Yes, PIGWEED is a Scrabble word!

Letter values in Scrabble US: P 3, I 1, G 2, W 4, E 1, E 1, D 2.

Is it a valid word?

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

common weedy European plant introduced into North America; often used as a potherb
leaves sometimes used as potherbs; seeds used as cereal; southern United States to Central America; India and China
leaves collected from the wild

Also: lamb's-quarters, wild spinach, chenopodium album, amaranthus hypochondriacus, lamb's-quarter.