricultural crop means that there is a great deal of genetic variation available for manipulation, but the potato of cultivation is generally tetraploid (having four sets of chromosomes), which makes the process of breeding, selection and cultivation of valuable new varieties especially complicated, time-consuming and labour intensive; at least eight years are required to trial and grow on potential candidates. Clarke built a tin hut, and later several greenhouses, to protect some of the thousands of seedlings which he grew every year, and from which he selected possible new varieties. He developed his skills of observation and evaluation as he tended cross-pollinated plants, choosing desirable traits from which to breed. He selected the most promising plants, and with help from the ministry's potato-testing facilities, from Bankhead, and from scientists elsewhere, developed his first new variety, certified as a seed potato in 1936 as Ulster Monarch. This was the first of thirty varieties that Clarke developed, all with the word 'Ulster' in their names; after a change in nomenclature rules, the Antrim man registered three more, called Dundrum, Dunluce and Dundrod. Several of the varieties were lastingly popular, Ulster Champion, Ulster Sceptre and Dunluce in particular, and breeding lines established by Clarke produced in the next generation particularly successful modern varieties like Maris Piper. The economically very important production of certified seed and ware potatoes in Northern Ireland owed a great deal to Clarke's work, and his contribution was recognised when he was awarded the OBE in 1969.
Clarke's other honours included the Lord Derby gold medal from the Ormskirk exhibition (for the Ulster Supreme variety, in 1947 and 1948), the Haigh Trophy in 1954 for his outstanding contribution to the potato trade, and the Belfast Telegraph Cup in 1957 for outstanding achievement in agriculture. In the mid 1950s he received the John Snell medal from the National Institute of Agricultural Botany, and in 1950 was awarded an honorary M.Agr.Sc. degree by QUB, perhaps the first ever awarded to a self-taught farmer; his presence at the graduation ceremony was greeted with lengthy applause. The degree was undoubtedly merited as well as celebratory; John Clarke from Broughgammon worked with a number of internationally known scientists, including the most important botanical geneticist of the day, Redcliffe N. Salaman (1874–1955), FRS. Salaman and others visited Clarke at Broughgammon, and Salaman commented that Clarke's grasp of the scientific method was outstanding, as was his flair for choosing promising crosses. Clarke and Salaman, and other collaborators, worked together in trying to develop varieties less susceptible to frost and especially to blight. He also was first to develop a variety that had resistance to a common strain of eelworm.
After his marriage in Dublin on 4 September 1947 to Angela Hayes, from Co. Kildare, who had come as a teacher to Ballinlea, Clarke bought a farm at the Giant's Causeway. However, the land had potato root eelworm, and the prevailing winds and lack of shelter shredded tender leaves every summer, so that the new farm proved unsuitable for potato-growing. Clarke therefore sold the Causeway farm, and bought another further inland, at Glassaneeran, near Mosside. There he grew eighteen acres of seed potatoes for sale, and carried on his breeding experiments. The Clarkes lived in Ballymoney for a year after they sold the farm, and then moved to Delgany, Co. Wicklow, in 1974.
Clarke died aged 91 on 28 May 1980, and is buried in Kilquade, Co. Wicklow. He was survived by his wife and their adopted daughter. He had been well known in Northern Ireland at the height of his career, but twenty-five years after Clarke's death, a local historian, Maurice McHenry, brought Clarke's achievements before a new audience, researching biographical publications and giving talks and presentations on local radio and television. A blue plaque was erected at 'Innisfree', the house on Clarke's former farm at the Causeway, and in 2013 volunteers organised the first festival in honour of Clarke's work and of the potato in Ireland; the Northern Ireland Potato Festival was successfully held at the Giant's Causeway, and is intended to be an annual celebration.