A perennial is a plant that continues its growth from year to year. In herbaceous perennials aerial parts die away in autumn, to be replaced by new shoots in the following year from underground structures, e.g., delphinium and rhubarb. In woody perennials, permanent woody stems above ground form the starting point for each new year's growth, a characteristic that enables some of them to reach a large size, e.g., shrubs and trees. Compare with annual and biennial.