Douglas fir is an excellent option for outdoor construction projects. The species grows faster than hardwood, and the lumber is less expensive. The wood features a natural resistance to insects and decay, making it a go-to choice for those who work with untreated lumber. When used in direct contact with the ground, such as in raised garden beds, you can expect a three to five-year lifespan. Raised structures that use untreated Douglas fir may last for 10 or more years.