When Tim O'Brien looks around the vegetable house at Prairie River Greenhouse , he sees "a sea of tomatoes." This makes sense; he and his wife Lynn are offering 70 varieties of heirloom tomatoes this year at their 2-person operation located on a city lot in the town of LaPrairie MN. Prairie River Greenhouse is a post-retirement "second career" for the O'Briens. Tim is a retired forestry researcher for the University of MN at its North Central Research and Outreach Center in Grand Rapids. Lynn