Take the Arch Druid's Seed Voucher to Mathrengyl Bearwalker in the Cenarion Enclave of Darnassus.
With what we call an Evergreen Pouch, we have cultivated Tharlendris seeds in the soil from Un'Goro Crater. These seeds blossom into an array of random, potent herbs. One result is morrowgrain, a mysterious herb we know little about; I intend to unravel this mystery.
Take this to Mathrengyl downstairs, and he will give you some seeds so you can get started. If you run out, you will need to purchase more from him.