The Ginger is available to high level users, and is placed between Purple Poppies and Cucumber on the last page of the game’s Seed tab. It is available to plant for 170 coins per square, and you’ll gain two experience points for doing so. After allowing the Ginger to grow for 18 hours, you can harvest your new crop for 320 coins per square.
As the Ginger is a permanent crop in the store, you’ll be able to master it, with the first level of Ginger Mastery being attainable at 650 harvests of the crop (remember, this can be made much easier by employing a bonus bushel from the Farmers Market, which gives you an extra point of mastery for each square you harvest, once you activate the bushel).
Head over to the game’s page to see if you’re a high enough level to plant this new crop.