Hardwood and softwood are broad biological terms used to describe species of wood. The terms have nothing to do with the physical hardness. Hardwoods come from broadleaved trees and softwood species from coniferous, evergreen trees. There is a wide range of species of wood within each category that are highly suitable for decking. The important factors for wood used outdoors is that it is durable and stable enough to resist fungal decay and not twist and warp. This means using a hardwood classified as naturally durable (some are not) or a softwood given extended durability through treatment. New technologies like ThermoWood
(heat treated timber) provide the ideal stability.