Almost any type of wood works fine for square or rectangular columns. For economy or paint-grade, use fir or yellow pine. For high-end columns, use hardwoods such as birch, mahogany or white or red oak. As with any type of exterior wood, proper sealing and maintenance is required.
Is mahogany a hardwood?
mahogany, any of several tropical hardwood timber trees, especially certain species in the family Meliaceae. One such is Swietenia mahagoni, from tropical America. It is a tall evergreen tree with hard wood that turns reddish brown at maturity.