The story
From grow + -th (abstract nominal suffix). Compare Old Frisian grēd ("meadow, pasture"; > North Frisian greyde (“growth, pasture”)), Middle High German gruote, gruot (“greens, fresh growth, shoot”), Old Norse gróðr ("growth, crop"; > Faroese grøði, Danish grøde (“fruits”), Swedish gröda (“crop, harvest”)). More at grow.
Etymology from Wiktionary, CC BY-SA 4.0.