Horsetail weed is an invasive perennial with fir-like shoots and spreading underground stems. Horsetail weed, or Equisetum arvense, reaches 10 to 20 inches high in the spring and 24 inches high in the summer. Horsetail weed is also known as mare's tail.
Its foliage is quite dense and difficult to eradicate when it has set root, as its underground stems grow as deep as 7 feet below the surface. Horsetail weed can be treated with regular mowing, with a strong weed killing agent or by pulling the weeds by hand. It may require many years of treatments before all remnants of the horsetail weed are out of the area.