William Nelson, nurseryman and plant collector, was the son of nurseryman John Nelson of Sheffield, England, and was educated at People's College, Sheffield. He worked for some time as a nurseryman at Bradfield, near Sheffield. In October 1876 he came to South Africa in the steamship Windsor Castle, which ran aground off the coast of Dassen Island without loss of life. After a few days on the island, during which Nelson did some botanising, the passengers were brought to Cape Town. On 31 October Nelso set out for Kimberley, a journey of four weeks by mule wagon. In letters to his family in England he provided a good description of the country he travelled through, including its fauna, flora and inhabitants. He spent the next year working as an overseer on the diamond diggings and during 1877 sent home a case of natural history specimens, including rocks and minerals, dried grasses, skins, birds' eggs, and land invertebrates. Around this time he also got to see the specimens that Dr Emil Holub* had collected on his travels into the interior.
Having earned enough to equip himself with a wagon Nelson left Kimberley in November 1877 and travelled first to Pretoria and then northwards, reaching the Soutpansberg district in January 1878. There he stayed with Antonio d'Albasini, the Portuguese Consul, and collected grasses, lichens, mosses, insects, shells and other natural history specimens, as well as cultural artifacts among the native population. From the Soutpansberg he went south to the Houtboschberg (near Haenertsburg), visited Mojadji's Kraal, and continued via Zebediela to Botsabelo, near Middelburg. There he turned east to Lydenburg and then went on to Natal, reaching Durban in May 1878. Soon afterwards he returned to England with his collections and a large number of photographs. His extensive collection of plants from the Transvaal and adjoining territories was presented to the Royal Botanical Gardens at Kew in August 1880. Many of his plants were new to science. One of these flowered in his nursery in Sheffield and was named Albuca nelsonii by N.E. Brown*.
In October 1881 Nelson signed a contract with the diamond pioneer Charles Newberry, in terms of which he was to plant 40 000 trees and shrubs annually on the farm Prynnsberg, east of Clocolan, in the Free State, for a period of four years. By the time the contract terminated in February 1885 Nelson had rented a farm in the Ladybrand district for four years. During this time he collected plants in the eastern Free State and Lesotho, including the type specimen of Kniphofia nelsonii. During the early 1890's he acquired an estate on the farm Turffontein, then just south of Johannesburg, where he developed one of the leading nurseries in the country. From there he supplied trees, shrubs and plants for many of Johannesburg's stately homes. He was contracted to plant trees along the streets of new property developments, including more that 100 km of street trees in the suburb of Kensington. Nelson's road in the suburb of Booysens was named after him.
In recognition of the many plants he sent to Kew Gardens he was elected a life Fellow of the Royal Horticultural Society, and a life member of the Council of Arboriculture of Belgium (LMCA). By 1903 he was a member of the South African Association for the Advancement of Science, but only for a few years. He married Clara de Gaars, with whom he had four children. One of his sons, Stanley, worked with him. Many plant species were named in his honour, including Indigofera nelsonii, Disperis nelsonii, Heliotropium nelsonii, Pelargonium nelsonii and Triaspis nelsonii, the last two by Dr J. Burtt Davy*. His original specimens are in the herbarium at Kew Gardens, while a set of duplicates as well as his own personal collection are now in the National Herbarium, Pretoria.