THERE IS NOT much that can compare to the thrill of a good challenge, whether it be bungee jumping or lion-taming. In my case, I get my thrills from the challenge of successfully growing plants in far northern Wisconsin, just about an hour’s drive from Lake Superior, where winters can be seriously sub-Arctic. Not just any plants, but the kinds of plants that evoke the response “I can’t believe THAT will grow here!” from most folks. Plants like bamboo, blue poppies, large-leaved rhododendrons and, yes, cacti. It is surprising how many people in the Midwest regard cacti as tender houseplants akin to African violets, not realizing that several kinds are extremely cold-hardy, growing nearly as far north as the Arctic Circle.