Remove the bottom leaves from the stalks before plunging them in water. Use a tall vase—the large seed heads look best in a vessel with some height. Paired with flowers such as dahlias, amaranths’ long, dangling seed heads add drama as spillers. Bloom stalks last 7 to 10 days in a water-filled vase. Or hang bunches of seed heads upside down in a cool place to dry.