If there's one design tip I've picked up while living in New York City, it's that a bouquet of flowers instantly elevates a space—no matter how small. When I invite friends over for drinks, I'll put together a simple glass vase of white and green hydrangeas in my living room for a touch of elegance. And sometimes, after a stressful week, I'll pick up bundles of dusty pink roses from the corner bodega. I like to make little arrangements using stemless wine glasses and dot them throughout my apartment to create a relaxed, yet sophisticated vibe. There's just one downside to buying bouquets: In a matter of a few days, they go from bright and beautiful to sad and withering. Hence, flower food was born—you know, those little packets that come wrapped up in the rubber band of a grocery store bouquet. A sprinkle of the stuff can work like a magical anti-aging serum (you know, the really expensive kind) for days-old flowers. But what do you do when you don't have ...