Opuntia polyacantha (Opuntiakaktus)

There are many succulent species in the American genus Opuntia. Many of them are frost tender subtropical plants but O.polyacantha can tolerate even the Canadian winters. I have one specimen in my garden, just as a desperate experiment. The critical factor seems not to be the winter temperature but the humidity. I have had to change the growth place several times and still I´m uncertain about the final result.