Microgreens have been popping up as garnishes at high-end restaurants, but they’re more than just a gourmet trick of the trade. The 2-to-3-inch seedlings of edible plants, which add a burst of color and flavor to food, are packed with nutrients and can make for a fun winter gardening project. At an average retail price of about $30 per tray, the little nutritional powerhouses can be prohibitively expensive to buy.