Christensen, K.I. 1992. Revision of Crataegus sect. Crataegus and nothosect. Crataeguineae (Rosaceae-Maloideae) in the Old World. Systematic Botany Monographs 35: 1–199.
Phipps, J.B., O’Kennon, R.J., and Lance, R.W. 2003. Hawthorns and medlars. Royal Horticultural Society, Cambridge, U.K.
Phipps, J.B.; O’Kennon, R.J. (1998). Three new species of Crataegus (Rosaceae) from Western North America: C. okennonii, C. okanaganensis, and C. phippsii. Sida Contributions to Botany. 18(1): 169–191.
Phipps, J.B.; O’Kennon, R.J. (2002). New Taxa of Crataegus (Rosaceae) from the Northern Okanagan-Southwestern Shuswap diversity center. Sida Contributions to Botany. 20(1): 115–144.
Phipps, J.B.; O'Kennon, R.J. (2007). Hawthorns (Crataegus: Rosaceae) of the Cypress Hills, Alberta and Saskatchewan. Journal of the Botanical Research Institute of Texas. 1(2): 1031–1090.
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