Willow trees are classified as a hardwood, but they are softer and contain more moisture than other hardwood species. If given enough time to cure and dry, you can use willow wood successfully as firewood. Never burn austree willow wood that has been treated with pesticides — the fumes can be harmful if inhaled. Willow wood will not put off as much heat as more traditional firewoods, such as oak or pine. It also burns relatively fast, requiring you to spend more time tending to the fireplace.