Even oak varies. I prefer red oak, but will gladly burn any oak.
Hickory is excellent, but usually hard to split.

Everything else around here is very secondary, and burned only because it is already there.
The secondary woods in MY order of preference are Cherry, apple, and walnut. If you are cold enough to need to burn poplar, just keep carrying the stuff around to get warm- works better than burning it cause you can't carry it in fast enough to keep the fire going!

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Regards,
Lee Jarrett