I've done that with softwood and soft-ish hardwoods like poplar, but they seem to go punky before they dry out. SOP up here is to split them while they're green, otherwise you lose most of the heat value.

Willow seems to fare better. Our local 'Manitoba Maple' too. And I don't know about really "hard" hardwoods. I took out some old ash trees two years ago and the small stuff (lingering in a big brush pile) is utterly dry and solid; I haven't cut the big stuff yet.

Any thoughts from southern neighbours?