Green wood is wood which has not been treated or seasoned, retaining a high moisture content. Typically, wood is considered green when it has been harvested recently, and when the moisture content is between 25 and 30%, depending on the tree. There are a number of characteristics associated with unseasoned wood which are important to consider when making a choice between green and seasoned wood.
When trees are alive, they have a high moisture content, thanks to their extensive vascular systems, which wick water and nutrients throughout the tree. Once a tree has been cut down, the circulation stops, but the wood remains very moist for months or even years, depending on the climate, unless the wood is dried. In warm regions, timber may be air-dried, and in other areas, it may be dried in kilns, for better control over the drying process. Some sawmills process their raw timber into lumber for drying, while others dry whole tree trunks.
Wood which has not yet been subjected to the drying process is known as green wood. This type of wood is much softer than seasoned wood, and less prone to splitting. Some people prefer working with green wood, because it is much more forgiving, and it is not as hard on tools as seasoned wood. However, there are some disadvantages to using wood which has not yet been cured.
The primary issue with green wood is that as wood dries, it shrinks and warps. If unseasoned wood is used in something like a structure, it will deform as it shrinks, potentially causing structural instability or reducing the airtight nature of the structure. Drying allows this process to happen before the wood is used, ensuring that the wood will remain straight and smooth when it is utilized in projects. Green wood is also easy to gouge or otherwise damage, creating structural or aesthetic defects which are not desirable.
This type of wood also performs very poorly in fires, because of the high moisture content. It burns inefficiently, and often uncleanly, generating a lot of smoke and debris which will clog the chimney. Green wood should be avoided when building fires, and ideally dense seasoned woods like oak and madrone should be utilized, since they burn very efficiently and generate a lot of heat.
Some lumber yards carry green wood for their customers, while others need to special order it. It is possible to purchase green wood and season it at home or in the workshop, although many people prefer to leave this process up to the lumber company. Even seasoned wood will need to be allowed to acclimate to the climate that it is going to be used in, which is an important consideration when buying a load of lumber for a project. The goal is to balance the moisture content of the wood with the moisture content of the air, reducing the amount of moisture which can be absorbed by the wood, and thereby minimizing the amount of swelling and subsequent warping and shrinkage of the wood.