Yes, BUCKBEAN is a Scrabble word!

Letter values in Scrabble US: B 3, U 1, C 3, K 5, B 3, E 1, A 1, N 1.

Is it a valid word?

BUCKBEAN is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

perennial plant of Europe and America having racemes of white or purplish flowers and intensely bitter trifoliate leaves; often rooting at water margin and spreading across the surface

Also: water shamrock, bog myrtle, marsh trefoil, menyanthes trifoliata.