USDA Equisetum Arvense Herbal Supplement Non-Irradiated Kosher Certified by QAI Horsetail is also known as scouring rush, shavegrass, bottlebrush or pewter wort. Because of its large quantity of silica, it was used to clean metal. Our horsetail consists of the sterile, aerial parts harvested in the summer months between June and August. A perennial grass that is dimorphic, having a fertile stem in the spring which dies back and is replaced by a sterile stem in early summer. The fertile stem is brownish in color, shorter and unbranched. The sterile stem is green with whorls of needle-like leaves and jointed stems.