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All answers having "Prickly plant" as clue:

3 Character Answers:

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RHO



5 Character Answers:

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BRIAR

BRIER

GORSE

THORN



6 Character Answers:

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CACTUS

NETTLE

TEASEL



7 Character Answers:

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BRAMBLE

THISTLE



8 Character Answers:

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ACANTHUS

KEEPSAKE