How long firewood takes to season, and how to prove it has

Every primary source we checked puts the target in the same place. The EPA's Burn Wise program says wood burns best below 20 percent moisture. Cornell Cooperative Extension says to burn only wood below 20 percent. University of Illinois Extension says below 20 percent, and University of Tennessee Extension calls moisture content the single most important property of firewood.

Green wood is a long way from that. Cornell puts freshly cut firewood at 100 percent moisture or more, which on its definition means half the weight of the log is water. Every pound of that water has to boil off in your stove before the wood itself burns, and that heat goes up the flue. Illinois Extension's summary: wetter wood gives more smoke and less heat.

The EPA's floor is at least six months. Illinois Extension says six to nine months at minimum after cutting. Cornell says many species drop below 20 percent within a year, while some, such as oak, can take two years or more. The reason for the spread is how much water each species starts with. The Maine Forest Service's comparison chart shows how many pounds of water the same volume of wood has to lose between green and air-dry:

The pounds are the Maine Forest Service's. The bands are our reading of them against Cornell's one-year and two-year marks, and they line up with the months in our firewood BTU chart, where ash is the quickest of the hardwoods and oak needs two summers. Willow is the warning case: Maine notes that half the green weight of willow is water.

• Oak

  The slow one. Red oak has more water to lose than ash or either maple on Maine's chart, and Cornell singles oak out at two years or more. Cut it this winter, burn it the winter after next.

• Ash

  The quick one. Green ash has less water to lose than either maple or red oak on the chart, which is why ash is the hardwood most often burned in its first winter.

• Maple

  The middle. Sugar maple and red maple sit close together, both a year's job when split and stacked properly.

• Birch

  Treat it like maple and split it promptly; any log left in the round dries slowly.

Species pages with the wood's other habits: oak, ash, maple and birch.

Time in the stack is a guess. A meter reading is a measurement. The EPA's advice is to test wood with a moisture meter before you burn it, and Cornell explains the catch that most people miss: a typical meter has two short pins that go in about a quarter of an inch, so it only reads the outside. The outside of a stacked split dries first.

1. Pick a split from the middle of the stack, not the top row.
2. Split it again, so you have a fresh face from the inside of the piece.
3. Push the pins into that fresh face, near the middle of the piece.
4. Read two or three pieces. Under 20% on the fresh face is ready.

No meter? The sources agree on the field signs, and none of them is as good as the number:

Splitting is the biggest lever you have. UNH Extension calls splitting unsplit wood "critical" to speeding drying, because it opens far more surface to the air. Then the EPA's storage rule: outdoors, off the ground, with only the top covered, because covering the sides traps moisture in.

• Off the ground

  Cornell suggests pallets as the first layer. Wood sitting on soil stays damp at the bottom of the stack.

• Two rows deep at most

  Cornell's limit for a loose stack, so air reaches every piece.

• Sun and wind

  UNH: an open, sunny spot with airflow on all sides. Cornell warns a shady site, or one near a stream or lake, can take much longer.

• Away from the house

  Cornell suggests about 30 feet, because rodents and insects live in woodpiles.

If a removal crew leaves your tree in rounds, ask for it bucked to stove length before they go. That costs some saw time on the day, which is how tree removal cost is built, and it puts the wood on the drying clock a season earlier. Splitting it yourself means a saw and an axe, so read chainsaw safety and PPE first.

Seasoning does not make firewood safe to move. Insects and diseases that kill trees travel in and on firewood, and the eggs can be too small to see. The Don't Move Firewood campaign's rule of thumb is that 50 miles is too far and 10 miles or less is best. Some states set it in law: Cornell notes that New York's limit is 50 miles.

Cornell is frank that most firewood sold is green, and its advice is to buy at least a year before you plan to burn it. Do two checks before you pay. First, measure the load: a full cord is 128 cubic feet, stacked 4 by 4 by 8 feet, and a face cord is only the split length deep. The cord of wood calculator turns a measured pile into cords. Second, split a piece and meter the fresh face in front of the seller.

Then decide on species with the heat in view. The firewood BTU chart ranks the common species by heat per cord. A dense wood that is still wet is worse than a light one that is dry, so the meter reading comes first and the species second.