You will need whichever-custom-crop you-want-to-plant's recipe book. To get the recipe books, you have to gain access to the BossWorld and kill lots of custom crop mobs, like e.g. Lemon Lizards for Lemon recipes, or Blueberry Badgers for Blueberry recipes. Both are from the Bronze Zone. After a lot of kills you will find they drop a book, which is your custom crop recipe book. Once you have the recipe book, use it, and you will be able to plant the crop that the recipe specializes. E.g. With Blueberry recipe, you can plant blueberries, buying the seeds from the Jartex Shop.