Yes, HYDRASTISES is a Scrabble word!

Letter values in Scrabble US: H 4, Y 4, D 2, R 1, A 1, S 1, T 1, I 1, S 1, E 1, S 1.

Is it a valid word?

HYDRASTISES is a valid word in the official Scrabble dictionary (US) and the UK Scrabble Dictionary, but not in Words with Friends (ENABLE).

small genus of perennial herbs having rhizomes and palmate leaves and small solitary flowers; of northeastern United States and Japan

Also: genus hydrastis.