You should buy your wood as early as possible, but no less than a couple of months before you intend to burn it. This goes for all types and species. The exception is if you purchase kiln dried firewood, which is ready to burn immediately. That benefit just comes with a much larger price tag.
Different wood species dry at different paces. Hardwoods tend to dry a little bit slower because they have a higher material density. The initial investment of time is worth it though, as hardwoods generally contain a larger amount of BTUs’ by volume than softwood species. Because of this, dry hardwood will burn hotter for a longer period of time, while also producing less residual waste in the smoke to create creosote in your chimney.
To be considered “ready to be burned”, the wood needs to be seasoned. In layman’s terms, it needs to dry out. To do this, the wood should be cut into sections and then split to expose the inside sections to the heat of the sun and circulating air. These elements help to speed up the drying process immensely. There is a terrific article explaining the necessity of seasoning here at Mother Earth News.
Living in New England, almost everyone has different experiences and guidelines. The funny thing is, everyone agrees that firewood should be cut, split, and stacked for an extended period of time before it is burned so that the wood has time to season.
A good benchmark to follow is to ensure that your firewood is given ten months from the time it is cut, split and stacked/piled to the time when it will be burned. For all intents and purposes, the longer the wood can be set out to dry the better.
When trees are felled or cut down, they are considered “green”. This means that the wood still contains a high moisture content and it is not ready to be burned. In ordered for this green wood to be ready, the moisture needs to be given time to dry out so the wood will light up easily and burn cleanly.
Many will tell you that “green” wood really shouldn’t be burned until the year after it is cut, thus ensuring that it is given enough time to fully dry out. A full ten months should be sufficient to let the wood burn with moderate efficiency, though.
Semi-seasoned wood is firewood that has been cut, split and seasoned for around five to six months. It is usually delivered to people at the halfway point in the seasoning process. Semi-seasoned wood should still be purchased a few months in advance so there is still plenty of time to stack it and let is breath for an additional four or five months before it is burned.
This will give it the time to release any additional “piling moisture” that is retained from the months that it sat in a pile. “Piling moisture” results from having the wood not always sitting on top of the pile where it is drying. If the wood sits at the bottom of the pile for six months, it has caught all of the water running down from the wood piled above it.
Seasoned wood has been cut, split, and set out to dry for ten months or more (if you’re lucky). This wood should set on fire easily and burn efficiently. It will have a silvery-grey color and the ends will have “checks” or little cracks. There will be virtually no residual moisture in properly seasoned firewood.
That aside, seasoned firewood should still be bought a month in advance to give you the time to stack it and ensure that the wood releases any remaining residual moisture before it is burned.
The answer to the title question is, “it depends on what you are buying for firewood”. To be safe though, you should buy firewood as early as possible. Every firewood seller has a different process and a different concept of what seasoned, semi-seasoned, and green firewood is. If you are looking for a way to see if the wood is ready to burn, here is a great resource from the Chimney Sweep Fireplace Shop to help you identify the dryness of the wood.
No matter what, the longer that the wood is stacked on your property, the better the wood will burn in the winter.