Publishing is a ferocious business and most magazines don’t survive five years, never mind 100. Boston-based Horticulture magazine, the country’s oldest gardening magazine, has reached that landmark, and there’s a fat centennial issue with a new cover design and new features currently on newsstands. “There’s a lot of competition. That it’s lasted speaks for itself,” said Leo Blanchette, an avid reader who runs the noted Carlisle perennial nursery Blanchette Gardens. He has collected old Horticulture copies going back to the 1950s.