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All answers having "Hardwood tree" as clue:

3 Character Answers:

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ASH

ELM

OAK



4 Character Answers:

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ACLE

EBON

IPIL

TEAK



5 Character Answers:

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ALDER

BEECH

BIRCH

EBONY

LARCH

MAPLE



6 Character Answers:

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PINOAK

REDOAK



7 Character Answers:

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HICKORY



8 Character Answers:

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MAHOGANY



11 Character Answers:

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SLIPPERYELM