Obtaining hemp
Hemp seeds are rarely dropped from tall grass. They can also occasionally drop from fully grown hemp crops.
Farming hemp
Hemp requires 2 blocks of vertical space to fully grow and maximum light level. This means that artificial lighting needs to be as bright as daylight. Hemp is planted on farm land just like wheat. Hemp crops first grow slowly until the first half is fully grown. A second half can grow on top and it instantly turns into a fully grown crop. This means that the initial growth is slow but will be a lot faster later on. It is not recommended to harvest the lower half because the seed drop chance is below 100% and would lead to unsustainable farms.