Dubbed the slowest of the performing arts, gardening can seem trend proof. After all, you can’t hurry an oak’s progress from acorn to shade tree, and making a garden isn’t like buying a new throw rug for your home but rather stitching a few glimmering threads of your own into nature’s rich tapestry. And yet tastes do change in gardening, as your once-obsessed African violet-growing parents or grandparents could tell you. Those who work with the buying public are especially attuned to what’s hot ...