List of sources:
Agricultural Journal (Union of South Africa). 1911-1913, Vol. 2, 4 and 6, articles by Hardenberg.
FamilySearch: Christiaan Bernhardus Hardenberg. https://www.familysearch.org/search/record/results?q.givenName=Christiaan Bernhardus&q.surname=Hardenberg
Janse, A.J.T. A short history of the South African Biological Society. South African Biological Society Pamphlets, 1939, No. 10, pp. 49-58.
Lounsbury, C.P. The pioneer period of economic entomology in South Africa. Journal of the Entomological Society of South Africa, 1940, Vol. 3, pp. 9-29.
National Union Catalogue, pre-1956 imprints. London: Mansell, 1968-1980.
Records of the Albany Museum, 1919, Vol. 3(3), paper by Hardenberg.
Roll of members [of the SA Biological Society]. South African Journal of Natural History, 1919, Vol. 1(2), pp. 274-277 and 1920, Vol. 2(2), pp. 293-301.
Royal Society of South Africa. Transactions, 1917, Vol. 6, lists of Fellows and members.
South African Association for the Advancement of Science. Report, 1916, paper by Hardenberg; 1918, list of members.
South African Journal of Science, 1922, Vol. 19, paper by Hardenberg.
The Transvaal Biological Society. South African Journal of Natural History, 1918, Vol. 1(1), pp. 22-32.
Union of South Africa. Public service list, 1914.