Annuals complete their life cycle in one growing season. Seed germinates in the spring, the plant grows, flowers, produces seed and then dies. Perennials live for more than one growing season. There are two types of perennials. Herbaceous perennials generally die to the ground at the end of the growing season but send up new shoots the following spring. Woody perennials, such as trees and shrubs, do not die back to the ground but get larger each year.
Most greenhouses have frames of wood or aluminum. Wood will lose no significant heat through the frame. Over time the wood fades unless you keep it painted or finished, and it may rot or warp. Aluminum frames retain shape well, won’t corrode, and are often color-coated. Some heat is lost through the aluminum.