Russell C., Felker P., Comparative cold-hardiness of Opuntia spp. and cvs grown for fruit, vegetable and fodder production, Journal of Horticultural Science, 1987, 62(4):545-550, doi: 10.1080/14620316.1987.11515819
The field trial compares 49 prickly pear materials after a series of frosts in Texas. It includes plants of different ages and planting dates.