Naturally grown Hemp plants can be usually found in forests, or near them, and in most deserts. Hemp Fibers can be harvested from Hemp plants; the wild kind are commonly found on all Maps in Experimental Rust. They can be harvested by targeting the Hemp plant and pressing the Use key(default "E"). A harvested plant yields 10 Cloth and 1 Hemp Seed, which can be then used to cultivate new plants in the future.
Holding down the Use key on Hemp plants grown from seed will give you the option of taking a cutting from a plant, yielding a Hemp Clone to plant where you choose.