is there any cactus that could survive 6a and 5b zone in london ontario. what the hardiest cactus for our zone that we can make it survive all winter. daves garden Smooth Mountain Prickly Pear atleast tells me 5b and lower! like 4a There are yuccas growing in our area hardy cactus should survive too. prickly bear is the one I want. check out daves garden plant files search cactus and you will find it. and it tells me that there are 128 cactus for 6a and 5b.