Yes, ROOTSTOCKS is a Scrabble word!

Letter values in Scrabble US: R 1, O 1, O 1, T 1, S 1, T 1, O 1, C 3, K 5, S 1.

Is it a valid word?

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

a horizontal plant stem with shoots above and roots below serving as a reproductive structure
root or part of a root used for plant propagation; especially that part of a grafted plant that supplies the roots